Groynes in Coastal Engineering. Guide to Design, Monitoring and Maintenance of Narrow Footprint Groynes

C793

Groynes are long, narrow structures built approximately perpendicular to the shoreline. They are designed to control longshore transport of sediment on beaches and to deflect nearshore tidal currents. Their principal purpose is to slow the longshore movement of beach material by interrupting the movement of material and trapping it on the updrift side of the groyne. They also help to build beaches that protect land behind the beach from flooding and erosion. Groynes may extend across part or all of the intertidal zone and are normally grouped together to form groyne engineering coastal in Groynes systems or fields. This guide: ? explains the role of groynes in the context of wider shoreline management and sediment transport processes ? provides guidance on setting the layout and profile of groynes to optimally influence transport of beach material and to retain the desired beach shape and profile ? provides a framework for the design and maintenance of all types of groynes with particular emphasis on timber and other 'narrow footprint' groynes. (Narrow footprint groynes are those whose width is very narrow in comparison with their length and are commonly supported on vertical piles.) Groynes in ? provides advice on good practice approaches to construction, routine and periodic maintenance and to the use of sustainable material during repair and replacement ? focuses predominantly on UK experience and situations (eg macro-tidal) but with appropriate coastal engineering reference to international experience. CIRIA

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Groynes in coastal engineering Guide to design, monitoring and maintenance of narrow footprint groynes

Jonathan Simm, Aurora Orsini, Belen Blanco HR Wallingford Alex Lee, Paul Sands Royal HaskoningDHV John Williams RSK Anthony Camilleri Mackley Construction Roger Spencer Arun District Council

Griffin Court, 15 Long Lane, London, EC1A 9PN Tel: 020 7549 3300 Fax: 020 7549 3349 Email: [email protected] Website: www.ciria.org Groynes in coastal engineering. Guide to design, monitoring and maintenance of narrow footprint groynes

Simm, J, Orsini, A, Blanco, B, Lee, A, Sands, P, Williams, J, Camilleri, A, Spencer, R

CIRIA

C793 © CIRIA 2020 RP1067 ISBN: 978-0-86017-898-9

British Library Cataloguing in Publication Data

A catalogue record is available for this book from the British Library.

Keywords Coastal and marine, environmental management, health and safety

Reader interest Classification Coastal engineering, shoreline management, Availability Unrestricted beach management, groyne design, construction Content Advice/guidance and management Status Committee-guided User Users, planners, designers, coastal and beach managers, material procurement managers, asset managers, designers, construction contractors, educational institutions

Published by CIRIA, Griffin Court, 15 Long Lane, London, EC1A 9PN, UK

Please cite this publication as:

SIMM, J, ORSINI, A, BLANCO, B, LEE, A, SANDS, P, WILLIAMS, J, CAMILLERI, A and SPENCER, R (2020) Groynes in coastal engineering. Guide to design, monitoring and maintenance of narrow footprint groynes, C793, CIRIA, London, UK (ISBN: 978-0-86017-898-9) www.ciria.org

This publication is designed to provide accurate and authoritative information on the subject matter covered. It is sold and/or distributed with the understanding that neither the authors nor the publisher is thereby engaged in rendering a specific legal or any other professional service. While every effort has been made to ensure the accuracy and completeness of the publication, no warranty or fitness is provided or implied, and the authors and publisher shall have neither liability nor responsibility to any person or entity with respect to any loss or damage arising from its use. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, including photocopying and recording, without the written permission of the copyright holder, application for which should be addressed to the publisher. Such written permission must also be obtained before any part of this publication is stored in a retrieval system of any nature. If you would like to reproduce any of the figures, text or technical information from this or any other CIRIA publication for use in other documents or publications, please contact the Publishing Department for more details on copyright terms and charges at: [email protected], Tel: 020 7549 3300.

ii CIRIA, C793 Summary

Groynes are long, narrow structures built approximately perpendicular to the shoreline (Figure 1.1). They are designed to control longshore transport of sediment on beaches and to deflect nearshore tidal currents. Their principal purpose is to slow the longshore movement of beach material by interrupting the movement of material and trapping it on the updrift side of the groyne. They also help to build beaches that protect land behind the beach from flooding and erosion. Groynes may extend across part or all of the intertidal zone and are normally grouped together to form groyne systems or fields. This guide: „ explains the role of groynes in the context of wider shoreline management and sediment transport processes „ provides guidance on setting the layout and profile of groynes to optimally influence transport of beach material and to retain the desired beach shape and profile „ provides a framework for the design and maintenance of all types of groynes with particular emphasis on timber and other ‘narrow footprint’ groynes. (Narrow footprint groynes are those whose width is very narrow in comparison with their length and are commonly supported on vertical piles.) „ provides advice on good practice approaches to construction, routine and periodic maintenance and to the use of sustainable material during repair and replacement „ focuses predominantly on UK experience and situations (eg macro-tidal) but with appropriate reference to international experience.

Groynes in coastal engineering iii Acknowledgements

This publication is the result of research project (RP)1049 produced under contract to CIRIA by HR Wallingford and subcontractors Royal HaskoningDHV, RSK, Mackley Construction, Arun District Council. The starting point for the project was a first draft of a guide on maintenance of timber groynes prepared for SCOPAC by the late Prof Dr Andrew Bradbury. This guide is dedicated to his memory, remembering his passion for groynes and coastal engineering more generally.

Authors

Dr Jonathan Simm

Dr Jonathan Simm is chief technical director for resilience at UK-based research and consultancy organisation HR Wallingford, with responsibilities for developments and applications in performance, risk, materials and sustainability. Originally trained as a coastal civil engineer, over the past 30 years Jonathan has been involved in the production of more than 20 technical guidance documents. He was technical lead for The International Levee Handbook (CIRIA, Ministry of Ecology, USACE, 2013) and is currently helping to edit international guidelines for the use of natural and nature based features. He chairs national and international committees and his passion is translating research and innovation into policy and practice. Chapters in this guide: overall editor and 1.

Aurora Orsini

Aurora Orsini is a principal coastal engineer and coastal lead at UK research/consultancy organisation HR Wallingford, responsible for coastal and maritime engineering consultancy and research. Aurora graduated in civil engineering in Rome, specialising in hydraulic and maritime engineering. She later specialised in environmental engineering for the effect of sea level rise at Southampton University, completing a master of research, and held a research position at L’Aquila University (Italy) on integrated coastal zone management. Aurora has over 20 years’ design experience working on UK and international civil engineering projects, with expertise in the design of coastal protection structures. Chapters in this guide: 2, 3, 4

Dr John Williams

Dr John Williams has over 20 years’ experience as a timber technologist working in the construction industry. His principal area of expertise is the assessment of structural timber in safety critical applications such as civil engineering projects and infrastructure. He has delivered several research projects aimed at promoting the use of timber in marine engineering projects and has developed fast-track screening tests to identify potentially-suitable, but less used, timber species. He is an experienced lecturer on the examination and assessment of structural timber with many specialist papers published on the subject. Chapters in this guide: 6, 10

Alex Lee

Alexander Lee is a chartered civil engineer, member of the Institution of Civil Engineers, and technical director at Royal HaskoningDHV. Alexander has over 20 years’ experience working on the appraisal and design of flood and coastal erosion risk management schemes, including for local authorities and the Environment Agency. The schemes have involved the design and construction of new timber groyne fields, the maintenance and refurbishment of existing groynes, as well as beach replenishment, recycling, and other forms of beach control structure. Chapters in this guide: 5, 6, 7

iv CIRIA, C793 Anthony Camilleri

Tony Camilleri is the managing director of Mackley and a Fellow of the Institution of Civil Engineers. He has 46 years’ experience in civil engineering and construction, delivering fluvial and coastal defence projects. He was project director for the Eastbourne Coast Protection Scheme between 1995 and 1999 which comprised of 106 large hardwood timber groynes and associated works. In 2000 Tony became the Mackley lead within the Pevensey Coastal Defence Consortium who were contracted to manage 9 kilometers of beach frontage under a 25-year private finance initiative (PFI) scheme. Since then he has been involved in the planning, programming and construction of hardwood timber groynes for many coast protection schemes along the south coast. Chapters in this guide: 8

Roger Spencer

Roger Spencer is a long-standing member of staff at Arun District Council and has come up through the engineering ranks, having latterly been the Council’s principal coastal engineer, and currently as engineering services manager. Besides his love of coastal defences, he seems to get ‘volunteered’ for anything wet (or potentially wet) within the Council’s boundary, and he brings to those tasks experience gained in looking after 24 km of coast with nearly 300 proactively managed timber groynes. He has been involved with monitoring, maintaining, designing and constructing timber assets for over 40 years but is yet to rebuild a groyne that he built! He was on the steering group for SMP2 guidance and is a vice-chair of the SE Coastal Group. Chapters in this guide: 9, 10

Other authors of this guide: Belen Blanco HR Wallingford (Chapters 3, 4) Paul Sands Royal HaskoningDHV (Chapter 6)

Some of the material in Chapters 5 and 7 was based on a technical note prepared by Simon Howard before his retirement from Royal HaskoningDHV in 2017. The note was based on 40 years’ experience in the design and construction of a wide range of groynes around the UK. The material from the draft SCOPAC timber groyne maintenance guide was principally used in Chapters 9 and 10, but excerpts of material were also used in other parts of the document.

Project steering group Mike Bekin Ecochoice Anthony Camilleri* J T Mackley & Co Ltd Samantha Cope Havant Borough Council Bryan Curtis Independent Consultant Peter Ferguson New Forest District Council David Harlow Bournemouth City Council Wayne Hope Denbighshire County Council Owen Jenkins CIRIA Peter Lawton St La Haye Consulting Engineers Ltd Alexander Lee* Royal HaskoningDHV Sue Manson Environment Agency Aurora Orsini* HR Wallingford Louise Pennington Natural Resources Wales Andrew Powell Environment Agency Nicholas Pritchard Natural Resources Wales Paul Sands Royal HaskoningDHV Jonathan Simm* HR Wallingford

Groynes in coastal engineering v Roger Spencer* Arun District Council Owen Tarrant Environment Agency Ian Thomas* Pevensey Coastal Defence Richard Thomas (chair) Independent Consultant Marek Weroniecki BAM Nutall John Williams* RSK Jack Young CIRIA * corresponding members

Project funders BAM Nuttall Ecochoice Environment Agency (project code SC160016)

CIRIA project team Owen Jenkins Project director Jack Young Assistant project manager Clare Drake Publishing manager

Other contributors CIRIA would like to thank Bridget Woods Ballard and Mike Dearnaley (HR Wallingford) and Uwe Dornbusch (Environment Agency) for detailed reviews of the guide and many helpful suggestions. Input and provision of photographs has also been provided by JBA Consulting, JBA Bentley, Mott MacDonald, the Environment Agency, Highways England, Transport Ireland, Welsh Government, and the Department for Infrastructure (Northern Ireland).

CIRIA would like to acknowledge the following with respect to some of the contents within the guide: Salvatore Russo HR Wallingford (student) Bryan Curtis Chair of coastal groups/Arun DC Rod Fox (deceased) Revaluetech Ltd

vi CIRIA, C793 Contents

Summary ...... iii Acknowledgements ...... iv Glossary ...... xiv Abbreviations and acronyms ...... xix 1 Introduction ...... 1. 1.1 Groynes in coastal engineering ...... 1 1.2 Background to the guide ...... 3 1.3 Aims and objectives ...... 4 1.4 Structure and use of the guide ...... 5 1.5 Target readership ...... 8 2 Groynes and shoreline management ...... 9 2.1 An introduction to groynes and shoreline management ...... 9 2.1.1 Groynes and shoreline management in the UK ...... 9 2.1.2 Long-term management of groynes ...... 12 2.1.3 Sediment trapping efficiency of groynes ...... 13 2.1.4 ‘Groynes’ formed by other structures ...... 15 2.1.5 Amenity and heritage value of groynes ...... 16 2.1.6 Groynes vs. alternative shoreline control structures ...... 17 2.1.7 Groynes used with other coastal defence measures ...... 20 2.2 Types of groynes ...... 20 2.2.1 Piled narrow footprint groynes ...... 20 2.2.2 Gravity groynes ...... 22 2.2.3 Bulk/rubble-mound groynes ...... 23 2.2.4 Permeable groynes ...... 24 2.2.5 Hybrid structure groynes ...... 25 2.3 Whole-life management of groynes ...... 27 2.3.1 Adaptive management of groynes ...... 27 2.3.2 Costs and funding ...... 28 2.3.3 Removal of a groyne field ...... 29 2.4 Licensing and consenting for groyne structures ...... 29 2.4.1 Planning permission ...... 30 2.4.2 Landowners’ permission ...... 31 2.4.3 Marine licensing ...... 31 2.4.4 Environmental and amenity constraints ...... 31 2.4.5 Permissions for use of materials ...... 31 3 Physical coastal environment ...... 32 3.1 Hydrodynamic and sediment transport conditions ...... 33 3.1.1 Water levels and waves ...... 33 3.1.2 Longshore currents ...... 35 3.1.3 Longshore sediment transport ...... 35 3.1.4 Tidal currents and influence on sediment transport ...... 41 3.1.5 Climate change influences ...... 42 3.2 Beach material and substrate type, grading and profile ...... 43 3.2.1 Introduction ...... 43 3.2.2 Sediment types and sources ...... 44 3.2.3 Beach types ...... 44 3.2.4 Beach substrate ...... 46 3.2.5 Implications for engineering design ...... 46 3.3 Site investigation ...... 47 3.3.1 Introduction ...... 47 3.3.2 Stages of investigations ...... 47 3.3.3 Use of trial pits in beach investigation ...... 48 3.3.4 Substrate type and profile ...... 48 3.3.5 Data logging ...... 49 4 Planning and assessment of groyne field layouts ...... 50 4.1 Understanding beach changes ...... 51 4.1.1 Effects of groynes on beach plan shape ...... 51 4.2 Overall layout considerations ...... 53 4.2.1 Beach width and orientation ...... 53 4.2.2 Groyne length ...... 55 4.2.3 Groyne spacing ...... 57

Groynes in coastal engineering vii 4.2.4 Groyne alignment ...... 58 4.2.5 The downdrift end of a groyne field ...... 59 4.3 Beach modelling for the planning of groyne fields ...... 60 4.3.1 Modelling approach ...... 61 4.3.2 Empirical approach to plan shape modelling ...... 61 4.3.3 Numerical plan shape modelling ...... 63 4.3.4 Physical modelling ...... 66 5 Design groyne profile ...... 67 5.1 Beach profile response to wave action ...... 68 5.2 Design beach profile ...... 71 5.2.1 Introduction ...... 71 5.2.2 Crest level ...... 71 5.2.3 Crest width ...... 72 5.2.4 Seaward slope ...... 73 5.3 Lowest and highest likely beach profiles ...... 73 5.3.1 Lowest beach profile ...... 73 5.3.2 Highest beach profile ...... 74 5.4 Groyne top profile and freeboard ...... 74 5.4.1 Groyne top profile ...... 74 5.4.2 Groyne freeboard ...... 75 6 Materials for narrow footprint groynes ...... 76 6.1 Selecting a material type ...... 77 6.1.1 Introduction ...... 77 6.1.2 General performance of materials ...... 77 6.1.3 Use of multiple materials ...... 78 6.1.4 Materials sources and lead-in time ...... 79 6.1.5 Cost ...... 79 6.1.6 Sustainability and environmental impact ...... 80 6.1.7 Ease of construction and maintainability ...... 80 6.1.8 Health and safety ...... 80 6.2 Timber ...... 81 6.2.1 Introduction ...... 81 6.2.2 Functional performance ...... 81 6.2.3 Agents of degradation ...... 82 6.2.4 Selection and sourcing of different timber species ...... 86 6.2.5 Timber preservation and modified timber ...... 88 6.2.6 Fixing materials ...... 88 6.2.7 Lead-in time ...... 89 6.2.8 Cost ...... 89 6.2.9 Sustainability and environmental impact ...... 90 6.2.10 UK Government timber procurement policy ...... 91 6.2.11 Health and safety ...... 91 6.3 Polymer composites ...... 92 6.3.1 Introduction ...... 92 6.3.2 Functional performance ...... 92 6.3.3 Agents of degradation ...... 93 6.3.4 Material sourcing ...... 94 6.3.5 Lead-in time ...... 94 6.3.6 Cost ...... 94 6.3.7 Sustainability and environmental impact ...... 94 6.3.8 Health and safety ...... 95 6.4 Concrete ...... 95 6.4.1 Introduction ...... 95 6.4.2 Functional performance and agents of degradation ...... 95 6.4.3 Selection and sourcing ...... 95 6.4.4 Concrete preservation and treatments ...... 96 6.4.5 Lead-in time ...... 96 6.4.6 Cost ...... 96 6.4.7 Sustainability and environmental impact ...... 96 6.4.8 Health and safety ...... 97 6.5 Steel ...... 97 6.5.1 Introduction ...... 97 6.5.2 Functional performance ...... 97 6.5.3 Agents of degradation ...... 98 6.5.4 Lead-in time ...... 98 6.5.5 Cost ...... 98 6.5.6 Sustainability and environmental impact ...... 98 6.5.7 Health and safety ...... 99 7 Structural design of groynes ...... 100 7.1 Groyne type selection ...... 101 7.2 Groyne bottom profile/embedment ...... 102

viii CIRIA, C793 7.2.1 Piled groynes ...... 102 7.2.2 Gravity groynes (wide footprint) ...... 105 7.2.3 Bulk groynes (wide footprint) ...... 105 7.3 Outline groyne structure design ...... 105 7.3.1 Piled groynes (narrow footprint) ...... 105 7.3.2 Gravity groynes (wide footprint) ...... 108 7.3.3 Bulk groynes (wide footprint) ...... 108 7.4 Groyne structure design details (piled narrow footprint groynes) ...... 108 7.4.1 Pile/post head rings ...... 109 7.4.2 Pile toes and shoes ...... 109 7.4.3 Joints ...... 109 7.4.4 Fixings ...... 110 7.5 Groyne structure details (wide footprint groynes) ...... 113 7.5.1 Gravity concrete groynes (wide footprint) ...... 113 7.5.2 Bulk rock groynes (wide footprint) ...... 114 7.6 Groyne ancillary features ...... 114 7.6.1 Interface with coastline or seawall ...... 114 7.6.2 Head of groyne ...... 115 7.6.3 Sediment bypassing structures ...... 115 7.6.4 Pile abrasion protection ...... 115 7.6.5 Vehicle and pedestrian access ...... 116 7.7 Designing for maintenance and adaptive management ...... 116 7.8 Health and safety aspects of design ...... 117 7.9 Environmental enhancements ...... 119 8 Groyne construction ...... 120 8.1 Construction risk management ...... 121 8.1.1 Introduction ...... 121 8.1.2 Planning for material procurement ...... 121 8.1.3 Minimising waste ...... 122 8.1.4 Tolerances ...... 122 8.1.5 Planning for site constraints ...... 123 8.1.6 Planning for ground conditions ...... 125 8.2 Stockpiling, handling and inspecting timber ...... 125 8.2.1 Storage and stockpiling ...... 125 8.2.2 Handling timber ...... 126 8.2.3 Pre-construction inspection and checking ...... 127 8.3 Construction of timber groyne elements ...... 127 8.3.1 Preparing king piles/anchor piles ...... 127 8.3.2 Installing king piles ...... 127 8.3.3 Pile verticality and centre spacing ...... 128 8.3.4 Walings and ties ...... 129 8.3.5 Installing planking ...... 130 8.3.6 Installing sheet piles ...... 131 8.4 Plant, equipment and labour ...... 131 8.4.1 Plant ...... 131 8.4.2 Portable equipment ...... 132 8.4.3 Labour ...... 133 8.5 Additional health and safety issues ...... 133 8.5.1 Introduction ...... 133 8.5.2 Lighting ...... 133 8.5.3 Access routes on and off the foreshore ...... 133 8.5.4 Segregating the public from the works ...... 133 8.5.5 Lifting operations ...... 134 8.5.6 Deep excavations ...... 134 8.5.7 Risk of drowning ...... 134 8.5.8 Traffic management plan ...... 134 8.5.9 Working with hardwoods ...... 135 8.5.10 Risks of buried services ...... 135 9 Monitoring and managing beaches with groynes ...... 136 9.1 Beach monitoring ...... 137 9.2 Managing functional failure of timber groynes ...... 138 9.2.1 Managing underrun of beach material ...... 138 9.2.2 Managing landward outflanking ...... 140 9.3 Adaptive management of groyne profiles ...... 141 9.3.1 Existing groynes significantly above existing beach profile ...... 142 9.3.2 Existing groynes at or below existing beach profile ...... 143 9.4 Groyne removal ...... 144 10 Inspecting and maintaining groynes ...... 145 10.1 Deterioration of timber groynes ...... 146 10.1.1 Introduction ...... 146 10.1.2 Predicting deterioration rates for timber groynes ...... 146

Groynes in coastal engineering ix 10.2 Groyne monitoring and inspection ...... 149 10.2.1 Introduction ...... 149 10.2.2 Nature, timing and frequency of groyne inspections ...... 150 10.2.3 Inspection logistics ...... 151 10.2.4 Specific inspection activities ...... 153 10.2.5 Recording inspections ...... 156 10.3 Maintenance practices for timber groynes ...... 158 10.3.1 Introduction: proactive and reactive maintenance ...... 158 10.3.2 Replacing worn/damaged timber piles ...... 160 10.3.3 Maintaining bolted fixings ...... 161 10.3.4 Plated connections between planks ...... 161 10.3.5 Pile abrasion protection systems ...... 162 10.3.6 Maintenance tools and equipment ...... 164 10.3.7 Materials supply for maintenance ...... 165 10.3.8 Temporary works – excavations and bunding ...... 166 10.3.9 Navigation markers ...... 167 10.4 Maintenance of non-timber structures ...... 167 References ...... 169 Statutes ...... 174 Further reading ...... 176 Boxes Box 1.1 Groyne design construction and management process: conceptual summary ...... 7 Box 2.1 Sources of information on funding ...... 29 Box 2.2 Licences, permissions and leases required for coastal defence works in England ...... 30 Box 3.1 T he use of joint probability to determine the range of wave conditions creating the same destabilising forces for design of rock groynes ...... 34 Box 3.2 Effect of NAO on longshore drift in Suffolk ...... 41 Box 3.3 Example of the effect of tidal currents on longshore drift rate and direction ...... 42 Box 4.1 Examples of past use of zig-zag groynes ...... 59 Box 4.2 Determination of beach plan shape using the parabolic bay shape equation (PBSE) of Evans and Hsu (1989) . 62 Box 4.3 Effect of wave chronology on estimated long-term trends in beach morphology ...... 64 Box 4.4 Application of a one-line model to beach management ...... 65 Box 5.1 SHINGLE-B beach profile model for gravel beaches ...... 69 Box 6.1 Reinforcement of seaward end of timber groynes with rock armour at Felixstowe, Kent ...... 79 Box 6.2 Abrasion vs. gribble attack at butt joints between groyne planks ...... 85 Box 7.1 Rotational failure of groynes under load ...... 102 Box 7.2 Hydrostatic and wave loading scenarios for groynes ...... 103 Box 7.3 Coachscrews vs. bolts and nut fixings for attaching planking to piles ...... 112 Box 7.4 Modification of fixing detail in an abrasive environment at Milford-on-Sea, Hampshire ...... 112 Box 7.5 Innovative plank fixing detail at Dawlish Warren, Devon ...... 119 Box 9.1 Evolution of the national coastal and beach monitoring programme ...... 137 Box 9.2 Adjustment of beach plan shape by adjusting groyne profiles at East Preston, West Sussex . . . . . 141 Box 10.1 Comparison of groyne deterioration curves ...... 148 Box 10.2 Inspecting a timber waling/sheet pile interface ...... 156 Box 10.3 Electronic groyne condition recording system ...... 157 Box 10.4 Pile protection at Milford-on-Sea, Hampshire ...... 163 Box 10.5 Reuse of groyne timber at Bournemouth, Dorset ...... 166 Figures Figure 1.1 Effect of a piled groyne on longshore beach movement ...... 1 Figure 1.2 Key elements of groynes and groyne fields ...... 2 Figure 1.3 Example of a groyne field, Cliff End, UK ...... 3 Figure 1.4 Flow chart of the groyne management process, with relationship to the structure of this guide . . . . .6 Figure 2.1 Spatial distribution of groynes around the coast of the UK ...... 10 Figure 2.2 Geographic locations of the second generation SMPs ...... 12 Figure 2.3 Schematic showing flow/sediment movement for a groyne system on a sand beach with oblique wave attack . 13 Figure 2.4 Sediment transport pathways past a groyne ...... 14 Figure 2.5 C ondition vs. time (a) and efficiency, E, vs. condition of a groyne (b). Groyne condition varies from very good (1) to very poor (5) ...... 15 Figure 2.6 Efficiency of the groyne as time progresses (in relation to initial sediment trapping quantity) . . . . . 15 Figure 2.7 Dymchurch wall showing groyne field, painted by Paul Nash in 1923 ...... 16 Figure 2.8 Recreational use of timber groyne at Bournemouth, Dorset, 2009 ...... 16 Figure 2.9 Burning damage to groyne waling from single-use barbeques ...... 16 Figure 2.10 Example of a timber groyne field at Sheringham, Norfolk ...... 21 Figure 2.11 Newly-constructed timber groyne at Southwick, West Sussex ...... 21 Figure 2.12 Timber groynes with buried ties (a) and struts (b) ...... 22 Figure 2.13 Concrete groynes at Hove, East Sussex ...... 22 Figure 2.14 Steel sheet pile and concrete at Seaford, East Sussex ...... 23 Figure 2.15 Variety of rock groyne shapes at Bexhill, East Sussex ...... 23

x CIRIA, C793 Figure 2.16 Rock groyne at Sidmouth, East Devon ...... 24 Figure 2.17 Example of a previous permeable concrete groyne now replaced with rock at Bournemouth, Dorset . . 25 Figure 2.18 Example of reinforced concrete permeable groyne along the Venetian coastline, Italy ...... 25 Figure 2.19 Composite concrete/timber groyne at Hastings, Sussex ...... 26 Figure 2.20 Hybrid groyne at Brackenbury, North Felixstowe ...... 26 Figure 2.21 Repairs to a timber groyne at Worthing, West Sussex ...... 27 Figure 2.22 Repair of a timber groyne head with armourstone ...... 27 Figure 3.1 Parts of groyne management process covered in Chapter 3 ...... 32 Figure 3.2 Wave diffraction at a groyne ...... 33 Figure 3.3 J oint probability of 10 000 years of modelled waves and sea levels at a coastal defence structure on the south-east Scottish coastline, highlighting events that have impact forces with a return period of between one and two years ...... 34 Figure 3.4 Waves breaking obliquely on a shoreline ...... 35 Figure 3.5 Cross-shore distribution of longshore velocity for a static water level scenario ...... 35 Figure 3.6 Direction of sediment transport ...... 36 Figure 3.7 Cross-shore distribution of longshore transport as a function of beach material ...... 36 Figure 3.8 C omputed cross-shore distribution of the longshore transport for a fixed water level, showing the difference in drift direction between the top and bottom parts of the beach ...... 37 Figure 3.9 Cross-shore distribution of longshore transport using a fixed water level ...... 37 Figure 3.10 Cross-shore distribution of longshore transport using a tidally varied water level ...... 38 Figure 3.11 E xample of annual variability of longshore transport. Note the change in drift direction between 1983 and 1989 ...... 40 Figure 3.12 Example of monthly and seasonal variability of longshore transport ...... 40 Figure 3.13 Example of longshore transport at various points along the length of a frontage ...... 40 Figure 3.14 C omparison between average annual net drift and the winter NAO index, time evolution (a) and scatter plot (b) ...... 41 Figure 3.15 Effect of tidal currents on longshore drift rates and directions ...... 42 Figure 3.16 R ange of sea level change (m) at UK capital cities in 2100 relative to 1981 to 2000 average for low (RCP2.6) medium (RCP4.5) and high (RCP8.5) emissions scenarios ...... 43 Figure 3.17 Examples of beach types and grain size distributions ...... 45 Figure 3.18 Exposure of London Clay substrate at Felixstowe, Suffolk ...... 46 Figure 3.19 Groyne protecting a flint and sand beach at West Runton on the North Norfolk coast ...... 46 Figure 4.1 Parts of groyne management process covered in Chapter 4 ...... 50 Figure 4.2 Example of beach alignment under one dominant direction of longshore transport (a) and under two dominant directions of longshore transport (b) ...... 52 Figure 4.3 Interaction of groynes with waves and currents ...... 52 Figure 4.4 Effect of a groyne field (a) or an individual groyne (b) on a coastline ...... 53

Figure 4.6 Definition of a minimum beach width in a groyne field min(Y ) ...... 54 Figure 4.7 E xample of beach realignment following northerly drift (a) vs. southerly drift (b) at Eastbourne, UK . . 55 Figure 4.8 Determination of the length of the groyne with the aid of the cross-shore distribution of the longshore drift . . 56 Figure 4.9 Acceptable and unacceptable groyne spacing ...... 57 Figure 4.10 D istribution of groyne lengths and spacings for the SCOPAC region of the English south coast, including the influence of sediment type ...... 58 Figure 4.11 Example of angled groynes at Whitstable, Kent ...... 59 Figure 4.12 Zig-zag groyne at Hunstanton, Norfolk ...... 59 Figure 4 13 Examples of zig-zag groynes at Calshot, Hampshire ...... 59 Figure 4 14 Close up view of zig-zag groyne at Calshot, Hampshire ...... 59 Figure 4.15 Transition from groyne field to natural beach as recommended by the USACE ...... 60 Figure 4.16 Definition sketch for PBSE ...... 62 Figure 4.17 Application of Evans and Hsu PBSE to a UAE beach ...... 62 Figure 4 18 Example showing effects of wave chronology ...... 64 Figure 4.19 Location and extent of model set-up ...... 65 Figure 4.20 M odel validation results shown relative to the measured shoreline from 2009 (model start position) and 2011 (model target shoreline). Groyne field west of Shore Road towards Sandbanks (existing rock groynes) (a), and at Branksome Dene Chine (new rock groynes, numbered 1 to 6, and timber groynes) (b) 65 Figure 4.21 Typical new groynes at Branksome, Poole Bay (a) and typical timber groynes at Bournemouth, Dorset (b) . 66 Figure 5.1 Parts of groyne management process covered in Chapter 5 ...... 67 Figure 5.2 Sandy beach response to stormy waves ...... 68 Figure 5.3 Shingle beach response to stormy waves ...... 68 Figure 5.4 Schematic shingle beach profile ...... 69 Figure 5.5 Predicted profile extracted from SHINGLE-B tool ...... 70 Figure 5.6 Beach profile terminology ...... 71 Figure 5.7 Groyne freeboard profile ...... 75 Figure 5.8 Crest of timber groynes showing unhindered pedestrian access at Herne Bay, Kent ...... 75 Figure 6.1 Parts of groyne management process covered in Chapter 6 ...... 76 Figure 6.2 Relationship between deterioration processes and climate change factors for a timber groyne . . . . 78 Figure 6.3 Missing sheet piles ...... 79 Figure 6.4 Armourstone reinforcement ...... 79 Figure 6.5 Narrow footprint timber groyne on Bournemouth beach, Dorset ...... 81 Figure 6.6 Abrasion of pile ...... 83 Figure 6.7 Abrasion of the top planks of a timber groyne at Withernsea, Yorkshire ...... 84

Groynes in coastal engineering xi Figure 6.8 Gaps and holes eroded by abrasion due to sediment passing between planks ...... 84 Figure 6.9 Scalloping of planks at Bournemouth, Dorset ...... 84 Figure 6.10 Scanning electron micrograph of a gribble (limnoria lignorum) ...... 84 Figure 6.11 Typical damage caused by gribble on a Greenheart plank, Felixstowe, Kent ...... 85 Figure 6.12 E xample of a shipworm removed from a Douglas fir waling attached to the Barmouth Viaduct in the Mawddach estuary at Barmouth, Gwynedd ...... 85 Figure 6.13 Shipworm damage observed on a Greenheart plank at Felixstowe, Kent ...... 86 Figure 6.14 Wet rot decay causing failure of softwood groyne timbers ...... 86 Figure 6.15 Polymer composite breastwork installed at Pevensey Bay, East Sussex ...... 92 Figure 6.16 Comparison of Greenheart and polymer abrasion on a groyne ...... 92 Figure 6.17 Spalling of polymer plank at Withernsea, Yorkshire ...... 94 Figure 7.1 Parts of groyne management process covered in Chapter 7 ...... 100 Figure 7.2 Overturned groyne due to beach loss and short piles ...... 102 Figure 7.3 Updrift loading of timber groyne causing groyne to deform ...... 102 Figure 7.4 Groyne construction at Bournemouth, Dorset ...... 104 Figure 7.5 Example of a timber groyne section ...... 106 Figure 7.6 Example of timber groyne anchor tie ...... 107 Figure 7.7 E xamples of alternative forms of ties and props. Steel tie/prop adopted at Dawlish (a) and short timber prop bolted into supporting pile at Herne Bay, Kent (b) ...... 108 Figure 7.8 Pile ring ...... 109 Figure 7.9 Pile shoe ...... 109 Figure 7.10 Butt joint ...... 110 Figure 7.11 Notched joint ...... 110 Figure 7.12 Scarf joint ...... 110 Figure 7.13 Plated/bracketed joint ...... 110 Figure 7.14 Bolts fixed with square washers ...... 111 Figure 7.15 Example of abrasion stripping the thread of a bolt. The double nutting reduces the risk of the working loose at Bournemouth, Dorset ...... 112 Figure 7.16 Coachscrew details, proud of surface (a) and countersunk (b) ...... 112 Figure 7.17 T imber groyne showing the manually-operated gap, when beach levels are low (a) and high (b) at Sheringham, Norfolk ...... 115 Figure 7.18 Steps over groyne which also act as an access to the promenade behind at Eastbourne, Kent . . . . 116 Figure 7.19 Scour hole at head of groyne also showing navigation marker at Dawlish Warren, Devon ...... 118 Figure 7.20 C oastal defences showing potential for overtopping of shingle onto promenade at junction with nearest groyne at Shoreham Port, West Sussex ...... 118 Figure 7.21 Coachscrew fixings ...... 119 Figure 7.22 Innovative plate connection ...... 119 Figure 7.23 Artificial rock pools attached to a timber and concrete groyne ...... 119 Figure 8.1 Parts of groyne management process covered in Chapter 8 ...... 120 Figure 8.2 Typical tidal cycle at three locations in the UK. Note the differences in vertical scales ...... 124 Figure 8.3 Tidal working with a rising tide ...... 124 Figure 8.4 Pre-augering ...... 125 Figure 8.5 Stockpiling ...... 126 Figure 8.6 Timber storage for construction ...... 126 Figure 8.7 Hoisting ...... 126 Figure 8.8 Installing a king pile by driving with an impact hammer ...... 128 Figure 8.9 Installing a king pile using a hydraulic vibrating hammer ...... 128 Figure 8.10 Tie heads above upper waling ...... 129 Figure 8.11 Planking installation ...... 130 Figure 8.12 Sheet piles ...... 131 Figure 8.13 Machines working on a beach in the south coast ...... 131 Figure 8.14 Boring ...... 132 Figure 8.15 Hand tools ...... 132 Figure 8.16 Examples of signage and portable fencing ...... 134 Figure 8.17 Plant on a public beach with warning sign ...... 134 Figure 8.18 Example of deep excavation ...... 134 Figure 9.1 Parts of groyne management process covered in Chapter 9 ...... 136 Figure 9.2 Beach lowering resulting in gaps beneath the groynes ...... 138 Figure 9.3 Falling beach levels have rendered this groyne ineffective ...... 138 Figure 9.4 As-built groyne elevation showing (in red lines) clay substrate (a) and recent beach profile (b) . . . .139 Figure 9.5 B each profile envelope within groyne compartment, showing substrate (bedrock) profile and recent beach profile ...... 139 Figure 9.6 Additional planks being added to the base of a structure, following undermining ...... 140 Figure 9.7 Sheet pile panel installation on a timber groyne ...... 140 Figure 9.8 Stranded groynes on a barrier beach frontage at Medmerry, West Sussex ...... 140 Figure 9.9 Stranded groynes on a soft eroding cliff frontage ...... 140 Figure 9.10 The step-back in seawall alignment behind the groyne at East Preston, West Sussex, taken in the 1950s . . 141 Figure 9.11 Reconstructed groynes at East Preston, West Sussex, c1983 ...... 141 Figure 9.12 Aerial photograph of groynes at East Preston, West Sussex, 1985 ...... 142 Figure 9.13 Aerial view showing straighter beach alignment at the old step-back at East Preston, West Sussex, 2016 . 142 Figure 9.14 Profile of a high timber groyne ...... 142

xii CIRIA, C793 Figure 9.15 High timber groynes at Bognor Regis, West Sussex ...... 142 Figure 9.16 Low groyne elevation showing sediment transport across the upper beach ...... 143 Figure 9.17 Profile of a low timber groyne ...... 143 Figure 10.1 Parts of groyne management process covered in Chapter 10 ...... 145 Figure 10.2 Corroded and abraded steel fixings ...... 146 Figure 10.3 Deterioration curve for Milford-on-Sea, Hampshire ...... 148 Figure 10.4 Deterioration curve for Bournemouth, Devon ...... 148 Figure 10.5 Deterioration curve for Dawlish Warren, Devon ...... 149 Figure 10.6 Deterioration curve for Withernsea, Yorkshire ...... 149 Figure 10.7 Measuring three-quarters of the pile perimeter ...... 155 Figure 10.8 Assessing scalloping abrasion of groyne planks ...... 155 Figure 10.9 Differential wear of end grain of sheet piles and adjoining waling ...... 156 Figure 10.10 Generic inspection form for timber groyne maintenance ...... 157 Figure 10.11 Timber groyne inspection form – dimension form ...... 157 Figure 10.12 Timber groyne inspection form – condition form ...... 158 Figure 10.13 Example of works completed section for the same groyne as shown in Figure 10.12 ...... 158 Figure 10.14 Planks joined with steel plates ...... 162 Figure 10.15 Recently attached pile protection units showing minimal wear ...... 162 Figure 10.16 Pile protection fitted to two sides (a) and three sides (b) of the pile ...... 163 Figure 10.17 Cross-section of timber protection to pile face ...... 164 Figure 10.18 Examples of worn pile protection system ...... 164 Figure 10.19 Full collection of hand tools ...... 165 Figure 10.20 Creation of sand bunds for temporary works ...... 167 Figure 10.21 Extensive temporary bund surrounding groyne under construction ...... 167 Figure 10.22 Erosion gullies on surface of polymer groyne plank at Withernsea, Yorkshire ...... 168 Tables Table 1.1 Previous publications providing background to this guide ...... 3 Table 1.2 ‘Level of detail’ hierarchy of guidance available for coastal/beach/coastal structure management . . . 5 Table 1.3 Structure and content of the guide ...... 7 Table 1.4 Potential users of the guide ...... 8 Table 2.1 Number of groynes around the coast of the UK ...... 11 Table 2.2 Three-tier approach to policy setting and planning of coastal defences ...... 11 Table 2.3 Link between groyne maintenance and efficiency to be assumed ...... 15 Table 2.4 Advantages and disadvantages of alternative shoreline control structures ...... 18 Table 4.1 Average groyne space and ratio in England ...... 58 Table 6.1 Timber use classes and where they occur on a groyne ...... 81 Table 6.2 Timber species with an established track record ...... 87 Table 6.3 Lesser used hardwood species strength classes ...... 88

Table 6.4 CO2e of UK cement, additions and cementitious material ...... 97 Table 10.1 Groyne condition grades ...... 153 Table 10.2 Visual assessment categories used to estimate marine borer attack ...... 154 Table 10.3 Visual assessment categories used to estimate abrasion ...... 156 Table 10.4 Reactive maintenance measures for timber groynes ...... 159 Table 10.5 Proactive structural maintenance or repair measures for timber groynes ...... 160

Groynes in coastal engineering xiii Glossary

Accreditation body A n independent authority which examines the operations and capacity of certification bodies. Accretion Accumulation of sediment due to the natural action of waves, currents and wind. Adaptive management A structured decision-making method, the core of which is a multi-step iterative process for adjusting management actions to changing circumstances or new information about the effectiveness of the projects or systems being managed. Alarm level/threshold The level before crisis level/threshold. This is often a predetermined value where the monitored beach parameter falls to within range of the crisis level, but has not resulted in systematic failure of the function being monitored. For example, if ongoing recession of a beach crest has eroded it to within 10 m of an asset and it has been predetermined that an extreme storm event could result in recession of 5 m. the alarm level in this example would be when the beach width has been reduced to a 5 m buffer. Increased monitoring would be required when an alarm level is compromised and intervention undertaken if deemed necessary. Managing alarm levels can be planned in advance. Armourstone L arge pieces of quarry stone (see BS EN 13383-1) normally placed to form a structure such as a groyne, including both in bulk and as a protective layer to prevent erosion of the seabed and/or other slopes by current and/or wave action. Barrier beach A sand or shingle bar above high tide, parallel to the coastline and separated from it by low-lying land which may be occupied by a water body. Baulk P iece of square sawn or hewn timber of equal or approximately equal cross-section dimensions of size greater than 100 mm x 125 mm. Backshore T he upper part of the active beach above high water affected by large waves occurring during a high tide (including any storm surge). Beach A deposit of non-cohesive material (for example, sand, gravel) situated on the interface between dry land and the sea (or other large expanse of water) and actively ‘worked’ by present day hydrodynamic processes (ie waves, tides and currents) and sometimes by winds. Beach control structures Beach control structures are used to inhibit or control the rate of sediment transport along the coastline. Beach management The process of managing a beach, whether by monitoring, simple intervention, recycling, nourishment, the construction or maintenance of beach control structures or by some combination of these techniques in a way that reflects an acceptable compromise in the light of available finance, between the various coastal defence, nature conservation, public amenity and industrial objectives. Beach management plan T his provides a basis for the management of a beach for coastal defence purposes, taking into account coastal processes and the other uses of the beach. Beach manager A beach manager seeks to manage or operate a beach as a natural/recreational resource, or as a means of coastal defence, while providing facilities that meet the needs and aspirations of those who use the beach. Beach nourishment (or recharge) Artificial process of replenishing a beach with material from another source. Beach plan shape T he shape of the beach in plan, often shown as a contour line, combination of contour lines or recognisable features such as beach crest and/or the still water-line. Beach profile Cross-section perpendicular to the shoreline. The profile can extend seawards from any selected point on the landward side or top of the beach into the nearshore. Beach recycling The movement of sediment along a beach area, typically from areas of accretion to areas of erosion. Bed load The proportion of sediment transport which is moved by surface sediment grains creeping over the bed by rolling and saltation. Berm A ridge located to the rear of a beach, immediately above mean high water. It is marked by a break of slope at the seaward edge. Bimodal wave period A frequency distribution (spectrum) of waves for a given sea state in which two peak frequencies are observed. Bole The stem, or trunk of a tree when over 200 mm diameter. Bone Gradient, slope of a section of groyne. Boxed heart A log converted so that the centre of the heartwood (perishable pith) is wholly contained in one piece and surrounded by durable heartwood. Breaching Failure of the beach head allowing flooding by tidal action.

xiv CIRIA, C793 Breakwater A structure projecting into the sea that shelters vessels from waves and currents, prevents siltation of navigation channel, protects a shore area, prevents thermal mixing (for example, cooling water intakes) or has a recreational purpose. In beach management, breakwaters are generally structures protecting areas from the full effect of breaking waves. Breakwaters may be shore attached and extended seawards from the beach, or may be detached and sited offshore, generally parallel to the beach, to provide sheltered conditions. Breastwork V ertical or steeply sloping structure constructed parallel to the shoreline or bank at or near the crest to resist erosion or flooding. Certificate A document that attests that a process or a product meets a standard. Certification body A n independent body that conducts inspection and verification, and which issues certificates (of conformity to a standard). Check Separation of fibres along the grain forming a crack or fissure that does not extend through the timber or veneer from one surface to the other. Climate change L ong-term changes in climate. The term is generally used for changes resulting from human intervention in atmospheric processes through, for example, the release of greenhouse gases to the atmosphere from burning fossil fuels, the results of which may lead to increased rainfall and sea level rise. Coastal cell Coastline unit within which sediment movement is self-contained. Coastal defence An overall term embracing both coast protection and sea defence. Coastal forcing (forcing factors) T he natural processes that activate coastal hydro- and morphodynamics (for example, winds, waves, tides). Coast protection M easures aimed at the “protection of the coast against erosion and encroachment by the sea” as defined by the Coast Protection Act 1949. Cohesive sediment Sediment containing significant proportion of clays, the physico-chemical properties of which cause the sediment to bind together. Contiguous pile Term often applied to cast-in-place concrete piles immediately adjacent to or touching each other. Sometimes used for plank piles. Crenulate bay Term describing characteristic plan shape of equilibrium beach formed between two fixed headlands. Crest Highest point on a beach face, breakwater or seawall. Crest level/height The vertical level of the beach relative to metres Ordnance Datum (mOD). Crest width The horizontal distance measured from the back of the beach to the top edge of the beach face slope – or on a barrier beach the distance between the top of the front slope and rear slope. Crisis level/threshold The level at which the function being monitored, such as the stability of the beach and/or any backing structures (seawall/promenade), could be compromised and emergency remedial action becomes necessary. An example would be, in the case described under alarm level/threshold above where it has been predetermined that an extreme event could result in 5 m of recession, that the beach crest recedes to within 4 m of an asset that requires protection. Cross-shore transport Movement of material perpendicular to the shore. Depth of closure The ‘seaward limit of significant depth change’ – it does not refer to an absolute boundary across which there is no cross-shore sediment transport. Drift reversal A change in the prevailing direction of littoral transport. Drift-aligned A coastline that is orientated obliquely to prevailing incident wave fronts. Empirical modelling A generic term for activities that create models by observation and experiment. Erosion Removal of the land or beach by the combined action of the motion of water and sediment. For the beach this can lead to beach retreat and/or beach lowering. Finger joint Pieces of timber end jointed by glued interlacing wedge-shaped projections. Foreshore The intertidal area below highest tide level and above lowest tide level. Forestry stakeholder An individual or organisation with a legitimate interest in the goods and services provided by a forest management unit. They are likely to include government representatives, forest scientists, forestry protection organisations, indigenous forest dwelling people and/or local people and their representatives, forest managers and the timber trade. Freeboard The (variable) height of the top of the groyne above the design beach profile. Geomorphology The study of landforms, their processes, form and sediments at the surface of the Earth. Green timber Timber freshly felled or still containing free moisture that is not bound to the cell wall. Groyne Narrow, roughly shore-perpendicular structure built to reduce or divert longshore currents and/or to trap and retain beach material. Most groynes are of timber or rock, and extend from a seawall, or the backshore, well onto the foreshore and rarely even further offshore. Groyne bay The compartment between two groynes.

Groynes in coastal engineering xv Groyne efficiency The fraction of the unconstrained instantaneous longshore sediment transport past a groyne which is retained by the groyne. This efficiency may reduce with time as the groyne deteriorates or as the groyne bay fills. Groyne head The seaward end of a groyne. Groyne root The landward end of a groyne. Hard defence General term applied to impermeable coastal defence structures of concrete, timber, steel, masonry etc, which reflect a high proportion of incident wave energy. Heartwood I nner zone of wood that, in a growing tree, has ceased to contain living cells and reserve materials. Hewn Timber section squared or levelled with an axe or adze. Infragravity waves Waves with a typical period of 25s to 250s much longer than the wind waves and swell waves from which they receive their energy (via the grouping of these source waves). They generally have a much lower height than wind and swell waves but can significantly influence shoreline processes such as run-up. Intermediate wood See Transition wood. Joint probability The probability of two (or more) things occurring together. Joint probability density Function specifying the joint distribution of two (or more) variables (eg wave height and water level). Joint probability analysis Analysis techniques to determine and use joint probability density functions. King pile or post A pile or post acting in cantilever to support a retaining structure. Knee pile (King) pile at the change of ‘bone’ in a groyne. Knot Portion of a branch embedded in the wood resulting in a discontinuity in the grain. Locally generated (wind) waves Locally generated short period and irregular waves created by the flow of air over water. Longshore transport Movement of material parallel to the shore – also referred to as longshore drift. ‘Mach stem’ waves A term originally used to describe a phenomenon associated with supersonic shock waves. It has been adapted to coastal engineering to describe the local amplification of a wave height and velocity at the (moving) point where the wave crest is being reflected at an angle from a (generally vertical) structure. Macro-tidal Locations where the tidal range exceeds 4 m. Meso-tidal Locations where the tidal range is between 2 m and 4 m. Mean high water The average of all high waters observed over a sufficiently long period. Mean low water The average of all low waters observed over a sufficiently long period. Mean sea level Average height of the sea surface over a 19-year period. Micro-tidal Locations where the tidal range is less than 2 m. Nearshore The zone that extends from the swash zone to the position marking the start of the offshore zone, typically to water depths of about 20 m. Numerical modelling Analysis of coastal processes using computational models. Offshore The zone beyond the nearshore zone where sediment motion induced by waves alone effectively ceases and where the influence of the seabed on wave action has become small in comparison with the effect of wind. Operating authority Authority responsible for local delivery of coastal protection or flood defence. Overtopping Water carried over the top of a coastal defence due to wave run-up exceeding the crest height. Overwashing The effect of waves overtopping a coastal defence, often carrying sediment landwards which is then lost to the beach system. Physical modelling The investigation of coastal processes using a scaled model. Plank pile Individual planks driven (normally vertically) next to each other to form large panels, often with the driving end cut at an angle or shod with metal for ease and to ensure that adjacent planks provide a tight fit. Planking Long flat timber boards (horizontal or sloping) attached to a frame, for example, forming a groyne or deck. Risk assessment method statements Commonly known as RAMs, these protect employees and their business as well as complying with the law (eg The Construction (Design and Management) Regulations 2015 (CDM 2015). They are intended to ensure that health and safety issues are properly considered during a project’s development so that the risk of harm to those who build, use and maintain structures is reduced. Return period A statistical measurement denoting the average probability of occurrence of a given event over time. Revetment Protective structure parallel to a bank, cliff or slope. Non load-bearing. River wall A structure that both protects and supports a river bank, either vertical or steeply sloping. Sapwood Outer zone of wood that, in a growing tree, contains living cells and food reserve materials (eg starch), generally lighter in colour than heartwood, but not always clearly differentiated.

xvi CIRIA, C793 Scour R emoval of underwater material by waves or currents, especially at the toe of a shore protection structure. Sea defence M easures referenced under the Water Resources Act (1991) aimed at protecting low- lying coast and coastal hinterland against flooding caused by the combined effect of storm surge and extreme astronomical tides. Sea level change T he rise and fall of sea levels throughout time in response to global climate changes and to local subsidence and tectonic changes. Seawall Structure built along the shore to prevent erosion and damage by wave action. Sediment Particulate matter derived from rock, minerals or bioclastic debris. Sediment grading D istribution defined by nominal and extreme limits with regard to size or mass of individual sediment grains. Sediment transport T he movement of a mass of sedimentary material by the forces of currents and waves. This can be either perpendicular (cross-shore) or parallel (longshore) to the shoreline. Shake Separation of wood fibres along the grain irrespective of the extent of penetration. Safety, health and To assist with compliance of CDM 2015. environment management Sheet pile Steel sheet piles of any section. Sometimes used for plank piles. Shore-parallel breakwater Shoreline erosion control structures placed parallel to the coast to reduce the wave energy attacking the coastline. Slope of grain Angle between the direction of the grain of timber and the axis of the piece (the greater the angle, the greater the strength-reducing effect caused by slope of grain). Split Separation of fibres along the grain forming a crack or fissure that extends through timber. Standard of protection The level of return period event which the defence is expected to withstand without experiencing significant failure. Standards Documented agreements containing technical specification of products or processes. Standard penetration test This involves driving a standard thick-walled sample tube into the ground at the bottom of a borehole by blows from a slide hammer with standard weight and falling distance. Still water level The level that the sea surface would assume in the absence of waves, but including the effects of tide and storm surge. Storm surge A rise in the sea surface on an open coast, resulting from reductions in atmospheric pressure during a storm including any amplification as the additional water is pushed into coastal bays. Straight grain Grain that is straight and parallel or nearly parallel to the longitudinal axis. Structural bay The unit of construction of a piled groyne from one king pile to the next. Strut A pile-sized structural member employed on the downdrift side of a narrow footprint groyne (and installed perpendicular to its orientation) which acts in compression to resist the tendency of the groyne to overturn under the weight of beach material on the updrift side. See also Tie. Surf zone T he zone of wave action extending from the water line (which varies with tide, surge, wave set-up etc) out to the most seaward point of the zone (breaker zone) at which waves approaching the coastline start breaking. Suspended load T he proportion of sediment transport which occurs as a result of sediment grains being supported within the water column by the effects of turbulence. Such sediment is washed along by currents. Swash T he area onshore of the surf zone where the breaking waves are projected up the foreshore. Swash aligned A coastline that is orientated parallel to prevailing incident wave fronts. Swell waves R emotely wind-generated waves (ie waves that are generated away from the site). Swell characteristically exhibits a more regular and longer period and has longer crests than locally generated waves. Terminal groyne L ast groyne at the downdrift end of a groyne field. This groyne will need to be a substantial structure to remain stable given the (potentially significant) erosion downdrift. Tidal current The movement of water associated with the rise and fall of the tides. Tidal range V ertical difference in high and low water level once decoupled from the water level residuals. Tide P eriodic rising and falling of large bodies of water resulting from the gravitational attraction of the moon and sun acting on the rotating earth. Tie A pile-sized structural member employed on the updrift side of a narrow footprint groyne (and installed perpendicular to its orientation) which acts in tension to resist the tendency of the groyne to overturn under the weight of beach material on the updrift side. See also Strut. Toe level T he level of the lowest part of a structure, generally forming the transition to the underlying ground.

Groynes in coastal engineering xvii Tombolo A beach formation created by sediment deposition in the lee of an island or offshore structure (such as a shore-parallel breakwater) whereby the island/structure becomes attached to the coastline by a narrow spit of beach material. Transition wood Found in a few species of tropical hardwood and sandwiched between the sapwood and heartwood. Transition wood is not as durable as heartwood, and its presence may be a contributory factor to the apparent failure/erosion of timber components. Transom An intermediate horizontal timber beam acting as a strengthening or supporting member in a structure. Underrun A process involving beach material passing under a piled groyne because the beach profile has fallen to levels that are below the lowest part of groyne planking or sheet piles. Waling A horizontal or sloping beam which distributes loading between the piles of a groyne or breastwork and provides a point of attachment for plank piles. Wane Term applied to converted wood in which the corner is missing at the circumference of the log, due to economical conversion. Wave climate A verage condition of the waves at a given place over a period of years, as shown by height, period, direction etc. Wave diffraction P rocess by which waves spread out as they pass around the end of a structure into the geometrical ‘shadow’ of that structure. Wave direction Direction from which a wave approaches. Wave height The vertical distance between the crest and the trough. Wave hindcast In wave prediction, the retrospective forecasting of waves using measured wind information. Wave induced currents T he movement of water driven by breaking waves that create a current travelling in an longshore direction. Wave period The time it takes for two successive crests (or troughs) to pass a given point. Wave reflection The part of an incident wave that is returned (reflected) seaward when a wave impinges on a beach, seawall or other reflecting surface. Wave refraction Process by which the direction of travel of a wave changes as the water depth decreases, as a result the wave crests tending to become more shore parallel. Wave run-up/run-down The upper and lower levels reached by a wave on a beach or coastal structure, relative to still water level. Wave set-up The increase in mean water level at the coastline due to the presence of breaking waves.

xviii CIRIA, C793 Abbreviations and acronyms

AONB Areas of Outstanding Natural Beauty ATV All terrain vehicle BIM Building Information Modelling BMP Beach management plan CCO Channel Coastal Observatory CDM 2015 The Construction and Design (Management) Regulations 2015 CERC Coastal Engineering Research Center CITES Convention on International Trade in Endangered Species of Wild Fauna and Flora DEFRA Department for Environment, Food and Rural Affairs DfT Department for Transport DLO Direct labour organisations DTM Digital terrain model EIA Environmental Impact Assessment EUTR European Union Timber Regulation FCERM Flood and coastal erosion risk management FEPA Flood and Environment Protection Act 1985 (Part II) FLEGT Forest Law Enforcement, Governance Trade FSC Forestry Stewardship Council GGBS Ground granulated blast furnace slag GPS Global positioning system HAVS Hand-arm vibration syndrome HDPE High-density polyethylene JPA Joint probability analysis LGA Local government association LiDAR Light detection and ranging LPA Local planning authority MAFF Ministry of Agriculture, Fisheries and Food MHCLG Ministry of Housing, Communities and Local Government MHW Mean high water MHWS Mean high water spring MLW Mean low water MLWS Mean low water spring MMO Marine Management Organisation mOD Ordnance Datum NAO North Atlantic Oscillation NGO Non-governmental organisation OD Ordnance Datum OPC Ordinary Portland Cement PBSE Parabolic bay shape equation PEFC Programme of Endorsement for Forest Certification PFA Pulverised fuel ash PFI Private finance initiative PPE Personal protective equipment PSO Partnerships and Strategic Overview RAMS Risk assessment method statement RBMP River basin management plan RMA Risk management authority SAC Special Area of Conservation SAND Shoreline and nearshore database system SCOPAC Standing Conference on Problems Associated with the Coastline SHE Safety health and environment

Groynes in coastal engineering xix SIG Special interest group SMP Shoreline management plan SoP Standard of protection SPA Special Protection Area SPT Standard penetration test SSSI Site of Special Scientific Interest SWL Still water level TTF Timber Trades Federation UAE United Arab Emirates UV Ultraviolet WFD Water Framework Directive

xx CIRIA, C793 1 Introduction

1.1 GROYNES IN COASTAL ENGINEERING

In 2010, CIRIA published C685 Beach management manual (Rogers et al, 2010). The manual defined groynes as long, narrow structures built approximately perpendicular to the shoreline (Figure 1.1). They are designed to control longshore transport of sediment on beaches and to deflect nearshore tidal currents. Their principal purpose is to slow the longshore movement of beach material by interrupting the movement of material and trapping it on the updrift side of the groyne. Through this process, groynes retain the beach updrift of each groyne and re-orientate the beach updrift and downdrift. They also help to build beaches that protect land behind the beach from flooding and erosion but can cause downdrift erosion impacts. Groynes may extend across part or all of the intertidal zone and are normally grouped together to form groyne systems or fields (Figures 1.2a and 1.3).

Figure 1.1 Effect of a piled groyne on longshore beach movement

Groynes in coastal engineering 1 Figures 1.2a and 1.2b define the key elements of groynes and groyne fields that are used in this document. Groynes may be constructed of timber, masonry or rock or may be of composite construction with different material at the seaward end (groyne head) or landward end (groyne root). Groynes may be partly permeable to currents and waves. They may also have various plan shapes as illustrated in Figure 1.2c, including at their head, such as fishtail groynes or T-shaped groynes, the latter most commonly constructed of armourstone (see also Figure 2.15).

Figure 1.2 Key elements of groynes and groyne fields

2 CIRIA, C793 Figure 1.3 Example of a groyne field, Cliff End, UK (courtesy Mackley Construction)

1.2 BACKGROUND TO THE GUIDE

Since the publication of CIRIA R119 (Fleming, 1990), groyne design assessment and management practices have evolved as research and practical experience have developed. For this reason, the Environment Agency, the Coastal Group network, the Standing Conference on Problems Associated with the Coastline (SCOPAC) and CIRIA have identified the need to update the existing guidance.

Table 1.1 lists prevous publications whose information has provided a useful background to this guide and are referenced as appropriate.

Table 1.1 Previous publications providing background to this guide

Publication details Author and date published Details Guide on the uses of groynes in coastal engineering Draws on work in the period 1982 Fleming (1990) (R119) to 1986 Groynes in coastal engineering: data on performance CIRIA (1990) of existing groyne systems (TN135) Manual on the use of rock in coastal and shoreline CIRIA, CUR (1991) Dealt with use of rock groynes engineering, first edition (SP83) The rock manual. The use of rock in hydraulic CIRIA, CUR, CETMEF (2007) No change in approach to groynes engineering, second edition (C683) Beach management manual, first edition (R153) Simm et al (1996)

Beach management manual, second edition (C685) Rogers et al (2010) Manual on the use of timber in coastal and river Work conducted under a DETR PII Thomas Telford (2004) engineering (timber manual) project Designed to complement the timber Guide on lesser used species Environment Agency (2011) manual The coastal handbook – a guide for all those working Environment Agency (2010) on the coast Beever and Flikweert Asset performance tools: guidance for beach triggers (2019)

Coastal morphological modelling for decision makers Blanco et al (2019)

Environment Agency research reports currently awaiting publication that have been referenced include Burgess et al (2018) (but see also Burgess et al, 2014).

Groynes in coastal engineering 3 In addition to these existing documents, this guide has benefitted from some previously unpublished recent work, in particular the following: „ Text on management and maintenance of timber groynes prepared for SCOPAC by the late Prof Andy Bradbury (Bradbury, n.d.). SCOPAC kindly agreed to make this material available as a befitting and suitable use of his unfinished work. They recognised that the present manual would serve to complete the work that SCOPAC had set out to achieve. It would also mean that it would conclude Prof. Bradbury’s guide, albeit in a different format. „ An Environment Agency funded review (Williams, 2018), carried out by timber expert John Williams (RSK Environmental Ltd) of the performance of groyne fields at Dawlish Warren, Withernsea, Cleethorpes, Felixstowe, Milford-on-Sea and Bournemouth. In particular, the removal and rationalisation of 30 year old groynes along the Bournemouth frontage in 2016 allowed the nature and rate of deterioration of groynes constructed of five different timber species to be evaluated. „ Unpublished results of trials by Pevensey Coastal Defence Ltd of several new materials (recycled plastics) which could be useful in groyne construction have been made available to the project.

The field data collected through the latter two projects have been used to develop new/validate existing performance, deterioration and whole-life cost curves for groynes to aid asset management planning and scheme appraisal.

International guides such as USACE (2002) are also referenced as appropriate.

1.3 AIMS AND OBJECTIVES

This guide has the following objectives: „ To provide a framework for the design and maintenance of all types of groynes with particular emphasis on timber and other ‘narrow footprint’ groynes. (Narrow footprint groynes are those whose width is very narrow in comparison with their length and are commonly supported on vertical piles.) „ To provide advice on good practice approaches to routine and periodic maintenance and to the use of sustainable material during repair and replacement. „ To update Fleming (1990), referencing key existing CIRIA manuals such as CIRIA, CUR, CETMEF (2007) and Rogers et al (2010). „ To incorporate operational experience and evidence gained through the collection and analysis of field data from the last 20 years. „ To focus predominantly on UK experience and situations (eg macro-tidal) but to make appropriate reference to international experience (eg from the Netherlands, USA).

The building of new groynes and maintenance of existing systems is often a significant proportion of both the capital and revenue expenditure of the relevant responsible operating authorities. The effective extension of the life of groyne structures through efficient maintenance may make the difference between financially sustainable systems and those that are not cost effective as a management solution. It is hoped that by sharing good practice and experience through the publication of guidance will lead to identifiable efficiencies in both capital and revenue expenditure.

The guide does not attempt to discuss broader coastal management or beach management issues, such as environmental considerations (eg water quality, conservation, ecology). For information on these topics, readers are referred to the following: „ The coastal handbook (Environment Agency, 2010) provides an overview of information relevant to coastal practitioners planning or undertaking work on the coast in England and Wales. „ Beach management manual (Rogers et al, 2010) provides a broader discussion of the principles and practice of beach management. Extensive reference to this manual is made throughout this guide.

In addition, because the focus of the guide is on narrow footprint groynes, the design of bulk and gravity groynes is not addressed in any detail. The reader is referred to CIRIA, CUR, CETMEF (2007)

4 CIRIA, C793 for the design of groynes constructed using armourstone and Dupray et al (2015) for gravity structures constructed of concrete. The ‘level of detail’ hierarchy of these guidance documents is summarised in Table 1.2.

Table 1.2 ‘Level of detail’ hierarchy of guidance available for coastal/beach/coastal structure management

Level Topic Guidance documents Coverage Coastal management Upper The coastal handbook (Environment Agency, 2010) All aspects of coastal governance in the UK Beach management Beach management manual, second edition Middle All aspects of beach management generally (C685) (Rogers et al, 2010) Guide to the use of groynes in coastal engineering All groynes, but with a focus on (C793) narrow footprint groynes The rock manual. The use of rock in hydraulic All armourstone structures including engineering, second edition (C683) (CIRIA, CUR, rock groynes Coastal structures CETMEF, 2007) Bottom including groynes Use of concrete in maritime engineering (C674) All concrete structures including (Dupray et al, 2015) concrete groynes Manual on the use of timber in river and coastal Provides additional information on engineering (timber manual) Crossman and Simm, timber structures 2004)

The focus of this guide is for practitioners in the UK (although some text refers specifically to practice in England). However, international readers should also find information which is of assistance to them.

1.4 STRUCTURE AND USE OF THE GUIDE

This guide contains information that is useful for both existing groynes and for the planning and construction of new groynes and groyne fields.

Summary descriptions of each chapter are presented in Table 1.3 following the structure of the guide. Figure 1.4 presents a high-level overview of the structure of the guide linked to the processes of design, construction and management of groyne fields, with markers showing how each chapter contributes information. The flow chart follows the classic plan-do-check-act cycle, but the cycle is adjusted to take account of the conceptual process summarised in Box 1.1. The entry points when following the flow chart are either of the two grey-shaded boxes.

Groynes in coastal engineering 5 Coastal management Functional requirements context and indicators (Section 2.1) (Section 2.1)

Physical coastal environment – hydraulic, geotechnical and morphological constraints (Chapter 3)

Start or continue No Groyne removal to use groynes? (Section 9.4)

Yes

Plan shape design of Evaluate and choose groyne field (Chapter 4) groyne type (Section 2.2) and material (Chapter 6)

Design groyne profile (Chapter 5)

Structural design or design review for groynes (outline and detailed) (Chapter 7)

Construction and/or major refurbishment (Chapter 8) Major

Minor Groyne repair and maintenance (Section 10.3) Groyne adaptation (Section 9.2) and adjustments (Section 9.3)

Small

Beach changes Routine monitoring Large (Section 9.1) (Sections 9.1 and 10.2) Structural deterioration (Section 10.1)

Post-incident monitoring (Sections 9.1 and 10.2)

Climate change Storm events (Section 3.1.4)

Figure 1.4 Flow chart of the groyne management process, with relationship to the structure of this guide

6 CIRIA, C793 Box 1.1 Groyne design construction and management process: conceptual summary

„ Functional requirements flow from the coastal management context (see Chapter 2) which may change if the policy or strategy changes. Functional requirements are likely to include a minimum beach volume per unit length or per groyne bay and a minimum beach crest width. „ The next step is to develop an understanding of the physical coastal environment (see Chapter 3, including forcing loads and processes, the existing beach and the geological and geomorphological context). If the situation is appropriate (including the transport of beach material being predominantly alongshore), a decision may be made at this stage to start or continue to use groynes (taking account of input from beach monitoring). „ If the decision is taken to proceed with groynes, the plan shape of the groyne field (see Chapter 4, including length and spacing of groynes) needs to be determined and/or confirmed, following which … … the groyne crest profile (seeChapter 5) can be determined for each groyne.

„ In the meantime the evaluation and choice of materials for the groyne (see Chapter 6) will need to have taken place. „ With all this in place detailed structural design of the groynes (see Chapter 7), including that of all component features, can proceed followed by … … construction of major refurbishment (see Chapter 8), depending on the circumstances.

„ Once the groyne field is in place monitoring begins, both on a routine basis and also after storms or other major incidents. The monitoring has two distinct aspects: Beach monitoring allows the effectiveness of the groyne field to be established and any necessary adjustments to the groynes determined and implemented (see Chapter 9). These may be small scale, but if larger scale may necessitate revisiting the main design and construction process. Monitoring deterioration of the groyne structures themselves allows necessary repairs and replacements to be identified and implemented (see Chapter 10). These may be minor or may involve setting up a further construction contract.

Table 1.3 Structure and content of the guide

Chapter Description Groynes and Explains the role of groynes in coastal management, in the context of policies, stakeholders, 2 shoreline management authorities and legislative requirements. The main types of groynes are summarised management and the approach to their whole life and cost management described. Explains the coastal and beach physical environment, information about which is required when designing or planning the management of a groyne scheme. The first part focuses on the effect of the physical environment (a) in creating the forcing that moves the sediment contained within a Physical coastal groyne field which in turn (b) applies the sediment level difference on the groynes. Aspects covered 3 environment include water levels, waves, longshore and tidal currents and sediment transport, and the way that these forcing effects may change with the changing climate (sea level rise, increasing storminess). The final parts of the chapter deals with the nature of the beach material itself and the beach substrate and how to understand and carry out investigations of these. Explains how to understand and analyse beaches in order to determine the plan-shape layout of Planning and a groyne field. The trapping efficiency of groynes is described and the likely effects of groynes on assessment 4 beach plan shape. Modelling approaches (empirical, numerical and physical) to assess the effects of groyne field of groynes are discussed linked to a description of the various ways of assessing and determining layouts groyne length and spacing. Explains how to assess beach profile response to wave action and determine the groyne crest Design of 5 profile taking account of the range of likely beach levels that may be experienced and the selection groyne profiles of a suitable freeboard. Describes the performance and procurement factors that should be considered when selecting and sourcing materials and/or combinations of materials for narrow footprint groynes. The most Materials for common material used in the UK is timber, however, polymer composites, concrete and steel 6 narrow footprint have and can also be used and are discussed. When choosing the most suitable material for a groynes groyne scheme, designers need to consider and evaluate a number of issues relating to material performance, material availability, material sustainability, environmental impact, and cost. Covers the design of all types of groynes, but gives most attention to narrow footprint groynes explaining the design of each component, including both aspects that relate to structural strength Structural and those that relate to maximising whole-life durability. The chapter explains how to determine 7 design of the structure outline (including the bottom profile) and how to design and select the structural groynes components and details including fixings. Ancillary features and the approach for design for adaptation and for minimising health and safety risks are also discussed. Overviews the planning and management of the construction of narrow footprint groynes, discussing the obtaining of materials, investigating the site working conditions, methods of Groyne construction of the various components, tolerances and accuracy and managing construction 8 construction risks. Repair methods are also discussed (linking to Chapter 10). Specialist equipment plant and labour required for timber groynes construction are described and health and safety issues for construction are discussed.

Groynes in coastal engineering 7 Chapter Description Describes the need for the monitoring of beaches (to support both effective initial groyne design Monitoring and ongoing groyne and linked beach-groyne system management strategies), approaches to and managing 9 managing scenarios where the function of the groyne is jeopardised as a result of physical beach beaches with and/or cliff processes, the use of ongoing groyne adaptation to improve beach performance, and groynes considerations for groyne removal. Provides information and guidance on the requirements for inspection and maintenance of a Inspecting and groyne field, examining approaches to both reactive and proactive maintenance. The chapter 10 maintaining considers inspection of specific materials, the durability of materials and maintenance and repair groynes techniques. The maintenance of non-timber structures is discussed briefly.

The following features have been designed to assist the reader in navigating the guide: „ Figure 1.4 provides a layout of the structure and contents. „ Table 1.3 presents an evaluation of the content from different users’ perspectives to assist the reader in finding information relevant to their needs. „ A summary box is provided at the start of each chapter which describes what is included in that chapter, and a flow chart that demonstrates links with other chapters electronic version: the guide is available to download from CIRIA’s website: www.ciria.org

1.5 TARGET READERSHIP

Potential users of the guide include planners, developers, structure owners, asset managers (from the relevant responsible operating authority), regulatory body personnel, designers, construction contractors, and educational institutions.

It is written to assist a technically competent practitioner with a broad (but not necessarily expert) knowledge of the field of application to arrive at the best approach for a particular groyne or groyne field. In this regard the guide aims to support decision making rather than to direct it. It will also provide clients with a technical background but no particular specialist knowledge) with sufficient background information to understand the main issues and general procedures likely to be followed by an experienced practitioner.

The guide has been written to address two key perspectives: 1 The needs of the manager of the relevant operating authority’s groynes who has responsibility for the overall programme of maintaining, upgrading, adding to and disposing of the authority’s stock of groynes. 2 The needs of the groyne designer who will be delivering the required planning and design of new groynes and ongoing maintenance works.

In addition, the guide provides some useful information for construction contractors (or other organisations) that may be working with the manager or designer in seeking to implement optimised and robust maintenance activities or new construction work (see Table 1.4).

Table 1.4 Potential users of the guide

Chapter Stakeholder/user 2 3 4 5 6 7 8 9 10 Planner/developer X O X Groyne owner X O X OOOOO X Groyne manager XXX O X O XXX Regulatory body OOOO X O XXX Design engineer O XXXXXXXX Construction contractor O X O XXX Educational institution OOOOOOOOO Key: X = directly useful for practice O = useful for information

8 CIRIA, C793 2 Groynes and shoreline management

This chapter explains the significance and role of groynes in coastal management with respect to policies, stakeholders, management authorities and legislative requirements. The main types of groynes are summarised and the approach to whole-life and cost management described. The sections of this chapter are:

„ An introduction to groynes and shoreline management (Section 2.1) „ Types of groynes (Section 2.2) „ Whole-life management of groynes (Section 2.3) „ Licensing and consenting (Section 2.4)

2.1 AN INTRODUCTION TO GROYNES AND SHORELINE MANAGEMENT

The key objective for a beach manager in providing or maintaining groynes on a shoreline is to create or preserve a beach, while the purpose of the beach in this context is to counter erosion, to reduce overtopping, or to meet some other purpose, eg to provide recreation. The measure of success of the groyne field is the extent to which the groynes retain a ‘healthy’ beach which is of sufficient size to achieve the desired objectives over the required service life, while avoiding as much as possible any downdrift erosional effects.

2.1.1 Groynes and shoreline management in the UK From the late 18th century (Owens and Case, 1908, and SCOPAC, 2012), structures such as promenades, groynes and seawalls began to be constructed along the UK coast to improve access and safety at coastal resorts and to maintain transport routes (particularly for railways initially). Groynes and groyne like structures can be seen on many paintings of coastal landscapes of the 19th century (see Websites box). Where groynes were adopted, often by individual landowners, it was in an attempt to follow the same principle as in rivers where groynes had been deployed to reduce current speeds along the banks (that had caused erosion) by diverting the flow offshore. A significant original objective for introducing coastal groynes was to control the tidal currents that were believed to be the primary driver for the longshore drift of beach sediment. As the population near the coast increased, large numbers of people and properties became at risk from coastal flooding and erosion, and defence of the coastline by individual landowners was not financially realistic. As a result, the cost of maintaining defences such as groyne fields fell to local authorities, river boards or similar bodies. Significant central government spending only started with the passing of the Coast Protection Act (1949), supported by the opening of UK coastlines to the public after World War II. The Coast Protection Act now needs to be read along with provisions of the Flood and Water Management Act 2010. (Legislative arrangements in the UK, particularly in relation to licensing and consenting, are discussed further in Section 2.4.

A recent study undertaken by HR Wallingford to support the development of this guide has assessed that there are now nearly 6000 groynes around the coast of the UK protecting some 350 km of coastline. Of these groynes, the vast majority are narrow footprint groynes (Figure 2.1 and Table 2.1). This information was derived by manually interrogating the coastal monitoring survey data and infilling with information from Google satellite images. The accuracy of the answer is limited by the methodology used (eg completely buried groynes would not be visible).

Websites Coastal and Geotechnical Services offers a range of advice and support in the fields of coastal zone management and landslide management: http://www.coastalandgeotechnicalservices.com/news_and_current_issues Watercolourworld is a free online database of pre-1900 documentary watercolours from public and private collections around the world: https://www.watercolourworld.org

Groynes in coastal engineering 9 Figure 2.1 Spatial distribution of groynes around the coast of the UK (courtesy HR Wallingford)

10 CIRIA, C793 Table 2.1 Number of groynes around the coast of the UK (courtesy HR Wallingford)

Rock groynes Narrow footprint groynes Region Number Coastline protected (km) Number Coastline protected (km)

Scotland 6 1.5 43 4.6

West 171 20.0 506 29.4

South West 34 2.5 134 7.3

South 204 19.5 1879 77.8

South East 51 6.6 1677 69.9

East 183 21.0 1081 80.8

Northern Ireland 1 – 36 1.6

Total 600 73.1 5356 271.8

In the past, the use of groynes and other structures led to some erosion issues being relocated further along a coast – potentially affecting a different operating authority or land owner. The solution to this only started to be addressed systematically with the introduction of the concept of shoreline management plans (SMPs) – a major focus of which was (and still is) the holistic management of sediment within a coastal cell or sub-cell. Here, a sediment cell is understood to be a length of coastline which is relatively self contained as far as the movement of beach material is concerned and where interruption to such movement should not have a significant effect on adjacent sediment cells. While subsequently modified slightly, the basis for the cells and sub-cells was initially defined by Motyka and Brampton (1993), with boundaries generally coinciding with major estuaries and prominent headlands. In many cases, coastal groups have formed based on these cells and/or groups of sub-cells. Figure 2.2 shows the geographic distribution of the second generation SMPs. During 2019 and 2020 the Environment Agency managed, with the support of the Coastal Group network, a ‘refresh’ of the entire suite of 20 SMPs covering the English coast. The refresh was not a third cycle of the SMPs, but an update to ensure that they remained fit for purpose.

In England and Wales, a simple three-tier approach (see Table 2.2) is operated for the management and planning of coastal defences or other development or management plans along the coastline (including SMPs), strategies and project or scheme delivery plans, which could include beach management plans, BMPs).

Table 2.2 Three-tier approach to policy setting and planning of coastal defences (after Rogers et al, 2010)

Stage SMP Strategy plan Scheme or project including BMPs

To verify and identify scheme To identify the detailed works to To establish a direction for the intervention or adaption options to Aim implement the scheme or course of future management of the coast. deliver or modify the generic policy action. options set out by the SMP. A large-scale assessment of the An implementation plan to allow Cost-effective implementation Delivers risks and opportunities associated RMAs to work with communities to options for the preferred scheme or with the coast. prioritise interventions. course of action. A generic policy framework (eg hold A cost-effective solution to deliver the line, advance the line) to adapt An agreed scoped, costed and the aims and objectives of the Output or reduce these risks to people, and specified recommendation for a strategy and SMP in a sustainable historic and natural environment in scheme or course of action. manner. a sustainable manner.

Schemes: of the order of 1 km. Scale Often over 100 km. 10 km to 50 km. BMPs: 10s of km.

Groynes in coastal engineering 11 Figure 2.2 Geographic locations of the second generation SMPs (courtesy Environment Agency)

SMPs are the basis on which the flood defence and coastal erosion risk management strategies (the strategies) for the protection of various coastal units are determined. These strategies become the starting point to determine if beaches have the potential to be part of the approach to shoreline management (as opposed to the use of hard structures or other interventions). If a beach is to form part of the approach, any associated groyne fields should be managed in accordance with the policy in the SMP and the strategy. How this is delivered by the relevant operating authority (authority responsible locally for delivery of the coastal defence strategy) will vary around the coast. Any particular portion of the beach is likely to be managed by individual landowners of various sizes, a coast protection authority (district or unitary council) or by area teams of a national body such as the Environment Agency. Whichever authority carries out the day to day management, co-operation with the others managing the coast is essential.

2.1.2 Long-term management of groynes The management of groynes will be informed both by the need to manage the beach (Chapter 9) as well as by the needs of the groyne structures themselves (Chapter 10). The strategy or BMP will indicate whether the groynes are intended to be adapted/adjusted over time to deliver a changing shoreline or profile (eg to accrete a wider/higher beach) or are part of delivering a more static shoreline with a ‘build and forget’ approach where it is anticipated that minimal post-construction maintenance/intervention will be required.

Whichever intended beach and groyne management regime is being followed, knowledge of how the coastline has evolved, or is likely to evolve, is essential. Local engineers and geomorphologists will have knowledge and experience of the behaviour of beaches on their respective frontages. When combined

12 CIRIA, C793 with information from other partners and organisations at both regional and national scales, this knowledge should indicate when groynes need to be repaired immediately and when they can be left until a package of works is formulated.

The degree to which groyne systems can be adaptively managed (see Sections 2.3.1 and 9.3) in a proactive manner with the available in-house or retained resources should be assessed. Resource considerations include: „ for timber groynes, the capacity to raise or lower groyne profiles at various positions by addition/ removal of planking (which may sometimes also necessitate extensions to king piles) „ for rock groynes, the capacity to extract and/or add large pieces of armourstone.

2.1.3 Sediment trapping efficiency of groynes Groynes can be used on both shingle and sand beaches.

With coarse materials (eg gravel, shingle), groynes act directly in trapping a fraction of the material that is moving along the shoreline.

For sand beaches with a significant amount of suspended sediment transport, groynes act more indirectly by affecting the longshore currents containing the sand. Longshore currents are created by either obliquely incident waves and/or tidal currents. The presence of groynes reduces the strength of currents within the groyne ‘bays’, creating circulatory patterns (Figure 2.3). Typically, these circulations result in seaward- flowing currents along the downdrift side of each groyne, this being a potential cause of scour.

Figure 2.3 Schematic showing flow/sediment movement for a groyne system on a sand beach with oblique wave attack (from Fleming, 1990)

The ability of a groyne to restrict the sediment transport (sediment trapping efficiency or groyne efficiency) depends on many factors, such as the type and condition of the groyne, the amount of sediment already trapped by the groyne since installation, the amount of sediment that can overtop the groyne, the wave climate, water level and tidal range, the sediment size and the groyne dimensions (ie length, height, spacing) and orientation in relation to the direction of sediment transport. Groynes on sand beaches may perform better in micro-tidal to meso-tidal environments where the spatial distribution of transport due to waves and tidal currents across the foreshore is more limited. This poses a challenge in the UK, because most of its coastline is meso-tidal to macro-tidal. As a result, on a UK sand beach, groynes do not generally aim to control all the drift, but should be long enough to control a sufficient part of the beach profile to encourage accretion or to ensure that a re-nourished bay is stable.

For this guide, the sediment trapping efficiency of a groyne is defined as the fraction of the unconstrained instantaneous longshore sediment transport past, a groyne which is retained, by the groyne. Possible sediment transport pathways are shown in Figure 2.4.

Groynes in coastal engineering 13 Key A = Aeolian transport (sediment transport due to wind) over the groyne in the emerged part of the beach. B = Sediment transport of beach sediment over the groyne crest. C = Transport of sediment past the end of the groyne by natural or mechanical bypassing. D = Transport through the groyne, especially if the groyne is permeable; this transport could be either in the emerged or in the submerged part of the beach. Figure 2.4 Sediment transport pathways past a groyne

The sediment trapping efficiency for the four pathways of sediment transport in Figure 2.4 is affected by the nature of the groyne: „ Pathways A and B are affected by the height of the groyne. However, only very high groynes such as substantial piers (Figure 2.14), can be sufficiently high to block all over-groyne transport. „ Pathway C around the head of groynes is mainly controlled by their length. Also, round head structures of rock at the groyne head, which diffract wave energy, can be used to encourage retention of sediment on the updrift side. Conversely, the updrift side of vertical structures is likely to be highly reflective and this can lead to scouring of sediment which may then be carried around the groyne head. „ Pathway D through the groyne structure depends on the structure itself and its tightness to sediment passage. For timber groynes this means that closeness of fitting of the timber planks is essential. Rock groynes are permeable, but rates of transport through them are modest. Dornbusch (2008) has shown that, for mixed beaches, the rate of transport through such structures is a function of difference in beach level between the two sides of the groyne.

For detailed discussion of the uses of this process information in modelling see Section 4.3.3.

As the condition of the groyne diminishes and/or the groyne bay fills, the efficiency of the groyne in trapping sediment will reduce as follows. „ Effect of structural condition changes on groyne efficiency (Figure 2.5). Narrow footprint groynes tend to be more structurally vulnerable to changes in condition than rock groynes. Because of the link with groyne condition, the efficiency is linked to the rate of deterioration of its structure in the prevailing environmental conditions and the level of maintenance provided to the structure. When assessing the performance of a groyne field over its design life using plan shape numerical modelling (see Section 4.3), care is required in relating the appropriate numerical assumptions about trapping efficiency to the condition of the groyne during the period of validation or that expected to occur in the future (see Table 2.3).

14 CIRIA, C793 a b Figure 2.5 Condition vs. time (a) and efficiency, E, vs. condition of a groyne (b). Groyne condition varies from very good (1) to very poor (5) (see Table 10.1)

Table 2.3 Link between groyne maintenance and efficiency to be assumed

Maintenance of groyne Numerical assumption about trapping efficiency Historic maintenance and associated changes in groyne Changes in groyne efficiency to match the known historic efficiency changes in condition Regular maintenance to sustain a fixed condition in the No changes in efficiency over the modelled life future Changes in groyne efficiency over time linked to structure ‘Build and forget’ philosophy involving no maintenance condition (Figure 2.5)

„ Effect of past sediment trapping on groyne efficiency (Figure 2.6). As sediment is accreted on the updrift side of the groyne, the efficiency of the groyne in trapping beach material will reduce in proportion to the reduction in the length and height of groyne still available to trap further sediment. If the accretion reaches the end of the groyne, bypass of sediment around the end of the structure will start. When the beach within the groyne bay has reached a swash-aligned position, although the beach will continue to build, no further realignment is likely to occur. The figure shows the slowly changing long-term average conditions (including any regular cyclical changes) with a decreasing capacity of the groyne to retain additional beach material updrift.

Figure 2.6 Efficiency of the groyne as time progresses (in relation to initial sediment trapping quantity) 2.1.4 ‘Groynes’ formed by other structures Many other structures may exist on the coast that act like groynes, but which have not been designed with the purpose of trapping longshore sediment transport. These include breakwaters, harbour piers, outfalls and boat slipways. Such structures, particularly if they are contained within a groyne field, can be of significant size (length and/or height) and can have a significant impact upon local coastal processes and how effective a groyne field might be. More information on such structures can be found in, for example, CIRIA/CUR/CETMEF (2007) and Cork and Chamberlain (2007).

Groynes in coastal engineering 15 2.1.5 Amenity and heritage value of groynes While not a main justification for their addition or retention, groyne structures can have significant heritage and amenity value for beach users.

Their heritage value is particularly evident with the timber groyne fields built in the early 20th century which have become an important part of the local landscape and history, celebrated in art (Figure 2.7) and photography. In some circumstances, amenity/heritage issues may be material considerations that will need to be taken into account in any planning decisions about the replacement of such structures. However, it should be possible to introduce replacement structures without creating a significant or dramatic change to the historical landscape. All types of groynes can accommodate many of the more passive amenity activities discussed in Rogers et al (2010), providing support and wind shelter to activities including sitting, walking, sunbathing and informal cooking. Recreational use of timber groynes is illustrated in Figure 2.8.

Figure 2.7 Dymchurch wall showing groyne field, painted Figure 2.8 Recreational use of timber groyne at by Paul Nash in 1923 (courtesy The Minories Colchester) Bournemouth, Dorset, 2009 (courtesy Jonathan Simm)

However, the actual groynes may present some health and safety hazards. For example, children may attempt to play on them and fishermen/jet skis may collide with them (see also discussion on health and safety in design (Section 7.8) and dangers of protruding/rusty fixings (Figure 10.2). They may also pose an obstacle for people who wish to walk along the beach where there is no promenade, ie necessitating climbing over the groynes. This should be discouraged and/or steps and ramps provided at appropriate locations. Informal cooking, eg with single-use barbecues, can lead to damage to walings and planking (Figure 2.9).

Figure 2.9 Burning damage to groyne waling from single-use barbeques (courtesy Uwe Dornbusch)

16 CIRIA, C793 2.1.6 Groynes vs. alternative shoreline control structures Where it is apparent that a natural or nourished beach would provide insufficient protection, it should not automatically be assumed that a groyne field will provide the necessary shoreline stabilisation. Instead, account should be taken of the alternatives available. Table 2.4 summarises some of the advantages and disadvantages of different types of coastal structures, their scope of applicability and associated health and safety issues.

Groynes will not prevent offshore losses during severe storm events, because they do not act on cross- shore processes. Shore-parallel breakwaters can reduce this issue, but can also have disadvantages. These include higher costs, how erosion/scour can develop on their seaward side, and how they interrupt longshore transport if a tombolo forms, causing erosion on the updrift and downdrift ends of the salients (cusp formation on the lee of a breakwater). Johnson et al (2010) give further guidance on design of shore-parallel breakwaters on sandy macro-tidal coasts.

Revetments and seawalls at the shoreline do not affect the longshore sediment transport on the beach, although they can significantly interrupt the supply of sediment from landward sources such as cliffs and dunes. Their aim is to protect the landward side from wave action and reduce the risk of flooding or coastal erosion. In situations where net loss of beach sediment is occurring and where there is no beach control, the seaward side of such revetments and seawalls will experience scour and progressive beach lowering, which eventually could cause toe failure. Beach or wave control structures, including groynes, can be considered as an option for mitigation of such impacts.

Groynes in coastal engineering 17 ). Disadvantages Can induce local currents and scour which increase erosion (particularly on sand beaches) and risks for beach users. Large differences from time to time in beach level across low footprint (vertical) groynes. Require nourishment or end effect management to avoid downdrift erosion problems (see Section 4.2.5 When constructed of they timber, will require maintenance. Significant visual impact particularly with macro-tides. May cause leeward deposition of fine sediment and flotsam. May cause localised hazardous rip currents between structures. Could be more expensive/difficult to construct especially in deeperwater. Difficult to balance the impact on both shingle and sand transport. Gaps between breakwaters will channel flow and sediment offshore. Have a major impact on cross-shore transport which can unintentionally cause them to become shore connected, with the consequent interruption to longshore transport and downdrift erosion. Large visual impact. Large structures more expensive to build. Limited design guidance available. Allows for variablelevels of frontage. protectionalong A lot of data about their performance is available. Can be built using shore-based equipment and so are less expensive. When constructed of rock, maintenance can be low. When constructed of the timber, crest profile can be adjusted. Allows for variablelevels of frontage. protectionalong Reduce existing sediment losses from onshore to offshore. When constructed of rock, maintenance can be low. Allows for variablelevel of frontage. protectionalong Can be used to increase the amenity value of the shore. Cross-shorecontrol. longshore and When constructed of rock, maintenance can be low. Advantages Generally positive as they retain beach material, but can impede water sports and constrain beach access. For some beach leisure activities they can create a submerged hazard and be a risk if climbed upon. Rip currents can be generated. Generally positive if they create valued clear pocket beaches through salient and tombolos which can be very attractive for recreational use. Create either partially or totally submerged hazard to craft and bathers. Can promote the accumulation of fines that can produce dangerous soft sand conditions. Rip currents can be generated. Generally built of rock. Can create amenity/pocket beaches. Often large and high structures built of rock that impede access both along the beach and from the water but given their wide spacing and flat profile can be relativelyunobtrusive. Can be used to create promontory type features with walkways. Principal amenityPrincipal health and safetyand implications et al (2010). Shingle/gravel in most tidal ranges due to dominant transport on upper beach. Some sand beaches. Degree of transport over crest and around head depends on length, height above beach and amount of updrift beach accumulation. Often suitable for beaches with either predominantly on or offshore movement or transport beyond the reach of groynes. Beaches can suffer storm damage when structures are ineffective. Have been more popular in micro-tidal than meso-tidal situations, although UK examples in macro-tidal situations exist Elmer, (eg Rhos on See Sea). guidance in Johnson Used where a combination of longshore and cross-shore control is required. General applicabilityGeneral dvantages and disadvantages of alternative shoreline control structures Control of transport longshore rates through physical impedance reorientation and of the shoreline, commonly via a system of multiple groynes. Alteration of nearshore wave climate to create more stable situation. sediment Often used to inhibit offshore sediment loss (eg Happisburgh to Winterton). Combination of physical impediment to movement of material and modification of nearshore wave climate. Principal intended Principal function A )

Groynes (see Section 2.2 Detached breakwaters and artificial reefs Shore connected breakwaters Type of structure Table 2.4

18 CIRIA, C793 Disadvantages They do not control drift at all. More reflective structures tend to exacerbate beach erosion. Will be unstable if due to erosion issues they suffer from scour or undermining. Can create a barrier to access to and egress from the beach. Significant visual impact. Will exacerbate coastal any squeeze. Generally, not suitable for macro-tidal situations (ie most UK situations) Poor performance under storm conditions. They could be a safety risk for beach users and vessels. Limited data available. Well-developed design methods. High level of flood and erosion protection. Make a calmer shoreline wave climate. Soft measure to protect the beach where there is a low/moderate wave climate. More aesthetically acceptable Advantages Provide important amenity function through the provision of promenades. If hard/vertical, then can cause erosion of amenity beach. If revetment, then footprint can impinge on beach. Structures can cause hazardous falls from heights. Generally positive if they successfully retain a perched beach. They can be a hazard to sea craft and bathers. Principal amenityPrincipal health and safetyand implications Historicallyused tofix coastline position, but often suffer problems due to consequent foreshore erosion. Often used in combination with other measures to stabilise material at the toe. Reduce wave action. Most suitable for micro-tidal environments and thus rarely practical in the UK. General applicabilityGeneral Hard edge to restrict erosion to line of defence. Provides higher certainty of defence standard as long as the structureremains intact. Support perched beaches. Principal intended Principal function Seawall/ revetment Sills Type of structure Note See also discussion in Rogers et al (2010).

Groynes in coastal engineering 19 If groynes are adopted, the downdrift erosion caused by the interruption of the sediment transport needs to be recognised and planned for. Acknowledgement and acceptance of such erosion may be noted as part of a managed realignment or no active intervention policy for the downdrift coast. Alternatively, the impacts may be mitigated by adopting one of the engineering solutions discussed in Section 4.2.5.

2.1.7 Groynes used with other coastal defence measures Groynes are often used with many other coastal defence measures.

A combination of groynes with hard defences such as seawalls and revetments is common. Groynes are often added to the beach fronting the hard defence to maintain a beach. This is sufficient to reduce wave overtopping rates to acceptable levels and/or limit the risk of the defence being undermined by beach loss.

It is also common to combine beach nourishment or recycling with groyne construction to quickly provide the required coastal defence and to allow for sediment to begin to bypass the groyne field from the outset, reducing the risk of downdrift erosion.

In micro-tidal environments such as on the Mediterranean coast, groynes are often used in combination with submerged breakwaters or sills when a new beach or a more stable beach is designed in order to support the desired beach profile where the existing seabed slope would be too steep otherwise to allow this.

2.2 TYPES OF GROYNES

There are many principal groyne types, such as piled narrow footprint, gravity, bulk/rubble-mound, permeable and hybrid. These are described in general terms in the following sub-sections. Further information on the structural design of the principal types of groynes is given in Chapter 7 (including a discussion of the health and safety issues associated with the various types, see Section 7.8).

2.2.1 Piled narrow footprint groynes While widely adopted in the UK (see Table 2.1) for their cost and flexibility, piled narrow footprint groynes (Figures 2.10 and 2.11) do involve more maintenance (see Chapter 10) than rock groynes and tend to be more reflective of wave action which can encourage localised scour problems. Decisions to retain timber narrow footprint groynes may be linked to their heritage or amenity value (see Section 2.1.5). There is significant diversity in the nature of existing narrow footprint groynes around the UK. The exact form adopted has been influenced by factors such as: „ available materials (see Section 6.1.4) at the time of construction „ individual preferences of the local authority or design engineer.

These vertical structures most commonly comprise a series of substantial ‘king’ piles (linked by walings), between which horizontal planking is fixed to provide the barrier to sediment movement (see Chapter 6 for details). Such groynes are generally built with straight profiles (Figure 2.11), although zig-zag profiles have also been used (see Section 4.2.4).

The most common material used for narrow footprint groynes is timber (see Section 6.2), which is the principal focus of this guide. However, polymer composites (see Section 6.3) have been adopted and/or are being considered for some elements of such structures, where environmental risks associated with the release of microplastics can be managed (see Section 6.3.8).

A direct alternative to timber involves reinforced concrete piles and planking (see Section 6.4). However, it is difficult to make concrete as abrasion resistant as timber. Concrete cracking and breakage from chloride-induced corrosion of reinforcing steel occurs as sea water penetrates through the abrading surface. Few examples of groynes with concrete piles and planking can now be found in the UK. Protruding or detached reinforcing steel forming part of deteriorating concrete groynes is also unsightly

20 CIRIA, C793 and dangerous to beach users, whether still attached to the groyne or subsequently scattered on the beach. Another, now rare, combination that has been used in the past is steel H-piles with concrete planks slotted between them or timber planks slotted between or bolted to their side. The latter type has been constructed using old train rails as piles.

A fundamentally different approach to forming a narrow footprint groyne is the use of steel sheet piling on its own (see Section 6.5). A modification that is occasionally adopted is to use steel sheet piling to provide a low-level cut-off wall beneath a predominantly timber structure, however, the complexity and cost of construction using multiple materials means that such a solution has only been adopted in a few situations.

Figure 2.10 Example of a timber groyne field at Sheringham, Norfolk (courtesy HR Wallingford)

Note the concrete steps for egress from the groyne bay Figure 2.11 Newly-constructed timber groyne at Southwick, West Sussex (courtesy Royal HaskoningDHV)

Piled groynes can be simple cantilever structures, relying entirely on their embedment into the underlying substrate for their stability or they can be propped/strutted or tied cantilevers. To reduce overall wear and tear, simple cantilevers are preferable but the height of the groyne or the weakness of the substrate may require more structurally complex tied cantilevers or propped cantilevers. The use of props (in compression on the downdrift side of the groyne) is less preferred compared with ties (in tension buried on the updrift side of the groyne) due to their greater exposure to abrasion during beach movements. (Note that the benefit of ties being buried disappears if the beach lowers to the level of the ties, so the ties can wear away more rapidly than props as they are on the side of the groyne with higher wave energy.) Examples of ties and props (historically referred to as ‘trees’, because the tie-members were suitably-sized tree trunks, often with their bark still intact) are shown in Figure 2.12. The maximum sensible height for a timber groyne is around 3.5 m (ie the height between the lowest likely beach level and the design beach level). However, in terms of exposed height to storm waves 3.5 m should be seen typically as a short-term situation over a short length of groyne. In practice, beach management should avoid excessive exposed heights of groyne that lead to increased wave reflections off them. This can often scour away beach material in their immediate vicinity.

Groynes in coastal engineering 21 a b

Figure 2.12 Timber groynes with buried ties (a) and struts (b) (courtesy Roger Spencer and Sarah Burrow)

Another fundamental difference exists between timber groynes ‘planked to the bottom’ (Figure 8.18) and those where planks are driven vertically into the beach/soft substrate (Figure 8.12) that are fixed to the groyne using a ‘lower wailing’ on the updrift side of the groyne (eg Figures 6.3 and 7.5). While the latter reduces the need for excavation and are easier to install, beach loss and erosion of the substrate into which the ‘sheeters’ were driven can lead to rapid failure (Figure 6.3) and complete beach loss due to underrun (see Section 9.2.1).

2.2.2 Gravity groynes Gravity structures are commonly constructed either of masonry (for historic structures) or concrete.

Masonry structures are discussed comprehensively in Cork and Chamberlain (2007). The guide covers small harbour breakwaters and many of the principles, and the structural design, construction and repair practices described therein are also applicable to masonry groynes.

Concrete groyne structures are rare (Figure 2.13). Example situations can include where the groyne has been combined with an outfall structure or where a more substantial ‘terminal’ groyne is required at the end of a groyne field. Such structures tend to be significantly larger than updrift groynes and may comprise a series of pre-cast elements.

Figure 2.13 Concrete groynes at Hove, East Sussex (courtesy Uwe Dornbusch)

Gravity groynes rely on their rigidity and weight for their stability (Figure 2.14). They can adapt to situations where there is a hard substrate under the beach, which is less suitable for piling, or when large beach height differentials are expected either side of the groyne. Due to their form of construction they do not typically adapt to situations where there is a need to periodically adjust the height of the groyne.

22 CIRIA, C793 Figure 2.14 Steel sheet pile and concrete at Seaford, East Sussex (courtesy Royal HaskoningDHV)

Some information on gravity groynes is given in the following sections, although note that this guide is focused on narrow footprint groynes.

2.2.3 Bulk/rubble-mound groynes In almost all cases in the UK, bulk/rubble- mound groynes are constructed out of armourstone. The armourstone may be single sized (typically a ‘heavy grading’ as defined by BS EN 13383-1), especially if the groynes are of small cross-section. More commonly, two or three sizes are used with BS EN 13383-1 ‘light-gradings’ forming the underlayers. Such groynes may be built with straight shape, but Y, T and L shaped structures (Figure 2.15) have also been used depending upon factors such as sediment type, drift rates, presence of tidal currents. Reasons Figure 2.15 Variety of rock groyne shapes at Bexhill, East Sussex for considering alternative shapes have (courtesy CCO) included a desire to: „ encourage diffraction at the head and thus encourage sediment deposition on the downdrift side of the structure reducing beach losses „ help to create a more stable bay shape between groynes (see Harlow, 2013).

While conventional groynes have little effect on onshore-offshore movement of sediment, groynes with a variety of head shapes can help to reduce onshore-offshore losses. This guide does not address the performance and structural design of these groynes as they are discussed in CIRIA, CUR, CETMEF (2007).

Rock groynes have the disadvantages of generally being more expensive and of occupying more of the beach than timber groynes. However, they have some advantages: „ Within the structure and at the structure-substrate interface, rock groynes may settle slightly over time. If there is erosion of the substrate, beneath or alongside the groynes, they tend to settle into the scoured area and self-heal – significant passage of sediment will not occur under these structures. Rock groynes may have reduced efficiency at their crest as a result of this settlement but they will never suffer in the same manner as an undermined timber groyne. „ If they are structurally stable, they have good long-term durability, which may be more suitable for a ‘build and forget’ philosophy.

Groynes in coastal engineering 23 As discussed in CIRIA, CUR, CETMEF (2007), bulk groynes rely on their size and mass for stability (Figure 2.16). Of the three types of groyne they have by far the largest footprint and are permeable to the movement of beach material, through the interstices between the rocks. When designing a bulk terminal groyne at the downdrift end of a groyne field, allowance should be made for ‘bypassing’ of beach material to the downdrift side of the groyne field. They are also suitable for construction seaward of low water mark where timber fixings or concrete placement would also be impracticable. However, if marine placement of armourstone is adopted, care should be taken to avoid any rock falling to the seabed during transhipment or placing operations that might pose a risk to future fish trawling operations.

Figure 2.16 Rock groyne at Sidmouth, East Devon (courtesy HR Wallingford)

Although in theory the height of rock bulk groynes can be adjusted by the addition or removal of rock, in practice this can be a major undertaking and, in comparison to timber groynes, far exceeds the work involved in adding or removing planks.

Due to the semi-porous nature of these structures it is necessary to construct the groynes with a significant freeboard (see Section 5.4). This adds to their overall size and footprint.

Also, due to their size, shape and semi-porous nature, bulk groynes can provide some attenuation of incident wave energy without causing significant wave reflection. This reduces the impact of ‘mach stem’ waves and leads to a more concave beach plan shape next to the groyne (Figure 2.16, and see also the discussion in Dornbusch et al, 2008). However, particularly on amenity beaches, there may be local lobbying to closely pack the bulk material (typically rock) for health and safety reasons, which reduces their ability to absorb wave energy and increases their reflectivity. Alternatives that have been adopted to avoid this issue, include pre-filling of the groyne voids with beach material and/or warning notices to stop clambering on rocks and avoiding the risk of becoming trapped. Localised scour can occur around large armour rocks creating pools of water, but this localised phenomenon is difficult to eliminate. For discussion of hybrid timber and rock groynes, see Section 2.25.

Due to their nature, bulk rock groynes do not suffer the same deterioration processes as timber or concrete groynes so they tend to have a longer life. They also lend themselves to dismantling and reuse of the rock.

Further information on bulk groynes will be limited in the following sections as this manual is focused on narrow footprint groynes.

2.2.4 Permeable groynes Some sediment passing through groynes is not uncommon, depending on the type of groyne, eg small amounts of sand transported mainly in suspension through timber groynes or shingle passing through rock groynes (Dornbusch, 2008).

24 CIRIA, C793 However, groynes that are deliberately permeable to flows and sediment have been designed with the aim of minimising the downdrift erosion, while still stabilising the updrift beach. Such groynes have not been used much in the UK, however there are examples of permeable groynes in Bournemouth (Figure 2.17) and in North Norfolk, both dating from more than 50 years ago. They seem to be more common in other countries, such as the example from Italy (Figure 2.18).

It is hard to gauge the effectiveness of permeable groynes because there is typically very little difference in beach profile either side of them. As this is the conventional way of assessing the effectiveness of impermeable groynes, it may appear that a permeable groyne is having no effect on a beach. However, the two large Makepeace-Wood groynes at Bournemouth shown in Figure 2.17 do seem to promote a wider beach than the conventional vertical timber groynes either side of them. This may be due to the frictional resistance and diffraction effects that they create.

Many originally impermeable groynes in the UK have become permeable following degradation – resulting in reduced efficiency and this may help to explain the reluctance to install deliberately permeable groynes.

Figure 2.17 Example of a previous permeable concrete groyne now replaced with rock at Bournemouth, Dorset (courtesy HR Wallingford)

Figure 2.18 Example of reinforced concrete permeable groyne along the Venetian coastline, Italy (courtesy HR Wallingford)

2.2.5 Hybrid structure groynes A hybrid structure groyne is built by combining two different types of groyne structure and/or material, such as the mixing of rock with timber seen at many places around the UK. Other options are also implemented such as timber/concrete (Figure 2.19), stone/concrete, steel/timber – often combining the benefits of a more robust gravity structure with the flexibility of being able to adjust the height of planking.

Groynes in coastal engineering 25 Figure 2.19 Composite concrete/timber groyne at Hastings, Sussex (courtesy Uwe Dornbusch)

In timber/rock hybrid groynes, timber sections are sometimes replaced by rock mounds to temporarily extend the life expectancy of the groyne. Generally, a first intervention in damaged or aging timber groynes is to substitute the outer damaged planks with new ones. However, structural bays of deteriorated planking and piles could be replaced by armourstone, particularly where most of the wear or damage has taken place. As well as providing some protection to the stability of the structure, this approach can help to avoid outflanking at the root of the groyne (see Section 9.2.2) and, if provided at the head (Figure 2.20), encourage Figure 2.20 Hybrid groyne at Brackenbury, North wave diffraction and sediment movement into the Felixstowe lee of the groyne.

Lower cost but much less durable alternatives to the use of armourstone here include rock-filled gabions or sediment-filled geobags.

The choice of a hybrid groyne presents a few challenges, such as: „ additional plant may be required „ more than one set of skills is required by the workforce „ the hybrid groyne requires an overlap between forms of construction and materials „ the armourstone in deeper water presents a hidden hazard to, for example, swimmers, boats/ jet skis. (While this is common to all rock groynes, it is less obvious a danger due to the sudden difference in groyne material.)

However, a timber/rock hybrid groyne may still be adopted for reasons such as: „ timber construction at the landward end, gives minimum impact on beach access and use „ armourstone construction at the seaward end more effectively dissipates wave energy and is practical because armourstone construction is possible into deeper water than with timber groynes „ although initially more expensive, the armourstone at the relatively inaccessible seaward end of the groyne generally requires less maintenance compared to timber.

A temporary repair of a timber groyne using armourstone is shown in Figure 2.21 and detail of repair of a groyne head in Figure 2.22.

26 CIRIA, C793 Figure 2.21 Repairs to a timber groyne at Worthing, West Sussex

Figure 2.22 Repair of a timber groyne head with armourstone 2.3 WHOLE-LIFE MANAGEMENT OF GROYNES

2.3.1 Adaptive management of groynes Despite the pressure to create ‘build and forget’ groyne structures, the capital cost of groyne replacement is very high (see Section 2.3.2) and groyne designs where regular/efficient maintenance and adaptive management are adopted may offer solutions that are more financially sustainable on a whole-life basis.

In planning and managing the groyne field life cycle (Figure 1.4), it is important to recognise that every beach with groynes will have a unique geography, geology, materials and coastal environment that will all strongly influence both objectives and needs. Important local factors to consider include: „ Beach orientation in relation to the strength and directions of the prevailing waves (Section 3.1.1) and tidal currents (Section 3.1.4) affecting the strength of, and balance between, longshore sediment movement and cross-shore sediment movement fluxes. „ Mean sea level and tidal range (Section 3.1.1) in relation to the beach profile. „ (Natural) sediment supply available (Section 3.1.2). „ Surface level and nature of the beach substrate (Section 3.3.4) which can affect beach volatility and constrain the form and nature of any groyne piles and piling procedures. „ Abrasiveness of the beach material (Section 3.2.2) given its size, shape and composition combined with the severity of the likely wave action.

In addition to understanding local impacts, groynes should be managed with consideration of how the whole section of coast or ‘beach’ is likely to perform or respond. Groynes can affect coastal erosion for a considerable distance up and downdrift of the groyne field itself and work on one groyne can have an effect on beach levels in adjoining groyne bays. Changes to a groyne (profile and/or efficiency) will affect the throughput of littoral material – even modest changes of beach volumes (sand, shingle or other material) available will affect the beach building ability or stability in nearby groyne bays as well as on immediately adjacent frontages without groynes.

Important characteristics of existing groyne systems to consider include: „ nature and materials of the existing groyne system and/or defence structures „ current performance of the existing groyne field „ condition/age of the existing groynes.

Groynes in coastal engineering 27 Important long-term influences to take into account include: „ decadal variations in the coastal forcing „ long-term changes in mean sea level „ the effect of beach nourishment and recycling as well as groyne deterioration on the amount of beach material retained „ whether the beach could reach a tipping point after which a substantial amount of material may be lost and after which it is possible that groynes may cease to be the optimum management approach and alternatives may need to be considered.

In the whole-life management of groynes, estimating the deterioration rates of the actual structures can be a significant challenge, especially in the case of timber groynes. The required frequency of inspection will depend on the aggressiveness of the loading (energy of the wave action combined with abrasiveness of the beach material) and on the current condition and age of the groynes. The Environment Agency (in press) study conducted in 2018 of six sites where timber groynes have been used showed that groyne replacement (or part-replacement) cycles varied from five years in the aggressive conditions at Milford- on-Sea to up to 30 years in the rather mild conditions at Cleethorpes with only modest abrasion and no biological attack. Deterioration rates are discussed further for timber groynes in Section 10.1.2.

The need to manage deterioration of groyne structures points to the importance of maintaining high quality processes for recording inspection and monitoring (see Section 10.2.5) as part of asset management planning.

2.3.2 Costs and funding The costs for coastal construction work (Hudson et al, 2015) can be three times greater than routine civil engineering construction works. This applies to both capital costs for initial construction or renovation and revenue costs for repairs and maintenance. A significant factor leading to these increased costs is the role tides play in restricting operations. If the tidal range is comparatively large, then works may be only practicable when the tide is low – meaning restricted working hours or working shift patterns (see Section 8.1.5). If the range is smaller, then working times may be less constrained but instead there may be a need for more expensive working in, or under, water.

Another key influence on cost will be the type of materials used (see more detailed discussion in Sections 6.1.5, 6.2.8, 6.3.6, 6.4.6, 6.5.5). At a location where groynes have not previously been used, it is possible to carry out detailed whole-life cost comparisons between groyne fields constructed of alternative materials. This should include consideration of appropriate lengths and spacings for each option (see Chapter 4), recognising that these dimensions will not necessarily be the same when moving from consideration of one material or structure type to another. Some materials, such as hardwood timber and (in some cases) armourstone, will require procurement from abroad (see Sections 6.2.9 and 6.2.10). This then introduces an additional exchange rate cost variable.

However, the decision in regard to material type may be constrained by historical practice along the frontage, including public perception of the amenity value of the alternatives and public resistance to change. In addition, the capital cost of changing, for example from timber to rock will be high and such changes may need to be made in a staged manner. Such an approach is also necessary to reduce beach loss during the change from one type of groyne field to another.

In England, funding for flood and coastal risk management is provided by Defra to the Environment Agency, in the form grant-in-aid. This is used by the Environment Agency to fund (see Box 2.1 for links to information) both new schemes (capital) and maintenance of their flood defences. Some of this funding is passed on to coast protection authorities as capital grants for coast protection improvements. Maintenance of coast protection is viewed by Defra as a ‘local issue’ and is funded by the coast protection authority themselves. Applications for grant-in-aid for capital works (both flood defence and coast protection) are administered by the Environment Agency, who require applications to be supported by full life-cycle assessment of costs (including carbon costs – see Section 6.1.5) and benefits. Since 2011, the Environment Agency has provided grant-in-aid as a proportion of the capital cost of each capital scheme, that proportion

28 CIRIA, C793 being determined (see Box 2.1 for links to information) on the basis of the scheme’s expected outcomes, ie the benefit cost ratio of the works, the number of households protected and the environmental obligations that are met. The promoters of flood and coastal erosion risk management (FCERM) capital schemes are, as a result, often required to supplement the grant with additional ‘partnership funding’ from other sources such as private beneficiaries (eg businesses that benefit), European Union funds, and other local and national funds. The methods of obtaining grant-in-aid are well known to most risk management authorities (RMAs) but advice and assistance can always be sought from the local Environment Agency FCERM manager or their partnerships and strategic overview (PSO) team.

Box 2.1 Sources of information on funding

The following information is available from the UK Government website: Business case development: https://www.gov.uk/guidance/flood-and-coastal-defence-appraisal-of-projects Guidance on scheme appraisal: https://www.gov.uk/government/publications/flood-and-coastal-erosion-risk-management- appraisal-guidance Calculation of available funding: https://www.gov.uk/government/publications/calculate-grant-in-aid-funding-flood-risk- management-authorities Calculator to determine necessary partnership funding: https://www.gov.uk/government/publications/fcrm-partnership-funding-calculator

The grant funding system in the UK rarely makes provision for funding of life-cycle maintenance works, which leaves operating authorities with a maintenance funding pressure and in practice can mean there is limited or no maintenance at some locations. This pressure creates a drive towards low maintenance/ build and forget options like rock structures (Dornbusch, 2019). There is clear evidence that, if not properly maintained, structures can degrade quickly and fail to provide the function for which they were originally designed. Spending constraints and pressures may mean that local authorities have moved away from a large, retained in-house workforce or direct labour organisations (DLO). Some authorities may have dispensed with the DLOs completely – outsourcing necessary operations on a retained (framework) or ad hoc basis, whereas others may retain skeleton teams that need to have multiple skills. Having access to the skills and resources needed to effectively manage groynes is essential to prolonging their asset life and is a key part of whole-life asset management.

One method of bringing some early certainty to cost, and to spread budgets for a works scheme over several financial years, may be to purchase some or all the materials in advance. This particularly applies to timber and steel sheet piling but depends on storage space being available and receiving approval for the advance purchase. As well as the financing advantage, this can also provide more certainty that the materials will be available when required for construction.

2.3.3 Removal of a groyne field Groyne removal rather than replacement may be considered when the groynes have reached the end of their useful lives. Considerations should not only include the practicalities of their removal discussed in Section 9.4, but also the implications for the coastal system. Consideration should be given to removal of groyne fields where, for example: „ it has been established since the original groyne construction that sediment movement is predominantly onshore-offshore rather than alongshore, so the groynes do not fulfil a useful function. „ alternative methods of protecting the coast are assessed to be more viable (see Section 2.1.6). „ a change in management policy for the coast is being implemented.

2.4 LICENSING AND CONSENTING FOR GROYNE STRUCTURES

Works on the foreshore (ie including groynes) may, or in many cases will, require licences or consents from various bodies (see example for England in Box 2.2). The nature of and need for these consents,

Groynes in coastal engineering 29 and the bodies responsible for them, changes from time to time. It is always wise to seek up-to-date advice from the following (in England): „ Marine Management Organisation (MMO) and their licencing processes (and information on Harbour Works Orders): https://www.gov.uk/topic/planning-development/marine-licences „ coastal manager at the local coast protection authority „ local Environment Agency area FCERM manager „ for planning permission, contacts at the local planning authority (LPA) (unitary or district council).

The equivalent information for Scotland Wales and Northern Ireland can be found at: https://www.gov.uk/government/collections/flood-and-coastal-erosion-risk-management-authorities

The coastal concordat is an agreement between the Department for Environment, Food and Rural Affairs (Defra), the Ministry of Housing, Communities and Local Government (MHCLG), the Department for Transport (DfT), the MMO, the Environment Agency, Natural England and the Local Government Association (LGA) coastal special interest group (SIG). The SIG was established in 2014 and should provide a framework within which the separate processes for the consenting of coastal developments in England can be better co-ordinated, between LPAs, government departments and the developer. However, by July 2019, not all local authorities had signed up to this scheme. MMO can advise on the currency of the coastal concordat.

Box 2.2 Licences, permissions and leases required for coastal defence works in England

1 Planning permission may be required under the Town and Country Planning Act 1990 for development above the low water mark. 2 A marine licence will be required under the Marine and Coastal Access Act 2009. This Act established the MMO, and they are responsible for the marine licence which incorporates: (a) the requirement for permission under S34 of the Coast Protection Act 1949 (as amended by S36 of the Merchant Shipping Act 1988) for: i the construction, alteration or improvement of any works on, under or over any part of the seashore lying below the level of mean high water springs (MHWS) ii the deposit of any object or materials below MHWS iii the removal of any object or materials from the seashore below the level of MHWS (b) the licence required under Part II of the Flood and Environment Protection Act 1985 (FEPA), for works involving the deposit of any articles or materials in the sea or under the seabed, either temporarily or permanently 3 A works (or equivalent) licence may be required from the statutory harbour authority under a relevant local act 4 A lease from the Crown Estate Commissioners may be required for works lying over the seabed 5 An appropriate assessment may be required under Council Directive 92/43/EEC (Habitats Directive) or Directive 2009/147/EC (Birds Directive) for works within Special Areas of Conservation (SAC) or Special Protection Areas (SPAs) respectively, or likely to affect the integrity of such sites. Natural England can advise on these situations and the UK equivalent of Sites of Special Scientific Interest (SSSIs) established under the Wildlife and Countryside Act 1981 and any legal requirements, including those under the Conservation of Habitats and Species Regulations 2017 6 An Environmental Impact Assessment (EIA) may be required under the relevant regulations implementing Directive 2014/52/EU (EIA Directive). The LPA or the MMO can advise on this requirement 7 A Water Framework Directive (WFD) compliance assessment is likely to be required, in accordance with the ‘clearing the waters for all’ guidance given in Environment Agency (2017). Note that heritage coasts are ‘defined’ rather than designated, so there is no statutory designation process like that associated with National Parks and Areas of Outstanding Natural Beauty (AONB), which were established to conserve the best stretches of undeveloped coast in England. A heritage coast is defined by agreement between the relevant maritime local authorities and Natural England. The national policy framework and objectives for heritage coasts were developed by the Countryside Commission, a predecessor of Natural England, and ratified by government.

2.4.1 Planning permission Planning authorities (typically district and borough councils, see Section 2.4 regarding the coastal concordat) should be pragmatic in their land-use planning approach when dealing with new, replacement or refurbished groynes, and their maintenance (often deeming them to be permitted development). As a rule, planning permission: „ is less likely to be required where the activity involves „ like-for-like maintenance or refurbishment

30 CIRIA, C793 „ replacement of old groynes for new on a like-for-like basis without any likely change in the landscape outcome „ is more likely to be required if new groynes are being installed or where the intention is to change the profile of the beach (ie with higher or longer structures).

The Environment Agency has permitted development rights where new works are within the footprint of an existing defence. If they choose to exercise these rights, then there is a statutory process of notices that needs to be followed.

In general, the LPA should be contacted for local pre-application advice. However, the existence of environmental designations on or close to the site and the policies set out in the local river basin management plan (RBMP) may be a strong influence on planning or marine licensing (see Section 2.4.3).

The existence and extent of environmental designations can be found on the MAGIC website: https://magic.defra.gov.uk

2.4.2 Landowners’ permission Much of the foreshore (the area between the MHW and mean low water (MLW) mark) is owned by the Crown Estate Commissioners. Many district and borough councils either buy or enter into regulating leases with the Crown to be able to better control activities at the coast. However, this should not absolve the owner or lessee from seeking permission from the Crown for major works (maintenance can be catered for within the lease). The Crown does not own all the foreshore and some sections may be owned privately, corporately or by councils or other public bodies.

Landowners’ permission may well be required for new works. The Coast Protection Act 1949 provides for situations where permission cannot be obtained and for powers of entry.

It is courteous to inform landowners of any planned requirements for entry onto land for maintenance operations but custom and practice may well mean that this is not formally required. It is sometimes the policy of the local operating authority to serve an entry notice unless a very close working relationship exists between the authority and the landowner.

2.4.3 Marine licensing Marine licences (see Box 2.2) are required for all works involving constructing, altering or improving any works in or over the sea or on (or under) the seabed ie within tidal waters. However, there are some relevant exemptions for coastal defences, emergency flood defences and harbour works which, if applicable, would mean a licence would not be required.

The MMO can provide guidance.

2.4.4 Environmental and amenity constraints Environmental designations (SACs, SPAs, and SSSIs), the local RBMP and marine conservation zones are likely to impose constraints on both construction and maintenance works. For amenity beaches, and beaches used for other activities such as fishing and boating, other constraints, eg seasonal and working hours, may apply (see Defra and Environment Agency, 2019, and MMO, 2013).

The LPA and the MMO can advise on these requirements.

2.4.5 Permissions for use of materials Materials should be responsibly sourced to avoid damage to the source environment, to meet sustainability targets and follow legal procurement procedures as described in Sections 6.2.9 and 6.2.10.

Groynes in coastal engineering 31 3 Physical coastal environment This chapter explains the physical environment of the coast and beach (Figure 3.1). Information about this environment is required when designing a groyne scheme or when planning its management. The first part of the chapter focuses on the effect of the physical environment in creating the forcing that moves the sediment contained within a groyne field and the impact of sediment level differences on the groynes. Aspects covered include water levels, waves, longshore and tidal currents, and sediment transport, and how these forcing effects may change with the changing climate (eg sea level rise, increasing storminess). The second part of the chapter deals with the nature of the beach material itself (including the beach substrate) and how to assess this. Where possible, the approach has been to signpost existing information, especially other CIRIA guidance. The sections of this chapter are:

„ Hydrodynamic conditions (Section 3.1) „ Beach material and substrate type, grading and profile Section( 3.2) „ Site investigation (Section 3.3)

Coastal management Functional requirements context and indicators (Section 2.1) (Section 2.1)

Physical coastal environment – hydraulic, geotechnical and morphological constraints (Chapter 3)

Start or continue No Groyne removal to use groynes? (Section 9.4)

Yes

Plan shape design of groyne Evaluate and choose groyne field (Chapter 4) type (Section 2.2) and material (Chapter 6)

Design groyne profile (Chapter 5)

Structural design or design review for groynes (outline and detailed) (Chapter 7)

Construction and/or major refurbishment (Chapter 8) Major Minor Groyne repair and maintenance Groyne adaptation (Section 9.2) (Section 10.3) and adjustments (Section 9.3)

Small Beach changes Routine monitoring Large (Section 9.1) (Sections 9.1 and 10.2)

Structural deterioration (Section 10.1) Post-incident monitoring (Sections 9.1 and 10.2)

Climate change Storm events (Section 3.1.4)

Figure 3.1 Parts of groyne management process covered in Chapter 3

32 CIRIA, C793 3.1 HYDRODYNAMIC AND SEDIMENT TRANSPORT CONDITIONS

3.1.1 Water levels and waves Water levels near the coastline in the UK are a combination of an astronomical tide level and a meteorological surge and are commonly described as still water level (SWL). The SWL at the coastline may be further increased by wave set-up. Actual instantaneous water levels at any point in time will be a function of the extent to which waves run up the beach.

Waves (wind waves, swell waves and infragravity waves) are the primary drivers for the transport of material in the nearshore zone and are the main factors in the movement of sediment on a beach and the resultant geometry of that beach. Waves are generated offshore and are transformed as they progress towards the shoreline by interaction with the seabed via the processes of shoaling, refraction and wave breaking. Additional processes illustrated in Figure 3.2 that are important for groyne fields are wave diffraction, which can take wave energy around the head of the groyne into its lee side, and wave reflection, which can exacerbate scour on the side of the groyne receiving wave action.

Figure 3.2 Wave diffraction at a groyne

For a detailed description of the wave characteristics influencing a groyne field, wave transformation processes (eg shoaling, refraction, diffraction and reflection) should be considered and there are several numerical models now available for this purpose. Detailed discussion of wave transformation processes and typical models can be found in Chapter 4 of Rogers et al (2010) and in Blanco et al (2019).

Wave and water level conditions, and the resulting run-up, are important for the assessment or design of a groyne field and the individual groynes within it including their individual elements. They influence two aspects of groyne field design: 1 Design of the groyne structures when identification of extreme wave and water level loading conditions at the groyne structures are important. Joint probabilities associated with combined wave and water level conditions may also be important (see Box 3.1). During low water conditions, the largest waves will often break before reaching a groyne and impact forces will consequentially be low. In contrast, when water levels are high relative to the level of the groyne, large wave forces can be generated, particularly for long period waves, even when offshore wave conditions are

Groynes in coastal engineering 33 insignificant. However, depending on the cross-shore position within the groynes, the most severe wave loadings may not necessarily occur at the highest water levels. 2 Design of the groyne field layout where the morphological effect of the wave and water level conditions on the beach sediment over a short (ie event) and averaged over a prolonged period of time becomes important. The long-term averaged wave climate can be assessed on a seasonal or annual basis and its effect on both the cross-shore and longshore response of the beach determined. a In a cross-shore sense, equilibrium beach profiles and onshore–offshore movement are dependent on the combined effect of wave height, wave period and water level. Such data form the basis for evaluation of the envelope of beach response profiles (see Section 3.3.4 and Chapter 4). b In a longshore sense, wave direction in relation to the alignment of the coast plays a dominant role in the generation of longshore currents, and the change in the orientation of the beach when a groyne field is present (see Chapter 4).

Detailed discussion of waves and water levels, including data sources and models, can be found in Chapter 4 of Rogers et al (2010) and in Blanco et al (2019).

Wave and water level data is vital to understand wave and current processes affecting the coastline, for example through the calibration of hydrodynamic models. In England, Defra funds the national network of regional coastal monitoring programmes, which collects coastal data such as waves, tides and topography. It is collected through six regional programmes (led by Scarborough Borough Council, East Riding of Yorkshire Council, Environment Agency, New Forest District Council, Teignbridge District Council and Sefton Council) and collated, archived and disseminated by the South-East Regional Monitoring Programme, the Channel Coastal Observatory (CCO), in collaboration with the University of Southampton and the National Oceanography Centre. This data is freely available through the Open Government Licence.

An important nearshore wave data source now available for England is that produced by the recent State of the Nation study (Gouldby et al, 2018) which created nearshore combinations of wave heights and sea levels at 1 km spacings around the entire coastline of England and Wales, calibrated against available data. An example of the use of this data for groyne design is shown in Box 3.1.

Box 3.1 The use of joint probability to determine the range of wave conditions creating the same destabilising forces for design of rock groynes

Figure 3.3 has been developed to show how joint probability effects could be considered in assessing wave forces at the toe of a rock structure after transformation across a beach. This figure highlights the c5000 ‘events’ (red dots) from a simulation of 10 000 years of wave conditions (all dots red and blue) impacting a groyne that would produce forces with a return period of the order of one to two years (ie 5000 events in a 10 000 year simulation). The figure shows that this level of force (one to two-year return period) is associated with a wide spread of wave heights and water levels. As a result, the same force is delivered with sea levels as low as 2.3 m Ordnance Datum (OD) and as high as 4.0 mOD and significant wave heights in the range 1.9 m to 2.4 m. The Figure 3.3 Joint probability of 10 000 years of modelled waves and sea levels reasons for the scatter in the wave at a coastal defence structure on the south-east Scottish coastline, highlighting forces show that different wave types events that have impact forces with a return period of between one and two years (wind or swell waves) and their associated periods and directional components will create different breaking conditions and of wave forces at a groyne for similar sized wave heights. Based on results from HR Wallingford (2015).

34 CIRIA, C793 3.1.2 Longshore currents Currents in the coastal zone include flows of water both parallel to the shore and perpendicular to it (eg rip currents – see Figure 4.3). Shore-parallel flows within the surf zone are generally called longshore currents and are generated by a number of mechanisms, including the action of the waves breaking obliquely on the shoreline (Figure 3.4), differential water levels due to local variation in breaking wave heights, tidal streams and direct wind shear. (Note that the wave breaking in Figure 3 4 driving the longshore currents is the more seaward line of white waves and Figure 3.4 Waves breaking obliquely on a shoreline (courtesy HR associated with the water depth being Wallingford) too shallow to sustain unbroken waves – the more landward line of breaking is that associated with the final breaking of waves on the shore.)

The basic theory for the generation of longshore currents by wave action on a straight and uniform slope was developed in the early 1970s by Longuet-Higgins (1972) and subsequently by Komar (1977) and Fleming and Swart (1982). The theory provides information on the distribution of currents across the foreshore as a function of wave height, wave angle and bottom roughness.

The distribution of longshore currents is required for the prediction of the longshore transport and for the assessment of the groyne length and related groyne efficiency (see also Chapters 4 and 6). Figure 3.5 shows a conceptual current velocity distribution across the breaker and surf zone due to wave action for a static water level. Note the maximum current lies slightly landward of the breaker Figure 3.5 Cross-shore distribution of longshore velocity for a static water level scenario (after Fleming, 1990) line, which can be determined from the wave height relative to water depth (Fleming, 1990).

3.1.3 Longshore sediment transport There are important differences between sediment transport on sand and shingle beaches. Shingle moves almost entirely as ‘bed load’, creeping over the surface of the beach/seabed and often driven by the direct action of breaking waves. Sand transport may have a bed load component, but also travels as ‘suspended load’. Suspended load occurs when sand grains are supported within the water column by the effects of turbulence and are washed along by the longshore currents.

The gross transport rate (Qgross) is defined as the scalar sum of the transport moving downdrift of the perpendicular to the beach (Qdowndrift) and the transport moving to the updrift (Qupdrift) as shown in

Figure 3.6. The net transport (QNET) is the vectorial sum.

Groynes in coastal engineering 35 Figure 3.6 Direction of sediment transport

Figure 3.7 shows equivalent conceptual cross-shore distributions of the longshore transport for both gravel and sand beaches. The figure illustrates the following: „ Gravel beaches are steeper than sand beaches, so the breaker line for the same water depth and wave conditions will be nearer to the top of the beach (Figure 1.2) than for sand beaches. „ The longshore sediment transport of gravel beaches is predominantly bed load and this transport is focused near the top of the beach, whereas the longshore sediment transport of sand beaches includes a significant amount of suspended load and thus is spread out over a much wider distance.

Figure 3.7 Cross-shore distribution of longshore transport as a function of beach material (from Fleming, 1990)

The modelling of the cross-shore distribution of the longshore drift may reveal changes in direction of the longshore transport across the profile (Figure 3.8). Changes in directions can be caused by the presence of currents such as, for example, tidal currents. Further details of how to assess the cross-shore distribution of currents induced by wave breaking and the resulting distribution of longshore transport can be found in Chapter 3 of Rogers et al (2010).

In estimating the distribution of longshore currents for the assessment of required groyne length, it is important to take account of water level variations throughout the tidal cycle. This principle can be illustrated by comparing Figures 3.9 and 3.10. Figure 3.9 shows a distribution of longshore sediment transport calculated with a fixed water level. As indicated by the blue dotted line, the cumulative

36 CIRIA, C793 Figure 3.8 Computed cross-shore distribution of the longshore transport for a fixed water level, showing the difference in drift direction between the top and bottom parts of the beach longshore sediment transport in the first 80 m from the shoreline is about 60 per cent of the total longshore transport. However, if it is assumed that the tidal range at this hypothetical location is about 6 m and take this into account in the calculation of the transport distribution, the amount of longshore sediment transport in the first 80 m drops to less than 30% (Figure 3.10). It will be evident that the difference in such longshore sediment transport calculations will be important when setting the length of any groyne (see Section 4.2.3).

Figure 3.9 Cross-shore distribution of longshore transport using a fixed water level

Groynes in coastal engineering 37 Figure 3.10 Cross-shore distribution of longshore transport using a tidally varied water level

Several attempts to directly measure the longshore sediment transport have been carried out, using traps (eg Cartier et al, 2013 for shingle beaches) and acoustic or optical backscatter instruments and tracers (eg Guza and Feddersen, 2011 for sand beaches). However, such measurement is not straightforward because of the obstruction caused by the traps themselves and the fact that they can only be deployed for a limited time period. There are also many uncertainties in making such measurements, for example, how representative are the measurements made at a particular position and time of what is happening in the wider area over a longer timeframe? Because of these uncertainties, the derivation of the longshore transport rates at a site is often carried out using empirical methods and formulations. These might involve a few different approaches or a combination as discussed here.

If the beach is long and straight and there is plentiful supply of mobile sediment, estimates of the longshore drift can be carried out by: „ measuring the build up of material against existing large structures, such as long groynes, breakwaters and harbour arms „ considering recorded rates of growth of natural features such as spits „ measuring infill rates or dredging volumes in harbour approach channels.

There are limitations to these methods, in particular because changes in beach cross-sectional area are related to the variations in longshore drift rather than to the total magnitude of the drift. Also, on sandy coasts, where most of the load is carried in suspension and where tidal currents may amplify the transport, half of the longshore transport may occur outside the breaker zone.

A more analytical approach to the assessment of the longshore transport can be adopted by assuming that the longshore transport rate depends on the longshore component of the energy flux in the surf zone, the inputs to the calculation are wave height, period and direction. This is the basis of both the CERC formula (USACE, 1984a) and the modified CERC formula (Ozasa and Brampton, 1980). The latter also accounts for situations where variations in breaker height along a coastline are relevant and important. There are several criticisms levelled at the formula because it considers bulk transport rather than explicitly relating this transport to other factors such as beach sediment size, beach slope, or beach permeability. Also, the approach gives no information on the likely cross-shore distribution. However, the simplicity and flexibility of the formula has meant that it is probably the most widely used formula for the calculation of potential longshore transport.

38 CIRIA, C793 There are other simple formulations, such as Kamphuis (1991), Kamphuis (1996), and Van Rijn (2014), which introduce variables such as bed slope, sediment grain size and/or wave period. However, the difficulty in measuring or defining these variables and the fact that a single value needs to be used to represent an entire area introduce additional uncertainty and necessary simplification.

These methods have the limitation of dealing with ‘potential’ sediment transport, ie assuming there is infinite supply of mobile sediments. Sometimes there is insufficient beach material to ‘satisfy’ the potential longshore transport that waves and tides could create, and this aspect needs to be taken into consideration. However, despite this limitation and the simplified approach, the methods still represent an important tool for the estimation of sediment transport rates, seasonal transport variability and sensitivity to drift reversals, which occur when the drift direction dramatically changes.

Variations in longshore sediment transport rates The effect of wave direction and the chronology of its natural variability is important when evaluating the longshore transport rates in either direction. Although longshore sediment transport rates may be assessed as annual net rates (net amount of sediment moving past a point on the beach in a year), any sediment transport study requires a good understanding of both the seasonality and annual variation of the potential transport rates and the actual rates. Actual rates can be estimated by validating the potential calculation against measured data, although the result will be subject to the limitations in the measurement methods previously discussed.

Longshore drift reversals occur when wave action drives sediment transport in the opposite direction (see Box 3.3). These changes can occur at different time intervals from short-term changes within a tidal cycle to more sustained changes that occur seasonally or annually in various places around the world. Radical changes in drift direction can affect a frontage protected by groynes, creating erosion where the design objective was to achieve accretion. Drift reversal will have a direct impact on the efficiency of the groynes and can cause erosion of the beach profiles/volumes and/or consequential severe flooding during storms. They may be identifiable from records, observations, and/or ongoing beach surveys/ post-storm surveys, so to avoid missing these drift reversals when modelling, it is preferable to carry out analysis using time series of wave conditions rather than an annual wave climate. An example of a significant and unexpected drift reversal situation is the stripping of the ‘normal’ shingle accumulation area at the eastern end of Seaford beach during a storm event in 2014. This stripping was the first time it had occurred since the beach nourishment project was installed in the 1980s.

Longshore transport estimation should consider the changes in wave height and direction along the frontage, drawing attention to potential areas of convergence or divergence of longshore drift, which will be linked to areas of sediment accretion or erosion. Examples of estimations of longshore drift in places around the UK based on hindcast wave data have been provided in Figures 3.11 to 3.13. These figures, calculated using the modified CERC formula, show examples of the annual (Figure 3.11), monthly/ seasonal (Figure 3.12) and spatial (Figure 3.13) variability of the longshore drift.

Groynes in coastal engineering 39 Figure 3.11 Example of annual variability of longshore transport. Note the change in drift direction between 1983 and 1989

Figure 3.12 Example of monthly and seasonal variability of longshore transport

Figure 3.13 Example of longshore transport at various points along the length of a frontage

Box 3.2 describes how the annual variations in the longshore drift on the Anglian coast of the UK have been found to be correlated to the North Atlantic Oscillation (NAO).

40 CIRIA, C793 Box 3.2 Effect of NAO on longshore drift in Suffolk

Changes in the bathymetry of the banks in the seabed offshore of Suffolk (and the resulting modification of the magnitude and reaction of the incoming waves) were thought to contribute to the variability of the nearshore wave climate and the erosion of certain areas. However, the effect of the changes in the nearshore banks on wave climate has been shown to be too small to affect the beaches nearby. Instead, the annual variations in the longshore drift on the Anglian coast of the UK have been found to be correlated to the NAO (Blanco and Brampton, 2017), see Figure 3.14. The NAO is a large-scale atmospheric a pressure see-saw in the North Atlantic region (related to the difference in pressure between the Icelandic low and the Azores high) which drives weather systems towards the UK in different directions. While the NAO could intuitively be considered to affect the western coastline of the UK, its influence in Suffolk is perhaps less expected. A correlation between the longshore drift and the winter NAO index for January, February and March was observed where years with a net longshore drift to the north were mostly correlated with years of a positive NAO index. Conversely, years with a net longshore drift to the south were mostly correlated with years of b a negative NAO index. Moreover, the increased erosion observed in the Figure 3.14 Comparison between average annual net drift and the winter NAO area since 2013 is linked to two high index, time evolution (a) and scatter plot (b) positive NAO index years following a high negative NAO index in 2013. Note that a dependence was found between the net drift and the winter NAO index, but such dependence was not seen between the gross drift and the winter NAO index. This dependence of the net drift on the NAO suggests that it is likely that it is the bimodality in the wave direction that is the key for this relationship.

3.1.4 Tidal currents and influence on sediment transport Tidal current conditions around many coastlines, particularly in the UK, are quite variable, with beaches which lie along or close to the mouths of tidal inlets or estuaries being subject to faster tidal currents. A tidal current on its own may be too weak to move sediment, but when combined with wave induced currents, may raise the total current to a level at which sediment entrainment and motion occurs.

Tidal stream data can often be obtained from existing tidal models (perhaps produced as part of a strategy study), from the local water company (who may well have modelled or recorded data undertaken for water quality studies), from the UK Hydrographic Office and may even be publicly available from the National Oceaonography Centre (via the anyTide app). anyTide app: https://noc-innovations.co.uk/software/anytide

With reference to the anyTide app, data on marine and estuarine current velocities can also be obtained by direct measurement or through the use of numerical models, although in the latter case measurements to provide boundary conditions (eg water levels) still may be needed (Rogers et al, 2010). However, caution should be exercised in using these models or measurements. In the more dynamic estuaries, where the flow patterns may be significantly influenced by positions of ebb and flood shoals at the mouths, any measurements or modelling needs to include up-to-date bathymetry. There may also be a need for higher resolution modelling nearshore, which may not have been included in the original modelling.

Groynes in coastal engineering 41 Strong inshore tidal currents, and associated changes in tidal levels, can be a major complicating factor in the prediction of sediment transport and the changing morphology of UK coastlines, and in the design and efficiency of coastal structures (Rogers et al, 2010). An example of this effect is where there are large spatial differences in either tidal times or magnitudes (tidal asymmetry) and significant tidal currents can be generated. Differences in tidal times are common near the mouth of an estuary or tidal inlet. The effect of spatial differences in tidal magnitude is illustrated by the North Norfolk coast where the rapid spatial changes in tidal range of approximately coincident timing are accompanied by strong longshore tidal currents. Here, at high tide, sediment on the upper portion of the inter-tidal zone experiences tidal currents flowing to the east and south, whereas at low water, when the ebb tide current is in the opposite direction, no effect is felt because the upper part of the inter-tidal zone is dry. In the second case, investigating the likely effects of the tidal flows on the transport of beach sediments, at least in a simple manner, is important to gain a better understanding of how groynes or other coastal defences might affect the beaches. It is common that field measurements and numerical modelling is required to support this sort of investigation. See Box 3.3.

Box 3.3 Example of the effect of tidal currents on longshore drift rate and direction

The effect of the tidal currents on the longshore drift rate and direction was analysed as part of a study by HR Wallingford (2001). Figure 3.15 shows the outcome of this analysis and the differences in the longshore drift rate that tidal currents might cause for a typical incident wave condition arriving at the coastline in the study area. The calculations have been averaged over a complete tidal cycle (of 12.4 hours) and so show the net effect of the flood and ebb currents. For the assessment of comparative drift rates, the wave condition was assumed to be constant throughout the tidal cycle. The figure shows the distribution of the longshore drift across the Figure 3.15 Effect of tidal currents on longshore drift rates and directions beach, ie as a function of the beach level relative to OD. The black line shows the beach level. The orange line shows the calculated drift assuming no tidal currents (but a rise and fall of the water level commensurate with a mean spring tide) while the blue line shows the same calculations but with the additional effect of the tidal currents. The following conclusions can be drawn: „ For the case without tidal currents (orange line) there is a southward (negative) transport of sand over the whole beach profile. „ With the tidal currents included (blue line) on the lower part of the beach profile, the predicted sediment transport for this wave condition is reduced or reversed, ie with a net transport to the west/north. This is the effect of the ebb tidal flow around the time of low water. At this time, waves are agitating the sand and although they also try to produce an east/south flowing current it is shown that this is countered by the stronger tidal flows. „ The peak negative drift on the upper part of the beach profile is increased by the effects of the flood tide near the time of high water. This effect is of the order of 7.5 per cent for the example situation considered. The implications of the tidal effect on sediment transport for any groyne field deployed in this situation will be that:

„ the downdrift effects (see Section 4.2.5) may be greater „ some reversals in sediment transport direction may take place.

3.1.5 Climate change influences Decision making for coastal management over a typical appraisal timescale of 100 years should take account of likely changes in climate to ensure that solutions are resilient to these changes and economically robust. In practice, however, small footprint groynes constructed of timber are only likely to have a structural life of about 5 to 40 years (see Box 10.1 for examples). In most cases, this means that adaptation to take account of actual climate changes need only be adopted each time the timber groyne field is renewed/replaced. The most recent information on climate change effects is now

42 CIRIA, C793 available from UKCP18, and the suite of UK climate predictions (Met Office, 2018) provides information on temperature, precipitation, wind, sea level rise and storm surge. Information can be extracted from the UKCP18 datasets for several scenarios representing different greenhouse gas concentration trajectories. The most significant changes relate to mean sea level increases, changes in storminess and wave directions (although temperature increases may lead to increases in deterioration rates of timber as discussed in Section 6.1.2).

Predicted changes in mean sea level are of most relevance to groyne design. Projections of mean sea level change over time around the UK and vary substantially by both emissions change scenario and geographic location (Figure 3.16). Current recommendations for sea level change allowances for flood risk management projects are given in Environment Agency (2019d). The significance of these allowances for groyne design will depend on the tidal range at any particular location. Additional information on sea level change has recently been published by the Figure 3.16 Range of sea level change (m) at UK capital Intergovernmental Panel on Climate Change (2019) cities in 2100 relative to 1981 to 2000 average for low (RCP2.6) medium (RCP4.5) and high (RCP8.5) emissions scenarios

Based on storm surge modelling work, it is suggested that there is unlikely to be any significant additional increase in the statistics of extreme water levels associated with atmospheric storminess change. Projections of average wave height suggest changes of the order 10 to 20 per cent and a general tendency towards lower wave heights. Changes in extreme waves are also of the order 10 to 20 per cent, but there is little agreement in the sign of change among the model projections. High resolution wave simulations suggest that the changes in wave climate over the 21st century on exposed coasts will be dominated by the large-scale response to climate change. However, more sheltered coastal regions are likely to remain dominated by local weather variability.

Other forcing conditions that may be affected by climate change include wave direction and sediment supply. There is significant uncertainty regarding the extent of these changes, but their relevance for the design of groynes fields is as follows: „ Wave direction. Changes in low pressure weather systems (ie storms) can have an influence on wind directions, affecting the resulting directions of wave attack on beaches. Even a small change in the mean wave direction can greatly alter longshore drift rates and also annual rates (or even directions) and beach widths. „ Sediment supply. Changes in rainfall and temperature will influence the weathering of the land surface and sediment supply to the coast and may increase (or reduce) the vulnerability of coastlines to erosion. In particular, increased rainfall can raise ambient groundwater levels creating additional cliff instability and resulting increased erosion.

3.2 BEACH MATERIAL AND SUBSTRATE TYPE, GRADING AND PROFILE

3.2.1 Introduction Information on sediment properties of the beach is essential for the assessment of the longshore and cross-shore transport and their distributions at the location of a groyne field, and for any assessment of erosion and scour. Knowledge of the surface profile and erodibility of the beach substrate is important both for the understanding of the behaviour of the overlying mobile beach. The nature of the beach substrate is important for engineering the piled or other foundations for the groyne.

Groynes in coastal engineering 43 3.2.2 Sediment types and sources The current situation of a beach depends on both natural (geological and geomorphological) changes and any human interventions. The key natural processes began towards the end of the last Ice Age as fluvial, glacio-fluvial, glacial and periglacial sediments that were deposited on the continental shelf were reworked and rolled back towards their present positions as sea levels rose. Modern sources of coastal sediments are cliffs, shore platforms, rivers and other sources, such as biogenic material (shells), industrial waste products etc. At any location, sources of sediment may well change with time, such changes influencing planform shoreline position, eg via shifts towards swash alignment when updrift sources are exhausted/cut off by defences. In addition, the historic and ongoing nourishment activities around the coast have added significant amounts of sediment and in many cases, these are not native to the beach location.

The sediment on a beach consists of rock fragments of a wide range of sizes and shapes (see Bluck, 1967) together with some shell fragments and other biogenic material. The most common sediment composition is quartz with some feldspar, but each beach has its own unique sediment, which is a product of its regional and local environment and geological evolution.

Source and mineralogy will influence the particle size distribution of the beach sediment and these will in turn influence: „ the cross-shore distribution of the longshore transport (see Section 3.1) „ the shape and steepness of the beach profile, which impacts on the design of the groyne length, crest profile and the amount of material that it aims to trap „ the rates of structural deterioration of the groynes, whichis affected by abrasiveness of the sediment type.

3.2.3 Beach types Rogers et al (2010) classifies beaches around the UK into four principal types: shingle, shingle upper – sand lower, mixed sand and shingle and sand beaches (Figure 3.17). All natural beaches have a variety of sediment sizes present across their surfaces and through their depths and strata. In general, categorisation of beaches is according to the surface material, although sometimes beaches defined as sand beaches can be characterised by deeper gravel formations or by deeper hard platforms. Various beach compositions can be recognised across a continuum – from beaches that are mainly sand to those that are mainly gravel. Within this spectrum, mixed beaches of different proportions of sand and gravel display a variety of longshore, cross-shore, depth and time variation in the sediment sizes present.

Figure 3.17 gives examples of beach compositions based on grain size distribution and typical beach profile. The sediment characteristics are defined through cumulative sediment grain size distributions, but they may be expressed more simply by d50 (mean grain diameter), d10 and d90, which can also be described as the intercepts for 10, 50 and 90 per cent of the cumulative mass distribution. From this classification it will be clear that the natural cross-shore profile of the beach is affected by the relative proportions of sand and gravel in mixed beaches and their resulting influence on the sediment transport across and along the beach. More information can be found in Chapter 3 of Rogers et al (2010).

When investigating sediment transport on a beach, as described in Chapter 4, it is important to appreciate the characteristics of the entire depth of the mobile beach, including the nature of any underlying layers of different sized materials. Sediment transport formulae generally assume that there is an unlimited supply of material on a beach that can be transported along and/or across the frontage. This may not be the case in many situations. If coarser material is buried underneath a sand layer, rates of transport can change once the sand layer is exhausted and the gravel is exposed. This will change the rates of sediment supplied downdrift, the beach slope and the dynamic of wave/beach interaction.

44 CIRIA, C793 Figure 3.17 Examples of beach types and grain size distributions (after Rogers et al, 2010)

Groynes in coastal engineering 45 3.2.4 Beach substrate Beaches may be formed on top of a shore platform the erodibility of which depends on the nature of the substrate (rock, clay etc). Shore platforms have different rates of erodibility depending on the geological formation, geotechnical properties and chemistry eg mudstone platforms will erode more readily than basalt. The platform will be weakened by biological action, frost and salt weathering. It will then be eroded and lowered by the direct action of the waves when exposed, by shattering due to the dynamic impact of boulders thrown about by the waves and by the abrasive action of thin layers of beach material moved by waves and currents.

Wave action can temporarily or permanently strip off all beach sediment exposing not only coastal defences but also the underlying substrate. Figure 3.18 shows a section of beach at Felixstowe where, at low tide, the London Clay substrate below the mobile sand and shingle beach is exposed over a wide area. The erosion of the shore platform will cause an increase in water depth at the toe of the beach, which allows bigger waves to act on the shore and on the defence of any cliff or upper foreshore platform behind and/or beneath. Without beach sediment to act as a natural buffer and to take energy out of the waves, there is an increased risk of defences being damaged.

The erosion of the beach substrate is an irreversible process that a functioning groyne field retaining a healthy beach could help to avoid. There are many examples of the use of groynes in this respect, including along the Lincolnshire coast, where extensive beach nourishment together with groyne fields have been used to replace the lost sand layer and to limit the platform erosion. Figure 3.19 shows a groyne between West and East Runton on the North Norfolk coast which is protecting a flint and sand beach over a Cretaceous Chalk wave-cut platform.

Notice the flint cobbles and sand sitting on top of the chalk platform Figure 3.18 Exposure of London Clay substrate at Figure 3.19 Groyne protecting a flint and sand beach at West Felixstowe, Suffolk (courtesy HR Wallingford) Runton on the North Norfolk coast (courtesy Peter Lawton)

3.2.5 Implications for engineering design The suitability of the beach and/or substrate for groyne foundations should be assessed at the preliminary stage of a design so that the engineering approach can take account of the findings and an appropriate estimate of costs can be made. Changes in structural design and piling systems at a later stage in the process can result in considerable cost increases. If site investigations are not carried out at enough locations along the frontage and to sufficient depths, cost estimates should reflect the level of uncertainty in the knowledge of the substrate (ie contingencies will need to be included). Key information required from ground investigations include thickness of sediment layers, substrate level and type of substrate.

Section 3.3 discusses the field investigations which should be carried out at the different stages of the design. When determining the type of piling to be used, ground information to a depth greater than the likely depth of piling needs to be acquired so that pile types and piling construction methods can

46 CIRIA, C793 be assessed. Trial trenches cannot alone provide enough information for this assessment, and boreholes should be used. However, trial trenches or pits can provide an initial assessment of potential issues, particularly with regard to the presence of a substrate. More information on piled groynes and the impact of substrate on the construction of groynes can be found in Chapters 7 and 8.

3.3 SITE INVESTIGATION

3.3.1 Introduction An assessment of site morphological, geological and geotechnical conditions is vital for the design of groynes. The establishment of the beach morphological conditions through monitoring is discussed in Section 9.1 and this is relevant for beaches with existing groyne fields and those where groynes are planned. This section focuses on the geological and geotechnical investigation and understanding of site conditions. Sound site investigation carried out in the preliminary stage of the project may even prove sufficient for later detailed design and construction.

The following sections of this chapter briefly identify the principal methods commonly employed in ground investigations and are intended to suggest general approaches and scopes for investigations. For more information on site investigation generally, it is advised to consult sources such as Clayton and Smith (2013) and BS 5930:2015.

If the site investigation is likely to involve tidal or floating working, site investigation contractors with experience and equipment for this type of work should be used to ensure sound risk management and good quality results.

3.3.2 Stages of investigations The extent of required investigations should be dictated by the quantity of data already available, the level of likely hazard at the site, and the nature of the existing or proposed structures. It is important to avoid assuming that at sites where vertical groynes already exist the ground conditions do not need to be properly investigated. Examples where weak ground formations have been found along frontages with existing timber groynes and, as a result, alternative types of groyne structure and/or revised groyne layouts had to be considered.

Geotechnical investigations for proposed sites are often divided into the following three separate phases to minimise costs and to develop the necessary data at each stage of the approval, design, and construction of a project: „ Preliminary investigations. These should provide sufficient information to justify selection of the type of structure, to facilitate preliminary cost estimates and, when necessary, to obtain regulatory approvals and environmental data. These investigations should: provide a first general description of the engineering and geological aspects of the proposed site start with a review of any existing ground investigation, beach monitoring and coastal erosion data and any as-built records for existing groynes/defences carry out necessary field work to include preliminary geologic mapping, sediment analysis and trial pits determine if further study of the site is required, by evaluating the data collected if required, plan the type, location, and amount of explorations and laboratory testing required for future, more detailed investigations and to prepare specifications. „ Initial design investigations. If preliminary investigations have identified the need for more refined investigation before detailed design or tender design, initial design investigations should be carried out, giving enough additional information to improve cost estimates. These investigations would identify the foundation characteristics of a site and to provide data for preliminary considerations of the design requirements and construction methods.

Groynes in coastal engineering 47 „ Detailed design investigations. This final phase of investigations should generate the information necessary for developing detailed plans and specifications, obtaining bids, and constructing the project. Such investigations would be: primarily composed of testing concentrated on specific areas of the selected project site specifically planned to provide the engineer with information that is necessary to design structures, estimate quantities, determine rates of construction progress, develop construction method statements and cost estimates, prepare specifications, and obtain bids.

In some cases, these phases may be combined for convenience of time or because of the prevailing/preferred contracting arrangements. For modifications or renewals of existing groyne fields, the extent of data needed may be relatively limited depending upon the adequacy of existing data and construction documentation.

In planning ground investigations, due attention should be paid to the safety of personnel and the public at all stages of the investigations.

3.3.3 Use of trial pits in beach investigation Shallow trial trenches or trial pits may be used in the coastal environment if the beach material can temporarily stand unsupported. Unsupported shallow pits can be dug using a hydraulic excavator and used for making a rapid check of the ground conditions, working down from the ground surface. The operator can make visual logs of the strata and take disturbed samples using the excavator bucket, or other appropriate methods (BS 5930:2015, BS 5930:1999+A2:2010). Restrictions regarding access in tidal areas should be considered (see also Section 8.1.5).

Hand-dug pits should generally be avoided as they can be expensive, can take time to excavate, are not always conclusive and can pose a significant health and safety risk due to undertaking the work near water and due to the nature of beach material.

3.3.4 Substrate type and profile The substrate type and profile may be determined by trial holes or, if the substrate is deep, by boreholes. Where possible, a grid of trial holes should be excavated to develop a contour plan and to check for any variability in the nature of the material.

In some situations, no distinct substrate layer is found within the practicable depth of the trial hole, so it is necessary to make a judgement on whether there is a critical substrate layer within the anticipated depth of construction. This can be based on: „ as-constructed drawings of any existing groynes (or seawalls) „ available local borehole data „ geological mapping.

Key to this investigation, especially where piling will be involved, is whether the substrate will act as an obstruction to the piling

In other situations, there is no change in the actual type of material (ie no distinct substrate layer) but instead a change in its composition (ie grading and/or compaction). On shingle beaches, for example, relatively ‘loose’ or recently disturbed layers of mobile shingle may be found overlaying well-consolidated gravel. Depending on its nature and level, and any signs of past disturbance, the consolidated material may be treated as a substrate.

The type of investigation boreholes required on a site varies depending on the type of ground to be penetrated and the depth of penetration needed. The holes will need to reach to or beyond the likely final penetration depth of groyne piles (see Chapters 6 and 7). A preliminary assessment of the probable ground conditions should be made before specifying the type of borehole investigation. Cable percussion

48 CIRIA, C793 drilling (shell and auger) methods are frequently used for geotechnical site or ground investigations in the UK and, depending upon access limitations and favourable ground conditions, boreholes of sufficient depth for foreshore/substrate investigations can be created using this method. Access and operating restrictions in tidal areas should be considered.

As discussed in Section 8.1.6, standard penetration tests (SPT) or equivalent, measuring the resistance to penetration offered by the soil at any particular depth, will need to be undertaken. This is to support the contractor in determining the type and size of piling equipment required to install the groyne piles (see BS 5930:2015).

3.3.5 Data logging Data logging of any kind of site investigation is very important – not only for the design and construction or refurbishment of a groyne but also for any future intervention. Data should ideally be logged within the asset database used by the responsible authority and should store (as a minimum) the location of the investigation, the date of the survey and the key conclusions from the investigation. (The process of evaluation of the condition of any existing groynes is discussed separately in Section 10.2.)

Groynes in coastal engineering 49 4 Planning and assessment of groyne field layouts As discussed in Chapter 3, the long-term plan shape evolution of a beach is driven by the rates of longshore transport of beach material along the coast, combined with the effect of any substrate, hard points and any existing coastal defences. This chapter (Figure 4.1) explains how to understand and analyse beaches in order to determine the required plan shape layout of the groyne field. The trapping efficiency of groynes is described together with the likely effects of groynes on beach plan shape. Modelling approaches (empirical, numerical and physical) to assess these effects are discussed, leading into a description of the various ways of assessing and determining groyne length and spacing. This chapter provides guidance:

„ Understanding beach changes (Section 4.1). „ Overall layout considerations (Section 4.2). „ Beach modelling for the planning of groyne fields Section( 4.3).

Coastal management Functional requirements and context indicators (Section 2.1) (Section 2.1)

Physical coastal environment – hydraulic, geotechnical and morphological constraints (Chapter 3)

Start or continue to No Groyne removal use groynes? (Section 9.4)

Yes

Plan shape design of groyne Evaluate and choose groyne field (Chapter 4) type (Section 2.2) and material (Chapter 6)

Design groyne profile (Chapter 5)

Structural design or design review for groynes (outline and detailed) (Chapter 7)

Construction and/or major refurbishment (Chapter 8) Major Groyne repair and Minor maintenance (Section 10.3) Groyne adaptation (Section 9.2) and adjustments (Section 9.3)

Small Beach changes Routine monitoring Large (Section 9.1) (Sections 9.1 and 10.2) Structural deterioration (Section 10.1) Post-incident monitoring (Sections 9.1 and 10.2)

Climate change Storm events (Section 3.1.4)

Figure 4.1 Parts of groyne management process covered in Chapter 4

50 CIRIA, C793 4.1 UNDERSTANDING BEACH CHANGES

The design of an appropriate beach management scheme cannot be achieved without evaluating the historical evolution of a coastline. The analysis of beach changes over time provides valuable input data for the estimation of sediment transport rates. Beach changes can be affected by several factors, information about which should be compiled and evaluated as follows. „ Previous historical sediment nourishments or extractions. Records of nourishment activities, recycling and bypassing including quantities, locations and timings should be reviewed, if available. Unfortunately, this information is not always available, and many assumptions often need to be made when assessing anecdotal evidence or historical records. „ Previous protection of updrift coasts (eg cliffs) leading to a loss of updrift sediment supply. „ Existing groyne structures (or structures functioning as groynes) in the area of interest and in the adjacent areas should also be compiled, including the layout, the time of construction, and data on the structural condition. An analysis of their functional performance will provide valuable information for the design of any future scheme. Also, the performance achieved by nearby existing structures can provide an indication of how any new scheme might perform (see earlier discussion in Section 2.1). As-built drawings, health and safety files, maintenance records and local knowledge are also very useful, particularly when phenomena such as beach lowering, or beach retreat are affecting the area. Groynes in an area may have a very long history which may be evident in old photographs and some coastal landscape paintings. „ Fluctuations in sediment supply. Assessing these fluctuations is important from the perspective of designing a groyne field. Fluctuations arise from factors such as seasonal or inter-annual variations in the direction and magnitude of the incoming waves. „ Changes to the orientation of the nearshore seabed contours that may have altered the direction of the approaching waves. As a result, the orientation of the beach, such as changes in the bathymetry of offshore banks or changes in the nearshore bathymetry, may be caused by the presence of structures.

Taking account of the influence of these factors, topographic and bathymetric monitoring survey data (see Section 9.1) can be used to assess beach changes, using historical maps to support understanding of longer term changes. An appropriate baseline, preferably seaward of the lower beach limit, should be used for the calculation of changes in beach volume. Consistency in the level datum used for surveys is required to avoid errors in estimating sediment volumes.

The analysis of beach changes is often separated into evaluation of plan shape changes (discussed in this chapter) and changes to the cross-section (section perpendicular to the shore), the latter commonly described as changes in beach profile (discussed further in Sections 5.3, 9.1 and 9.3). Beach changes can also be expressed as changes in beach volume. Changes to the beach plan shape can be directly associated with changes in the beach cross-shore profiles. Beach profiles should be analysed to investigate if the erosion of the upper beach has resulted in accretion of the lower beach or if sediment has been moved further seawards or further downdrift. A good reference check is to see whether there have been changes in the horizontal distance between the high and low water lines.

4.1.1 Effects of groynes on beach plan shape On a shingle beach, groynes form a physical barrier to the sediment movement along the beach. Here the shoreline will locally re-orientate parallel to the dominant lines of breaking wave crests, creating the typical saw tooth plan shape (shown in Figure 4.2a). There is often a resulting significant difference in beach elevation between the updrift and downdrift side of each groyne, particularly in the case of narrow footprint vertical reflective structures. The difference in beach level between the updrift face and the downdrift face of a timber groyne should be monitored (see Section 9.1), because over time the overloading of one side can cause the failure of a structure (see Box 7.1).

Groynes in coastal engineering 51 When installed on sandy beaches, the saw tooth shape may still be seen because some material may still be carried as bed load. However, the mechanism to retain much of the sediment relates to the way that the groynes interfere with the longshore currents created by the obliquely a breaking waves which carry sandy sediment as suspended load. Here, the groynes form ‘bays’ within which these currents are forced to move in a circulatory pattern. This reduces the ability of the waves to carry sand as suspended load along the coastline within each groyne bay. In addition, the currents within the groyne bays are usually slower than those just b beyond the groynes. This leads to the Note that in (a) the difference in beach orientation from that of the average shoreline has been exaggerated. deposition of sandy sediment in at Figure 4.2 Example of beach alignment under one dominant direction least some parts of the groyne bays, of longshore transport (a) and under two dominant directions of longshore particularly in one or both of the transport (b) ‘corners’, as illustrated in Figure 4.3.

An important consideration in the design of groynes is to avoid seaward-flowing ‘rip’ currents that can carry sediment offshore and out of the groyne bays. These currents typically occur on the downdrift side of groynes and can lead to scour alongside and near the tips of groynes. Reflective and overly high structures can encourage the formation of such currents. Scour holes can also occur at the head of groynes. For example, on the downdrift side these tend to be caused by diffracted wave energy plus any eddies it produces, while on the updrift side these are caused by wave reflection effects. (Risks of groynes being outflanked landward Figure 4.3 Interaction of groynes with waves and currents (from Trampenaut et al, 2004) of their roots are discussed in Section 9.2.2.)

Groyne systems can only trap a finite quantity of sediment and once the groyne bay is filled, longshore transport is likely to resume around the toe and over the crest of the structure. This dynamically stable situation is the eventual aim for an effectively designed and/or managed groyne system (Fleming, 1990).

As discussed in Section 4.2.5, the trapping of sediment within the groyne compartments may cause erosion downdrift of the groyne field where sediment is not being trapped. The erosion is most noticeable locally (ie immediately down stream of the last groyne – see Figure 4.4) but some erosion may also to take place further along the coast because of the reduced available sediment loads. Groyne planning and design will need to take account of these potential impacts and determine appropriate mitigation measures such as beach nourishments (see Section 4.2.5 for more details).

52 CIRIA, C793 a

b

Figure 4.4 Effect of a groyne field (a) or an individual groyne (b) on a coastline 4.2 OVERALL LAYOUT CONSIDERATIONS

4.2.1 Beach width and orientation When designing groynes, every beach will be different and although lessons learnt from other locations are very important, transferring the designs themselves is not advisable. The main objectives of a groyne field should be established at the start of any project together with clear design criteria.

Along frontages with a groyne system, the beach width may be described in terms of an average beach width. However, it is important to identify and specify a minimum beach width and/or a minimum beach volume to be retained along the frontage early in the design process. The presence of the beach may be a key element for the protection of the backshore (fore dune, cliff, hard defence structures or vegetation) and/or to deliver amenity value. While a minimum dry beach width may not always be possible, it can be defined, for example, as the minimum beach width to be achieved/retained along the frontage or within an individual groyne bay when the water level is at a specific contour. Figures 4.5 and 4.6 show the definition of beach width in relation to a baseline.

The minimum beach width and associated shoreline shape should be clearly linked to the required beach volume and eventually to the desired beach profile (including crest width and seaward slope) discussed later in Section 5.2. If it is desired to use the beach to limit wave overtopping on the frontage, the beach profile including its associated storm response shape (see Box 5.1) combined with the minimum beach width should be set in relation to a criterion on tolerable overtopping discharges. This information should be included in any BMP, which is normally produced at the detailed design stage and then periodically revised. For operational and maintenance purposes, the BMP can define the relevant shoreline trigger positions and, if the shoreline erodes landward of these trigger lines, it can recommend that action is initiated. Further information on monitoring beach position and profile is given in Section 9.1 and for further guidance on developing and using beach trigger lines/levels see Environment Agency (2019a).

Groynes in coastal engineering 53 Figure 4.5 Diagram showing minimum beach width (Ymin)

Figure 4.6 Definition of a minimum beach width in a groyne field min(Y )

Once the design criteria for any minimum beach width at the relevant contours have been set, the design of the groyne field can progress by determining the expected resultant beach alignment. Depending on the stage of the design, the long-term beach orientation can be estimated using different approaches. At feasibility stage the observation of the beach alignment of nearby groyne compartments can provide useful information. In some cases, use of empirical long-term equilibrium plan-shape models (see Section 4.3.2) can be considered. Estimating future shoreline evolution can be based on two complementary approaches: 1 Extrapolation of historic time series data. Where there is an existing groyne field on the beach of interest or a similar nearby beach, a time series set of digital terrain model (DTM) surfaces, ideally using many years’ worth of data, can be created. Additional historic beach contour/ orientation information can be obtained by an analysis of historic maps or from aerial photography if appropriate, including photogrammetric analysis. This can be used to infer likely future beach changes, but the reliability of the resulting estimates can be of variable quality. The approach requires a degree of interpretation and may need to be supported by modelling if the groyne field configuration is being changed. 2 Plan-shape modelling. During preliminary and detailed design stages plan shape modelling is routinely used to support the design process by providing a more accurate estimate of the shoreline evolution. The modelling involves initial validation against existing beach change data, followed by modelling with the new or modified groyne field introduced into the model (see Section 4.3). However, the accuracy can be more limited when there is a broad range of sediment sizes, a limited supply of sediment, and/or geological forms (eg shallow platform) which limit the volume of the beach. Plan-shape modelling can provide a valuable assessment of the long-term shoreline evolution and variability of beach alignment and beach width, particularly if it includes the evaluation of the effects of ‘wave chronology’ (the sequencing of waves, see Section 4.3.3). Combined with the results from short-term cross-shore modelling in response to design storms or to sequences of storms (see Chapter 5), the results from the plan-shape modelling provide the required information to assess the variability of the beach width during the life of the scheme. From this variability, the estimated/required beach width can be assessed and related to any design criterion for the lifetime minimum beach width.

If the longshore transport has significant periods of drift reversals (see Section 4.2) the likely symptoms of this will either be an area of accretion being seen to form on either side of the groyne or complete realignment of the beach within the groyne compartment (see lower image in Figure 4.2). Which of these responses occurs, and the extent to which it occurs, will depend on the following: „ width of the groyne bays

54 CIRIA, C793 „ persistence of the various wave conditions „ nature of the groynes – the changes are more pronounced with smooth narrow footprint groynes than with rough porous groynes such as those created with armourstone.

Wiggins et al (2019) report significant storm-related beach plan-shape realignments or rotations within a coastal embayment. However, as discussed by Dornbusch et al (2008), it is possible to over-predict the likely degree of beach rotation within groyne bays. Figure 4.7 shows an example of beach realignment or rotation at Eastbourne.

a b

Figure 4.7 Example of beach realignment following northerly drift (a) vs. southerly drift (b) at Eastbourne, UK. Note the realignment is particularly evident in the near-shoreline shingle beach (courtesy CCO)

Capturing the range of potential drift reversals is important when analysing the behaviour of the frontage, as these may dramatically affect the frontage and its potential for flood protection. To achieve this, an hourly time series of waves (heights, periods and directions) of duration of the order of 10 years should be used. In this way, seasonal and longer-term drift reversal can be captured by the modelling. The process of simulating the beach plan shape evolution may be simplified for initial assessments (as opposed to detailed design) by selecting a year of wave data, which is more representative of the long- term wave climate, and repeating the same annual wave series for the entire period of the wave climate time series. However, this approach will not expose the shorter-term episodic drift reversals or long-term gradual changes in forcing conditions. This may be important as sometimes sustained stormy weather from a particular direction can cause critical loss of beach material. Additional useful information on this topic is available in Burgess et al (2014), with one example at Littlestone (in Section 3.2.5) exposing the dangers of ignoring local knowledge and historical mapping evidence. Ideally any analysis should be informed by local knowledge of any particular wave direction that cause problems or, where this does not exist, data records should be examined carefully to determine likely critical directions of wave attack.

4.2.2 Groyne length The length of the groyne should be selected based on the desired extent of the beach as controlled by factors, such as those discussed as follows, including the expected drift rates, wave angles, tide ranges and tidal currents. Other factors include existing beach profile envelope (see Section 5.2) and site geometry. 1 Expected longshore transport rates and their control. Groynes function by intercepting the longshore transport, so the length of a groyne (see definition in Figure 1.2) affects the capacity of a groyne field to retain or intercept sediment moving along the coast. The cross-shore distribution of the longshore drift will need to be analysed to establish the width over which the majority of sediment travels (see Section 3.1.2). Figure 4.8 gives an example of how an assessment of the groyne efficiency as a function of its length can be made by using the beach profile and the

Groynes in coastal engineering 55 cumulative cross-shore distribution of the longshore transport. The figure shows that by increasing the length of the groyne by 40 per cent, it can potentially trap 35 per cent more of the sediment being transported alongshore. In initial assessments of the required groyne length, it can be assumed that most of the longshore drift occurs on the upper part of the beach profile although sand is still mobile under both wave and tidal currents at relatively large water depths.

Figure 4.8 Determination of the length of the groyne with the aid of the cross-shore distribution of the longshore drift

To control sediment transport, groynes on sand beaches need to be longer (for the same tidal range) than those constructed on shingle beaches. On gravel and shingle beaches the transport occurs by rolling in the swash, so groynes can be shorter and still effective, the adopted length being a function of tidal range and beach slope. On sand beaches groynes need to be much longer as sediment will move anywhere within the breaker zone. Van Rijn (2004) states that in the UK, groynes on shingle beaches do not generally exceed 60 m whereas on sand beaches the length may be in excess of 100 m. In the Netherlands (where typically the aim is to trap a greater proportion of the sediment transport), groynes on sand beaches can be up to 200 m long. 2 Dominant angle of breaking wave approach. Breaking wave crests arriving at between 40o and 50o to the shoreline are most effective in driving longshore transport. In these situations, groynes will need to be longer than in locations where the angle of approach is smaller. Groyne spacing can also be adjusted to improve the efficiency of a groyne bay in retaining material (see Section 4.2.3). 3 Tidal range. As explained in Section 3.1.2, in macro-tidal environments taking account of the effect of the varying tidal water levels is important when assessing the cross-shore distribution of the longshore transport, and important when making decisions about the resulting required length of the groynes. 4 Tidal currents. At some locations around Britain (see Section 3.1.4), tidal currents may also be an important factor in determining the inshore flow field. In these cases, groyne length may need to be extended compared to a situation where transport is dominated by wave-induced currents only.

Other practical considerations for groyne length include the following: „ To avoid sharp changes in the amount of sediment being trapped, the length of new groynes may need to be modified to reflect the lengths and performance of any existing or nearby groynes. „ On sandy beaches, the maximum groyne length is determined by the locations of the inner backstop (eg seawall) and by the MLW line in tidal environments. However, the seaward end should always be within the surf zone to allow some sand to pass around it (Van Rijn, 2004) to reduce downdrift impacts. „ The need to accommodate irregularities in the coastline and potentially smooth out the alignment of the front of the beach crest across the frontage (see also discussion on beach crest width in Section 5.2.3).

56 CIRIA, C793 4.2.3 Groyne spacing The optimum spacing of the groynes is affected by the following: „ grading of the beach material (Bird, 1996) „ structure length „ dominant direction of wave attack to which the beach plan shape tends to align „ trapping efficiency of the groyne.

Setting the groyne spacing is driven by delivering the required minimum beach width on the downdrift side of the groyne as shown in Figure 4.9. In this regard, rock groynes may be more efficient than timber groynes as the wave diffraction that they generate tends to reduce the amount of sediment erosion on their downdrift side. As a result, the spacing of rock groynes may be larger than timber groynes for the same prevailing conditions.

The groyne spacing should decrease with increasing wave angle to ensure a uniform distribution of sediment within the compartment (Bird, 1996).

If the groyne spacing is set too small, Figure 4.9 Acceptable and unacceptable groyne spacing the groyne bays fill very rapidly, whereas if the spacing is too large, erosion may occur at the root of the groyne on the downdrift side and beach width may be lost.

In relation to Figure 4.9, there are important differences in the design of groyne spacings (and lengths) between shingle and sand beaches. The ratio spacing/length (S/L) is influenced by previous experience and engineering practice, the approach to which varies from one country to another. In the Netherlands, S/L is typically between 2 and 4 (Van Rijn, 2004), more characteristic of sandy beaches, while in the UK, the spacing/length (S/L) ratio is between 0.8 and 3, given the presence of more shingle and mixed beaches along the coast. (See Rogers et al, 2010.) „ Sand beaches do not re-orientate themselves as quickly as shingle beaches during storm conditions. The central part of a groyne bay on a sand beach is less likely to realign so there is reduced advance/retreat at either end of a groyne bay compared to a shingle beach. A rule of thumb is that the spacing should be about two to four times the groyne length in order to prevent the generation of rip currents and excessive erosion between the groynes (Van Rijn, 2004). „ Shingle beaches stand at much steeper gradients than sand beaches, both perpendicular and parallel to the shoreline. Shingle beaches also realign to the wave crests to a greater degree than sand beaches, forming straight parallel contours between groynes. The steep seaward gradient means that significant level differences can build up across a groyne, so to avoid the resulting overturning forces becoming too great the spacing of groynes on shingle beaches needs to be closer. The rule of thumb for the spacing to length ratio for shingle beaches is generally 1:1, the validity of which was confirmed some years ago in a physical modelling research programme (Coates and Dodd, 1994).

Closely-spaced timber groynes tend to generate many small rip currents, which makes the groyned beach safer for recreational use. In contrast, a beach with widely spaced rock groynes will tend to generate fewer but larger rip currents. However, if groynes are further from each other, more regular beach re- nourishment may be needed, as smaller groyne bays should retain the sediment more efficiently.

Groynes in coastal engineering 57 Figure 4.10 shows the relationships of actual groyne length to spacing adopted across the SCOPAC region (south of England) for a range of sediment sizes and types. The figure, extracted from the work of the late Prof. Andy Bradbury, shows considerable scatter in the calculated groyne length to spacing ratio. The graph shown in Figure 4.10 largely confirms the data on spacing for different sediment characteristics shown in previous guidance (Fleming, 1990) and summarised in Table 4.1.

Note that the lines are of equal length to spacing ratio (as shown in the ratio label on each line)

Figure 4.10 Distribution of groyne lengths and spacings for the SCOPAC region of the English south coast, including the influence of sediment type

Table 4.1 Average groyne space and ratio in England* (from CIRIA, 1990)

Beach type Average groyne length L (m) Average groyne spacing S (m) Range of S/L

Shingle 60 60 0.5 to 1.7

Shingle upper/sand lower 50 50 0.5 to 1.5

Shingle/sand mixed 70 85 0.6 to 2.4

Sand 95 130 0.8 to 2.7

Note * Extracted from the assessment of a large number of sites in England undertaken in 1990 and reported in CIRIA (1990).

4.2.4 Groyne alignment While most existing narrow footprint groynes have been designed and constructed straight and perpendicular to the shoreline, alternative alignments can be considered. As shown in Figure 1.2, groynes may be inclined/angled, curved, hooked or T-shaped with a shore-parallel structure at their seaward end.

With small footprint groynes the most common modification is to have them angled to the perpendicular as shown in the example at Whitstable in Figure 4.11. Adopting angled groynes on beaches may be useful (by analogy with a riverine situation) where the drift is strongly unidirectional, such as near the mouth of estuaries and tidal inlets. However, despite the greater use of angled groynes in the late 19th and early 20th centuries, there is no real evidence of any strong performance advantages of angled groynes over perpendicular ones. For angled groynes, the disadvantages are that they: „ need to be longer to reach the same depth of water (which affect the longshore currents and sediment transport to the same extent) „ may be more vulnerable to wave impact pressures.

Zig-zag timber groynes have been constructed in the past (Figure 4.12 and Box 4.1). The historic reasons for adopting a zig-zag alignment is not entirely clear, but it was believed to provide inherent stability and to increase the effective width of the structure to resist lateral ‘earth pressure’ forces. The additional bracing against lateral instability provided by the zig-zag configuration often avoids the need for ties or

58 CIRIA, C793 struts (see Section 2.2.1). The zig-zag design creates ‘compartments’ to trap sediment, which is helpful at the top of shingle beaches. The zig-zag design also aims to reduce wave and/or current action running seaward along the downdrift side of the structure and causing scour (Bradbury, 2010). As with angled groynes, however, such zig-zag groynes may well experience greater wave impacts.

Figure 4.11 Example of angled groynes at Whitstable, Kent Figure 4.12 Zig-zag groyne at Hunstanton, Norfolk (courtesy GooglePro) (courtesy Uwe Dornbusch)

Box 4.1 Examples of past use of zig-zag groynes

Figure 4 13 Examples of zig-zag groynes at Calshot, Figure 4 14 Close up view of zig-zag groyne at Calshot, Hampshire (courtesy GooglePro) Hampshire (courtesy New Forest District Council Before about 1960, some groynes were built with a zig-zag footprint. Earlier groynes with this footprint type were simply built using tree-trunks as ‘piles’ butted together. Examples of both could have been found at Dawlish Warren (a sand beach) and Minehead (a shingle beach). One remaining example of zig-zag groynes is at Calshot (Figures 4 13 and 4 14), where 55 timber groynes are in place along a 0.9 km frontage to maintain the beach. The beach material is mixed, with an abrasive potential, although the area is sheltered from south-westerlies and has maximum significant wave height values of about one metre. A low-cost capital defence scheme was adopted, with emphasis on low maintenance costs, so the groynes were designed to be short but effective. The short length allowed for minimum impact on water-sports in the area.

4.2.5 The downdrift end of a groyne field A groyne system has to end somewhere, and coastal erosion is highly likely downdrift of the system (see Section 4.1.1). Different mitigation measures can be used to address this. 1 Artificial nourishment of the groyne bays. This may be carried out so that the overall littoral sediment budget is not adversely affected (Komar, 1998). It is considered by scientist and engineers to be the best solution to the downdrift effect, but it implies regular expensive maintenance. 2 Artificial nourishment of the downdrift coast. This is an alternative to nourishing the groyne bays themselves, with a similar long-term effect, but again requires a commitment to continue to nourish into the future. 3 Progressive use of permeable/semi-permeable structures at the downdrift end of the groyne field to slowly increase the rate at which beach sediment passes the groynes, avoiding a sharp change in transport rates. 4 Progressive reduction, or ’tapering’ of groyne lengths. Observations show that sediment which bypasses the end of a groyne does not immediately come ashore but carries on travelling parallel

Groynes in coastal engineering 59 to the shoreline for some distance. These observations suggest that the downdrift ends of groyne systems may be designed by tapering off their seaward ends to ensure a smooth transition from the groyne to an ‘un-groyned’ portion of a beach. The progressive reduction of the groyne length, at both the updrift and downdrift end of a groyne field, allows longshore currents to change smoothly in relation to the shoreline, avoiding the danger of diverting a stream of sediment away from a beach and offshore. The advice in the USA (Bruun, 1952) in predominantly micro-tidal situations is that the groyne shortening should make an angle of about 6o with the natural shore alignment (Figure 4.15). Experience in East Riding, Yorkshire suggests that flatter angles, perhaps nearer to 3o, may be necessary in some places. However, this may lead to a need for an excessively long length of coast downdrift of the main project being protected with the progressively smaller groynes and may be economically unrealistic. For example, aerial photographs show that ,before 2003 in Hornsea, the shortening of the groynes was about 3o. New groynes were then built and in 2017 the shortening was estimated from aerial photographs as 6o.

Figure 4.15 Transition from groyne field to natural beach as recommended by the USACE (from USACE, 2002)

5 Provision of hard protection downdrift of the groyne field. A stretch of coast along the downdrift area of a groyne field can be protected with some form of linear protection. Any hard measure adopted to reduce the downdrift erosion will reduce local sediment supply and should be carefully assessed because it may transfer the erosion pressure even further downdrift. The extent of coast adversely affected by the groynes downdrift of the last structure and the potential for landwards retreat can be assessed using plan shape modelling or through review of relevant coastal monitoring data. A timeline can also be associated with the retreat, which helps to time maintenance and to simulate the mitigating effect of periodical beach nourishment or other measures. 6 Acceptance of downdrift erosion. If there is a length of coast downdrift of a groyne field where a no active intervention or managed realignment policy can or could be adopted, then downdrift erosion may be accepted and limited hard protection only provided to mitigate the risk of outflanking of the defended frontage. In this case, the most downdrift groyne should have its root set well back into the land to mitigate for risk of landward outflanking when erosion begins (see Section 9.2.2).

4.3 BEACH MODELLING FOR THE PLANNING OF GROYNE FIELDS

Beach modelling offers value over and above data collection and analysis as it allows prediction of the effects of introducing a groyne field into an existing situation or modifying an existing groyne field. Building on the discussion of the assessment of longshore transport rates and distributions in Section 3.1.3, this section sets out the overall approach to modelling the plan shape of beaches with groyne fields, discussing the use of empirical, numerical and physical models. The Environment Agency has recently published a guide on morphological modelling (Blanco et al, 2019) where more information is provided on model types, assumptions, outputs and uses.

60 CIRIA, C793 4.3.1 Modelling approach The assessment of the plan shape of a groyne field through modelling should be performed following these steps: 1 Set out the objectives for the modelling and the modelling approach based on overall project requirements. 2 Review both the local and wider geomorphology and coastal processes influencing the site (see Chapter 3). 3 Review the availability of model input datasets (including validation data and other supporting observations). 4 Review information on historic impacts on and changes to the beach that might be relevant for the modelling, eg any historic inputs or extractions of material, or changes over time to the defence structures. 5 Define the modelling area. 6 Select the appropriate model. 7 Build and validate the model against available data. 8 Undertake modelling of scheme options and optimise the preferred option(s) using sensitivity testing. 9 Use modelling to assess requirements for supporting beach management.

Post-project evaluation of the accuracy of model predictions is also advised.

4.3.2 Empirical approach to plan shape modelling Empirical beach models are parametric models of beach plan shape or profile derived by comparison with observed data. As described here, empirical models can be used as a first estimate in a beach assessment study. Even though these models do not require validation, it is still necessary to follow most of the steps listed in Section 4.3.1. When designing a new beach, it is desirable to avoid or minimise future maintenance commitments. For this reason, during the preliminary stages of design, empirical methods may be applied to specific situations in which it is possible to estimate a theoretical stable equilibrium beach plan shape.

One such situation of a stable beach plan shape with little movement of the beach contours over time is where a beach is held between two large erosion resistant headlands. This approach has been mainly used around the world (eg Mediterranean coasts) and in the UK to deliver stable sand beaches. However, it has also been used at several shingle beach locations (eg Medmerry, Elmer and Folkestone). One commonly-used approach is that described by Evans and Hsu (1989) (see Box 4.2), to assess beach plan shape between large rock groynes or fishtailed groynes. Other approaches and applications include those by Raabe et al (2010) and Elshinnawy (2018a and b). More details are given in Rogers et al (2010).

Groynes in coastal engineering 61 Box 4.2 Determination of beach plan shape using the parabolic bay shape equation (PBSE) of Evans and Hsu (1989)

Figure 4.17 Application of Evans and Hsu PBSE Figure 4.16 Definition sketch for PBSE (from Hsu et al, 2004) to a UAE beach An application of the most widely-used formulation for representing long-term plan shape evolution using empirical methods is given here. The PBSE proposed by Evans and Hsu (1989), was applied to a waterfront in the United Arab Emirates (UAE), where a series of groynes and nourished amenity beaches were planned as part of a masterplan development. The method is appropriate at conceptual stage design. At later stages, full process modelling is likely to be required to test the seasonal variations and the long-term requirement for any beach nourishment maintenance. The PBSE is a second order polynomial equation derived by observations of 27 mixed cases of prototype and model bays believed to be in static equilibrium (with no littoral drift). Wave heights and periods are not included in these expressions, but the direction of the main wave sectors is included:

where:

Rβ = Control line length R(θ) = Radius to a point along the curve at an angle θ β = Wave obliquity C = Constants generated by regression analysis to fit the peripheries of the 27 prototype and model bays θ = Angle between wave crest and radius to any point on the bay periphery in static equilibrium

The two basic parameters are the reference wave obliquity β and control line length Rβ (Figure 4.16). The variable β is a reference angle of wave obliquity or the angle between the incident wave crest (assumed linear) and the control line, joining the up-coast diffraction to a point on the near straight beach downcoast point. The wave crest direction used for estimating the beach plan shape should be representative of the direction of the persistent swell. The radius R to any point on the bay periphery in static equilibrium is angled θ from the same wave crest line radiating from the wave diffraction point up coast. The three C constants, generated by regression analysis to fit the peripheries of the 27 prototype and model bays, differ with reference angle β. These C values are bounded within 2.5 and -1.0 for the usual range of angle β from 10° to 80° applicable in most field

conditions. For θ = β, the condition R = Rβ has to be met which forces C0 +C1 +C2 = 1. Values of non-dimensional ratios

R/Rβ versus increments of 2° of β from 20° to 80° have also been tabulated for manual application (Hsu and Evans, 1989).

For a bay beach with a given set of β and Rβ, locations for pairs of R and θ can be marked on the existing shoreline, and a curve can be drawn for the static equilibrium bay shape prediction. The tidal shoreline which the PBSE represents is not clear but can be interpreted to represent the mean water shoreline or MHW line. The main limitations of the PBSE are:

„ The fitting of the parabolic shape was found by Martinoet al (2003) to be sensitive to the initial estimate of the focus position, and so a trial and error procedure is needed to be followed until a good adjustment is obtained. „ Placing of the downcoast control point and the tangent is also a matter of great subjectivity, which depends on visual interpretation. „ The data upon which the equation is based are principally limited to β >22°. Also, the PBSE is intended for application for β ≤ θ ≤ 180°. The physical location of the wave diffraction point (ie the tip of the headland or the tip of the breakwater or groyne) is used as the centre of the co-ordinate system for the parabolic equation. Figure 4.17 shows the application of the approach for a frontage in the UAE using the dominant wave direction. The method was applied to each groyne bay to develop the artificial stable beach plan shape.

62 CIRIA, C793 4.3.3 Numerical plan shape modelling Numerical plan shape models can be used to evaluate the long-term evolution of the beach with and without a proposed scheme, and the efficacy of the proposed solution. The outputs of one-line models are generally provided as shoreline positions at appropriate intervals of time and as rates of longshore transport along the frontage. The shoreline position can be expressed as the mean position but also as maximum and minimum deviations from the mean position. In this way, alternative design options can be tested, and their outcomes compared in a relatively limited time. Such models may be less reliable where there is a very limited supply of mixed sediment on an eroding platform and, in such cases, if modelling is essential, it may be necessary to consider the use of more detailed two or three-dimension (2D, 3D) computational modelling (see Blanco et al, 2019) or physical modelling (see Section 4.3.4). The latter type of modelling can be used to further refine a groyne field, in terms of groyne lengths and spacings, and to optimise the design of the structures and of any beach nourishment required.

While some multi-line (or multiple contour) plan-shape models are available, the one-dimensional (or one-line) coastal morphology model is simplest and relatively straightforward to set up and run. This uses a mass continuity approach to link the changes in calculated longshore sediment transport (see Section 3.1.2) to the changes in a specified shoreline contour (eg MHWS) and for beach plan shape. All one-line models are based on the simultaneous solution of two simple one-dimensional equations: „ One-dimensional morphology equation, which expresses the conservation of mass and calculating the shoreline changes as a function of the distance longshore. „ Equation of sediment motion, given by a bulk sediment transport rate formula, which expresses the longshore sediment transport rate as a function of the relevant wave climate and beach parameters (eg USACE, 1984a, and Ozasa and Brampton, 1980, already discussed in Section 3.1.2).

More information on one-line models can be found in the Chapter 3 of Rogers et al (2010) and in Blanco et al (2019).

The main differentiators between the various models available (eg ARIES,BEACHPLAN, COVE, GENESIS, UNIBEST, LITPACK) are the longshore transport formulations, the assumptions on the shape of the cross-shore distribution of the longshore transport, the co-ordinate system and the application of the different structures and their interaction. All the models require validation in order to be applicable to the area of interest. Sensitivity analysis and probabilistic modelling should also be considered when applying the models. Once validated, the models are best at providing a relative comparison of the effects of different management methods at a specific location rather than giving absolute values.

Input data and assumptions for one-line models The most important assumption of this type of model is that the beach profile is of a constant shape and that it slides along a horizontal base, located at the closure depth dc. This is a depth at which the longshore sediment transport and beach profiles are assumed to be no longer affected by prevailing wave conditions.

One-line models require as inputs, parameters such as slope, beach crest and closure depth and the sediment transport parameters. The models also require information on the geometry and efficiency of the coastal structures present along the frontage, such as groynes, seawalls and breakwaters. Any information on sources (eg replenishments) and sinks (eg mining) of beach material are also required to be included in the input.

A specific challenge for one-line models is to address the issue of wave chronology or sequencing of wave conditions. The issue is that future sequences of wave conditions are unknown even though the overall future wave climate in the short term (eg next decade or so) can be predicted with reasonable accuracy (by adopting the principle of stationarity).

Groynes in coastal engineering 63 The approach to this challenge is sensitivity testing by running the one-line model typically about 30 times with each run using the same starting conditions, but with a re-ordered sequence of input wave data. The results (for example, in terms of beach levels or coastline positions) are processed to give the statistics of the shoreline position (mean, maximum, minimum etc) across the set of model runs. The methodology has been used with plan shape models for periods of several years. While the approach ignores chaotic mechanisms, it remains the best method available at present for addressing the issue of predicting medium-term morphodynamic variability on long-term trends. An example application is shown in Box 4.3. Any results from applying one-line model outputs should be subject to specialist interpretation and comparison with expert geomorphological judgement.

Box 4.3 Effect of wave chronology on estimated long-term trends in beach morphology

Wave chronology can have a significant effect on long-term model predictions. Figure 4.18 shows the results of a sensitivity testing exercise using a plan shape model to establish the effects of wave chronology following construction of a groyne. The resulting plot shows the envelope of changes for a straight beach. In this example the same beach was forced with the same five year wave sequence, but the sequence of events was re-ordered for each of the 40 model runs. For each wave sequence, the maximum (most seaward) position of the beach during each of the 40 five-year sequences was plotted and 40 separate envelopes of maximum beach position were drawn. The shaded area shows the region covered by these envelopes. The continuous line shows the envelope of maximum beach positions from one of the wave sequences. Note that the wave conditions were distributed approximately evenly about the groyne direction, accounting for similar accretion patterns on both sides of the groyne. Figure 4 18 Example showing effects of wave chronology

Representation of groynes within one-line models The way that structures are represented within one-line models (see Box 4.4) is one of the main differences between various models available. When representing groynes, the following factors are key to the correct simulation of their performance: „ full description of groyne dimensions „ groyne efficiency, in relation to the different sediment pathways past a groyne (see Section 2.1.3) „ diffraction and reflection of the waves at the structure „ cross-shore distribution of the longshore transport „ definition of types of groynes, including permeability.

A fully-efficient groyne in a plan shape model will have the longshore transport (Q) from left to right past the groyne set to be zero, making it an impermeable barrier that will stop all sediment transport through or around it. This is an extremely simplistic approach and it will overestimate the effect of the groynes on the transport, unless the structure is a terminal groyne or a port breakwater, which extends far enough offshore to be considered as an infinite barrier. A modeller needs to allow for the degree of sediment bypassing of groynes, once the updrift beach position reaches the end of the groyne (Figure 4.3). Box 4.4 describes an application of a one-line model to a frontage where different types of groynes were present.

To ensure stable simulation of shoreline evolution, plan shape models should not deposit the entire volume bypassing or passing through the groyne into the beach section immediately downdrift of the structure (as there is very little drift on the downdrift side of a groyne capable of being removed). Instead it should distribute the bypassed sediment between several beach cross-sections.

64 CIRIA, C793 Box 4.4 Application of a one-line model to beach management

Poole Bay frontage, Shore Road to Branksome Dene Chine (from HR Wallingford, 2015) This case study describes how the performance of a groyne field was assessed. The groynes were located between the entrance to Poole Harbour at Sandbanks and the boundary with Bournemouth Borough Council near Branksome Dene Chine. This frontage has a high amenity value which is vital for the tourist industry in Poole. The beaches are backed by a seawall dating from the 1930s with a promenade running the length of the frontage. At the rear of the promenade are soft cliffs about 25 m to 30 m high with high-value residential properties and a highways network on top. During the winter of 2005 to 2006, an opportunity arose to nourish the beaches in Poole Bay using sand excavated during a deepening of the entrance channel to Poole Harbour. The nourishment buried the previously existing short groynes. Given the outcome of an earlier study (HR Wallingford, 2003), which established a comprehensive assessment of the sediment regime in the area, a decision was made to follow this nourishment by building six new rock groynes at Branksome near the eastern end of the frontage, and not to replace the groynes lost as a result of the nourishment operations. The new rock groynes were completed in early 2009. Their location is shown in Figure 4.19. Six years after the implementation of the scheme, a post-project evaluation and review of the performance of the groynes was undertaken (HR Wallingford, 2015), by comparing survey data from the regular beach monitoring and the modelled predictions. The post-project evaluation also had the aim of assessing future management requirements. Figure 4.19 Location and extent of model set-up Previous beach plan shape modelling had been carried out in 2007 to 2008 (HR Wallingford, 2008). As part of the post- project evaluation this previous beach plan shape model was validated using shoreline surveys taken in May 2009 and May 2011 together with a time series of hindcast waves covering the same time period. The survey data were collected by the CCO and Poole Borough Council. The model used the 2009 shoreline as a starting position, subjecting it to the time series of waves and running the model up until 2011. The results are shown in Figure 4.20.

a b

Figure 4.20 Model validation results shown relative to the measured shoreline from 2009 (model start position) and 2011 (model target shoreline). Groyne field west of Shore Road towards Sandbanks (existing rock groynes) (a), and at Branksome Dene Chine (new rock groynes, numbered 1 to 6, and timber groynes) (b)

The validation considered the presence of the newly-built rock groynes and differences between their performance and the performance of the existing narrow footprint timber groynes. The new rock groynes at Branksome were modelled as impermeable, horizontal-crested diffracting structures extending seawards at 90 degrees from the shoreline. The Bournemouth narrow footprint timber groynes that were also included in the model were represented as impermeable shore-normal, non-diffracting structures with sloping crest levels. Although the model could differentiate between types of structures, by reproducing the effect of diffraction, the simulation of the stretch of frontage with the existing rock groynes at Sandbanks, at the south west boundary of the model, required some judgment and experience. The modelling needed to schematically incorporate the complexity of the onshore feed from Hook Sands and the tidal influence affecting the drift in and around the entrance of Poole Harbour. Previous hydrodynamic and sediment transport studies (HR Wallingford, 2003) provided valuable information to the modeller to support key decisions on how to include these effects into the model.

Groynes in coastal engineering 65 Figure 4.21 shows typical new groynes at Branksome and typical timber groynes at Bournemouth included in the modelling.

a b

Figure 4.21 Typical new groynes at Branksome, Poole Bay (a) and typical timber groynes at Bournemouth, Dorset (b)

Although the main emphasis of the re-modelling was on the performance of the six new rock groynes at the eastern end of Poole Borough Council’s frontage, the existing rock groynes at Sandbanks were also included in the model domain. The modelled shoreline is accurate to within 10 m of the 2011 MHW line around the groynes at sandbank, and within 5 m of the 2011 MHW line around the new rock groynes at Branksome. As seen in Figure 4.20, after running the model for two years, the shoreline had been predicted to retain its gentle curvature between the rock groynes at Branksome Chine and those at Sandbanks. It also replicated the shoreline changes near the sensitive point around Shore Road where the beach width is much narrower. The transition between the timber groynes on Bournemouth’s side of the boundary and the rock groynes on the council’s side of the boundary had been replicated. The differences in the performance of the two types of groyne is noticeable both in the model predictions of the shoreline, as well as in the MHW shoreline extracted from the topographic survey data. The five bays between the six new groynes near Branksome Dene had all showed signs of accretion between 2009 and 2011, with seaward advances of the shoreline of up to 18 m. The MHW shoreline had also advanced past the tips of the most western of these rock groynes suggesting that the groyne bays were filled to their capacity and longshore drift bypassed to the next groyne compartment at the same natural rate as would have occurred without these groynes. Given the successful validation the model could then be used with confidence to provide advice on requirements for future beach management options such as recycling and nourishment.

4.3.4 Physical modelling Physical modelling is an important tool for the assessment of the effect of engineering works in the coastal environment and involves the reproduction of a prototype (ie real case) situation through a scaled-down version. Scaling of sediment transport can be challenging. If the material is scaled down geometrically the experiments would have to be carried with such fine material (cohesive) that their sediment transport mechanisms would be completely different. So, to select a practical sediment size a distortion of timescale is sometimes necessary. It is also necessary to ensure that the beach profile response is realistic and to run the model with so-called ‘morphologically averaged’ wave conditions, which represent the net effect of all the wave conditions (height, period and direction) likely to be experienced. For groynes, physical modelling is generally undertaken to refine the spacing and length of the groynes and the plan shape of complex groyne geometries.

Physical modelling is often used in association with numerical modelling to exploit the strength of both types of models. Numerical and physical models can be nested as described here. „ A physical model of a whole coastal cell, at a suitable scale, is impractical. „ A numerical model can be used to model the full length of the cell at a coarse resolution. The gross variation in transport rate along the cell may be calculated and major shoreline changes predicted. „ The large-scale numerical model may be used to identify major problem areas for more detailed studies using a physical model. „ The physical model only represents a small section of the sediment cell and so the updrift longshore transport rate, and also the volume of material entering the physical model, needs to be calculated. This input transport rate can be provided by the numerical model. „ The physical and numerical models can then be run together to ensure that the net transport rate of the material through the section represented by the physical model is modelled correctly. This is particularly important for physical models that use lightweight material where the cross-shore transport timescales will differ from the longshore transport timescales.

66 CIRIA, C793 5 Design groyne profile

Given decisions about the layout of the groyne field (length and spacing) using material in the Chapter 4, this chapter explains how to assess beach profile response to wave action (Figure 5.1). It also discusses how to determine the groyne crest profile taking account of the range of likely beach levels that may be experienced and the selection of a suitable freeboard. The main sections of this chapter are:

„ Beach profile response to wave action Section( 5.1) „ Design beach profile Section( 5.2) „ Lowest and highest beach profiles Section( 5.3) „ Groyne top profile and freeboard Section( 5.4)

Coastal management Functional requirements context and indicators (Section 2.1) (Section 2.1)

Physical coastal environment – hydraulic, geotechnical and morphological constraints (Chapter 3)

Start or continue to No Groyne removal use groynes? (Section 9.4)

Yes

Plan shape design of groyne Evaluate and choose groyne field (Chapter 4) type (Section 2.2) and material (Chapter 6)

Design groyne profile (Chapter 5)

Structural design or design review for groynes (outline and detailed) (Chapter 7)

Construction and/or major refurbishment (Chapter 8) Major Groyne repair and Minor maintenance (Section 10.3) Groyne adaptation (Section 9.2) and adjustments (Section 9.3)

Small

Beach changes Routine monitoring Large (Section 9.1) (Sections 9.1 and 10.2) Structural deterioration (Section 10.1) Post-incident monitoring (Sections 9.1 and 10.2)

Climate change Storm events (Section 3.1.4)

Figure 5.1 Parts of groyne management process covered in Chapter 5

Groynes in coastal engineering 67 5.1 BEACH PROFILE RESPONSE TO WAVE ACTION

The mechanism of cross-shore transport is completely different to the longshore sediment transport and is dominated by storm wave action. Longshore transport tends to build up over a period of time (although can rapidly be reversed especially on gravel and shingle beach through temporary drift reversals – see Section 3.1.3) whereas cross-shore movement can be rapid and extreme. In some cases, it can strip the upper beach of significant amounts of sediment during a single tide. Groynes have very little effect on cross-shore transport, but it remains important as it can move sediment into or out of the immediate zone in which groynes influence longshore transport. For a comprehensive assessment of groyne performance, it is necessary to consider the envelope of possible beach profile variation as well as the average profile.

The response of the beach profile to incoming waves varies significantly depending on the sediment size and composition: „ Sand beaches have low permeability. Milder wave conditions tend to move sand onto the beach whereas storm waves (larger and more energetic than summer ones) tend to move sand seawards. The resulting differing profiles (Figure 5.2) are expressions of the often seasonal cycle of wave energy. Where there is an offshore steepening of of the underlying bathymetry (eg off the North Norfolk coast), sand may be permanently lost to deep water during large storm events.

Figure 5.2 Sandy beach response to stormy waves

„ Shingle beaches have high permeability and respond quickly to changes in waves and water levels, making them one of the most effective natural coastal defences, capable of dissipating in excess of 90 per cent of all incident wave energy (Powell, 1990). Shingle beaches will respond to a storm by accumulating a crest or ridge of material on the subaerial (ie above the water level) part of the profile. Large storms can also draw material down the profile creating an offshore step (as shown in Figure 5.3). Shingle beaches frequently overlie sand beaches at lower level (see classification in Section 3.2.3). Similarly the seaward extent of a groyne on a shingle beach (or a shingle upper/ sand lower beach) needs to reflect the lowest limit of the steeper shingle beach gradient, typically revealed after a winter storm, sometimes with removal of the sand at the base of the shingle.

Figure 5.3 Shingle beach response to stormy waves

Note that Figures 5.2 and 5.3 are general representations of the beach response to a storm and actual post-storm profiles may differ.

68 CIRIA, C793 Where there is insufficient beach profile monitoring data or in situations where significant changes are being made (eg beach nourishment), it may be helpful to make use of numerical or physical beach profile modelling to model the cross-shore beach profile response. Although none of these models consider groynes or the effect of the groynes on the beach profile, they are directly relevant in the design of groynes as they will be used to determine the crest profile of the groynes and also inform the determination of the groyne length. The types of model available focus on the response of the beach profile to storm conditions and they cannot deal with the recovery period/s in between storms. Two types of models are commonly used depending on the sediment on the beach: „ For sandy beaches, one-dimensional process-based models, such as COSMOS, C-SHORE and XBEACH are often applied. A storm or a series of storms are modelled with an initial profile to ascertain the morphological development of the beach profile to those conditions. These models are run for storms lasting two to three days (maximum of the order of one week) and tend to erode the upper part of the profile and to create a sand bar seaward. „ For shingle beaches, Powell (1990) developed an empirical model, SHINGLE, based on physical laboratory tests over a range of different input conditions. This model outputs the response of the beach profile to the wave conditions inputted. The model was recently replaced by SHINGLE-B, an online tool where the input waves can be separated into the sea and swell components. This model was developed from a new set of laboratory tests undertaken after realising that in some parts of the UK (in particular in the South East) the damaging storms in 2013 had a considerable amount of swell as well as sea conditions (in some cases up to 40 per cent of the total wave energy spectra) which were not taken into account in the original SHINGLE model. A useful starting point in this situation is to make use of empirical beach profile models. An example of a recently updated model for establishing shingle beach profiles is given in Box 5.1. If it is desired to move from empirical simulations to full process-based approaches, models such as XBEACH-G can be adopted.

Further explanation and examples of other models that can be used to predict changes in beach profile under management interventions such as groynes is provided Blanco et al (2019).

Box 5.1 SHINGLE-B beach profile model for gravel beaches

An empirical framework, based on an extensive 2D physical model study was developed to examine the profile response of gravel beaches to bimodal wave spectra, characterised by swell wave and wind wave periods in various combinations. Based on this 2D physical model study, a new parametric model for predicting gravel beach profile response was derived by Polidoro et al (2018). In contrast to the SHINGLE model (Powell, 1990), the suggested beach profile schematisation adopts four curves (Figure 5.4), defined by their vertices as:

„ Curve 1: landward displacement and beach crest. „ Curve 2: beach crest and start beachface point. „ Curve 3: beachface point and top edge of step. „ Curve 4: top edge of step and lower limit of profile deformation.

Figure 5.4 Schematic shingle beach profile

Groynes in coastal engineering 69 Box 5.1 SHINGLE-B beach profile model for gravel beaches (contd)

The most influential wave variables for the beach profile evolution are the spectral waveH ( m0), wind-wave period (Tp,wind),

swell-wave period (Tp,swell) and swell component percentage (S%). The final equations describing each parameter, and hence the profile curve, as a function of bimodal wave variables are reported as follows:

1 Crest width = 3.92 + 0.31STp,swell

2 Crest position = −8.80 + 9.10Hm0 + 0.66STp,swell

3 Crest elevation = −1.88 + 0.81Hm0 + 0.31(1 – S)Tp,wind + 0.37STp,swell

4 Beachface position = −11.66 + 8.63Hm0 + 0.52STp,swell

5 Beachface elevation = −0.65 + 0.71Hm0 + 0.12STp,swell

6 Steo point position = −17.76 + 8.67Hm0 + 0.83STp,swell

7 Step point elevation = −1.19 + 0.51Hm0 + 0.06STp,swell

8 End profile position = 12.23 + 1.5Hm0 9 Run-up elevation: calculated with Polidoro et al (2013) The parametric model proved to be an improvement over existing gravel beach prediction models (SHINGLE, XBeach-G) under bimodal sea states. The SHINGLE-B model also provides a quantifiable measure that can guide users on the allowable regions of input data that will lead to a valid application of the model. The model should be used with the selected design storm parameters and be evaluated across a range of water levels in order to generate a profile envelope. Example application Input data:

„ Beach slope: 1 in 8 „ Hm0: 3.5 m „ SWL: 0 m „ Tp,wind: 7.5 s „ Tp,swell: 20 s „ Swell percentage S: 20% Results

Figure 5.5 Predicted profile extracted from SHINGLE-B tool

Erosion: 21 m² Accretion: 21 m²

Parameter Estimate (m)

Crest width 5.16

Crest position 25.69

Crest elevation 4.29

Beach face position 20.62

Beach face elevation 2.31

Step point position 15.91

Step point elevation 0.835

End Profile elevation 6.88

Run-up elevation 4.73

The SHINGLE-B tool is available from the CCO website: https://www.channelcoast.org/shingleb

70 CIRIA, C793 5.2 DESIGN BEACH PROFILE

5.2.1 Introduction Various design methods for beach nourishment are given in Section 14.4 of Rogers et al (2010). Although these mention crest height, width and slope focus, they focus primarily on nourishment volume. This section gives more information on these geometrical characteristics because they are an important starting point for groyne design.

Various types of beach profile may be identified: „ Design beach profile. The nominal profile of the ‘required’ working beach to protect a frontage. Its definition comprises a crest with a defined seaward level and width and a seaward slope (Figure 5.6), and it should mimic as far as possible the natural profile at the location in question. To ensure a practical and cost-effective design life, the design beach profile needs to be able to protect a frontage when it has been eroded back to a minimum acceptable profile. „ Minimum acceptable profile. The minimum profile that will prevent erosion of a cliff toe or limit the overtopping rate of a seawall to an acceptable maximum for the design flood defence standard for that frontage. In the context of beach management and associated triggers for intervention, the minimum acceptable profile is similar to the ‘crisis trigger value’ (as defined in Environment Agency, 2019a). „ ‘Alarm trigger’ profile (also identified in Environment Agency, 2019a). The beach profile above the minimum acceptable (or crisis) profile, which provides enough ‘buffer’ time for intervention.

Cross-shore movement of beach material during periods of high wave action may lead to sediment being drawn-down from the pre-existing beach crest during storm wave attack. This material can often be ‘temporarily’ deposited lower down the beach profile, close to or lower than the low water line, before being transported back onshore by subsequent milder wave action (provided the material has not been drawn too deeply offshore).

The following sections discuss how to determine the design beach seaward crest level, crest width and seaward slope (Figure 5.6). These somewhat artificial parameters should be related to the actual or predicted beach behaviour. Actual behaviour can be established from beach surveys, which are discussed in more detail in Section 9.1, and predicted beach behaviour from the models discussed in Section 4.3.

Figure 5.6 Beach profile terminology 5.2.2 Crest level The crest level of a beach is a function of various combinations of extreme water level, coincident inshore wave height, wave run-up and type of beach material.

For shingle beaches, when there is adequate mobile sediment available the beach will often find its natural ‘full’ height for the most severe antecedent storm, assuming no subsequent human intervention. The crest level is dictated predominantly by wave run-up (the height above the still water elevation

Groynes in coastal engineering 71 reached by the swash), which can transport sediment upward and landwards. When the beach’s pre- storm crest level is lower than the level of the highest run-up, the crest line can naturally ‘roll back’ in a landward direction under severe storm conditions, as the mobile shingle is washed over the crest and deposited on the rear face of a shingle ridge by the wave run-up. Under these conditions, breaching of the shingle ridge can be a real possibility (although this breaching may not be permanent when sufficient sediment is available for natural recovery of the beach).

Sand beaches tend to lose sand to seaward with the high-water line migrating landward. The crest level of the beach may remain unaltered unless erosion effects retreats the crest line of the beach to a higher or lower area within the original beach profile. Windblown sand can affect sand beach (crest) levels throughout the year.

The best way to help identify the naturally occurring ‘full’ crest level, assuming the beach is not affected by anthropogenic structures or influences, is from survey information. This can be from a survey of the beach in question, existing similar ‘full’ beaches, or from historic survey information of the beach in question (guidance on beach monitoring is given in Section 9.1).

Higher design crest levels than the natural ‘full’ height can be selected, which can provide greater benefits in terms of the reduction in overtopping during storm events. However, beaches with crest levels higher than more natural elevations are likely to be more prone to steepening during storm events and can encourage greater losses of beach material offshore.

In contrast, when nourishing the beach within groyne bays, the initial crest of the beach nourishment may be set below the eventually desired level, leaving the sea to reshape the beach to a more acceptable, natural profile.

The design beach crest level should not be defined in isolation from other influential criteria. These include beach crest width, slope, beach material type, offshore bathymetry, capacity for the crest line to be able to retreat, wave climate, extreme water levels, freeboard (ie height of any beach management or other infrastructure above design beach profile), effects of climate change and, importantly, the desired design standard for the scheme.

5.2.3 Crest width The minimum design crest width is primarily determined by the required standard of protection against overtopping.

This width can be obtained from numerical or empirical modelling or potentially assessed and/or validated from monitoring records of a healthy similar beach.

For most frontages, the beach crest does not generally remain parallel to the coastline between groynes due to the incident angle of the dominant wave fronts. The dominant wave front, when not running in parallel to the shoreline, will tend to cause the beach material to realign itself within the groyne bay. This typically results in a wide crest width at the one end of the groyne bay, with a narrower crest width at the other end of the groyne bay. In such situations the design minimum crest width becomes the minimum crest width (see Section 4.2.3 for further discussion on minimum crest widths).

Also, for many frontages the alignment of the coastline varies along its length, eg in terms of local embayments, headlands and pinch points or step changes. The crest line of the beach may not follow such irregularities in the coastline but follow a ’smoother’ alignment. The crest width may increase at embayments and could be compromised at local promontories. See Section 4.2.3 for further discussion.

Consideration also needs to be given to how the beach may behave because of the minimum crest width being reduced yet further (ie sensitivity testing). For example, if the waves can reach a (vertical) seawall at the rear of the beach then the wave reflection may precipitate a rapid reduction in beach crest level, which may not be subsequently recoverable through natural processes.

72 CIRIA, C793 5.2.4 Seaward slope The seaward slope of a beach not only depends on (and varies with) the hydraulic conditions, but also the size and grading of the beach material, in particular the ‘mobile layer’ at the surface of the beach.

The nature of the beach material in a cross-shore direction can vary (eg when moving seawards changing from shingle to mixed sand and shingle and then again from mixed material to sand) and this will give rise to varying surface gradients across the width of the beach.

The natural seaward slope for any given beach material at a location along a frontage is best determined from survey data of the beach in question or a nearby similar beach. The time of the year and antecedent weather conditions that preceded any survey need to be considered in any analysis, as beach profiles tend to build in calmer conditions and be eroded during more stormy conditions (further guidance on beach monitoring is given in Section 9.1).

Design slopes should mimic average post-storm natural profiles as far as possible (so it is important to time any reference beach surveys to capture these). Overly-steep slopes, which tend to be formed within the upper profile of shingle beaches after an individual storm, are more reflective of wave energy and may promote cross-shore sediment loss. Overly-flat beaches, which can be formed within the lower profile of a post-storm shingle beach, may steepen and promote material accretion after the storm has subsided.

5.3 LOWEST AND HIGHEST LIKELY BEACH PROFILES

Lowest and highest historic beach profiles may be extracted from beach monitoring data (see Section 9.1), but for design lowest and highest profiles additional considerations need to be taken into account as set out in this section.

5.3.1 Lowest beach profile The anticipated lowest beach profile under working conditions will define the worst-case scenario in terms of the design stability of the groyne (ie below this level there is potentially no ‘guarantee’ of the groyne being structurally stable or functioning satisfactorily).

The lowest likely beach profile will often occur on the downdrift side of a groyne. Any limiting groyne stability scenario may, however, also be exacerbated by increases in beach profile that could form on the updrift side of the same groyne.

This profile needs to be realistic but not overly cautious, otherwise it will lead to an excessive groyne over-design with its attendant higher construction costs and sustainability implications.

It should be determined based on as many as possible of the following: „ Analysis of historical survey data for the beach in question. The longer the survey period and the better the quality control on the survey datum etc, the more confidence can be placed in this data. „ Analysis of historical survey data for the nearest existing low beach profile. „ Substrata (which may act as a natural limiting factor). „ Toe level of any adjacent seawall (which may act as the ‘datum’ for the worst-case scenario, ie beach levels close to or below this datum would have an adverse effect on the overall defence system and not only the groynes). „ Overall context of the groynes in terms of the wider beach processes and management, eg an isolated groyne field within an otherwise unprotected frontage may be more vulnerable to low beach levels than say groynes within a heavily managed frontage. „ Intended future beach management practices.

It should also be recognised that during storm conditions it is likely that beach material can be disturbed to a depth well below the surface levels observed/surveyed during the low tide periods.

Groynes in coastal engineering 73 5.3.2 Highest beach profile This is the anticipated highest beach profile under working conditions and the design groyne crest levels should normally be defined to exceed this level.

The highest beach level is generally self-controlling since anything higher that the crest level of the groyne could spill over into the adjacent groyne bay.

5.4 GROYNE TOP PROFILE AND FREEBOARD

5.4.1 Groyne top profile The groyne profile should cover the entire beach profile to prevent alongshore loss of beach sediment during stormy conditions. This means that the root of the groyne should connect with the landward backstop at the foot of any cliff/dunes or the linear defence if present.

For groynes with a stepped longitudinal top profile, eg timber groynes, the ‘effective’ top profile of the groyne is the line directly below the steps in the top profile of the groyne. For bulk rock groynes the effective longitudinal top profile is difficult to define in general terms, as it is dependent on several variables, which include: „ rock size(s) used in the construction of the groyne „ relative top level of the differing rock sizes used „ height difference of beach material on either side of the structure „ varying width of the groyne structure at various levels „ obliquity of the waves to the shoreline „ permeability of the rock structure to the beach material being retained (Dornbusch, 2008).

Where a beach has many slopes or ‘bones’ (eg the level crest section, sloping face and shallow foreshore of a mixed sand shingle beach) and the decision has been taken to have the planks running parallel with the beach slope, it will be necessary to have continuous vertical planking joints at change of bone piles. Otherwise it is better to have overlapping joints – much like the bonding of brickwork and if the choice is made for horizontal planking this is also possible at change of bone piles.

Selection of the groyne top profile, although strongly linked to the design beach profile and required freeboard (see Section 5.3.2), will often need to be adapted (see Section 9.3) based on experience, eg too high a profile at the seaward end will generate excessive wave reflections and affect the stability of the beach profile. The use of horizontal planks in timber groyne construction together with suitably- oversized king piles (ie for initial design conditions) means the height of the groyne can be adjusted either temporarily or permanently in response to changing beach levels (see Box 9.2). Oversizing king piles during the initial design offers a greater flexibility for future adaptation and reduces the potential impact of ‘necking’ due to abrasion on the sections of pile exposed at or above mobile zone of the (varying) beach level (see Sections 7.6.4 and 10.3.5 for further discussion on protection against necking).

Often, the aim is to only have one to two horizontal planks above the beach level to avoid the effect of waves reflecting off the face of the groyne and to avoid obstructing access along the beach. The need for any exposure of the bare planking is principally geometrical – beaches have a sloping profile while the planks are horizontal, so their top profile necessarily reduces in level in ‘units’ of the plank width being used. For structural integrity reasons, the planking layout on the face of the groyne is in a ‘brick work’ pattern avoiding coincidence of vertical joints on adjacent planking lines. Regular inspections and adjustments to groynes may reduce the risk of large differences in beach levels across groynes which can pose a significant risk of trips/falls to beach users as well as restricting access along the shoreline unless over-steps are included within the design.

74 CIRIA, C793 5.4.2 Groyne freeboard The freeboard (Figure 5.7) is the distance above the design beach profile to the effective top profile of the groyne.

Figure 5.7 Groyne freeboard profile

This freeboard needs to be provided to account for uncertainties in the future behaviour of the beach compared to theoretical analysis (eg as defined by models), such as: „ erosion and accumulation of beach material within and adjacent to the groyne field including anticipated longshore and cross-shore fluctuations (see also Chapter 4) „ efficiency of the groyne(s) in trapping beach material „ effects of climate change on sea levels „ other unforeseen human influences.

As indicated earlier, any freeboard will reflect the incident waves, which is not desirable and can cause a reduction in the beach profile. So when setting the freeboard, a balance has to be reached between retention and reflection, which will be influenced by the beaches’ volatility at any specific location. Typical values based on experience are as follows: „ For sandy beaches it is recommended that the freeboard between the groyne profile and the maximum beach profile is about 0.5 m on the updrift side of the groyne. „ For shingle beaches the freeboard should be increased to about 1 m, to accommodate the larger fluctuations in the beach profile levels. „ For rock groynes the effective freeboard should be approximately half the diameter of the top layer of rock (CIRIA, CUR, CETMEF, 2007), but generally can be decreased in relation to a widening of crest width.

At the crest of the beach, while maintaining connection with the landward backstop, it may be advisable to avoid any freeboard to minimise the trip hazard to pedestrians walking along the crest of the beach (Figure 5.8).

Figure 5.8 Crest of timber groynes showing unhindered pedestrian access at Herne Bay, Kent (courtesy Uwe Dornbusch)

Groynes in coastal engineering 75 6 Materials for narrow footprint groynes

This chapter describes the performance and procurement factors that should be considered when selecting and sourcing materials and or combinations of materials for narrow footprint groynes (Figure 6.1). The most common material used in the UK is timber, however, polymer composites, concrete and steel, which can also be used, are also discussed. When choosing the most suitable material for a groyne scheme, designers need to consider and evaluate a few issues relating to material performance, availability and sustainability, environmental impact and cost. The main sections of this chapter are:

„ Selecting a material type (Section 6.1) „ Timber (Section 6.2) „ Polymer composites (Section 6.3) „ Concrete (Section 6.4) „ Steel (Section 6.5)

Coastal management Functional requirements context and indicators (Section 2.1) (Section 2.1)

Physical coastal environment – hydraulic, geotechnical and morphological constraints (Chapter 3)

Start or continue to No Groyne removal use groynes? (Section 9.4)

Yes

Plan shape design of groyne field (Chapter 4) Evaluate and choose groyne type (Section 2.2) and material (Chapter 6) Design groyne profile (Chapter 5)

Structural design or design review for groynes (outline and detailed) (Chapter 7)

Construction and/or major refurbishment (Chapter 8) Major Groyne repair and Minor maintenance Groyne adaptation (Section 9.2) (Section 10.3) and adjustments (Section 9.3)

Small

Beach changes Routine monitoring Large (Section 9.1) (Sections 9.1 and 10.2) Structural deterioration (Section 10.1) Post-incident monitoring (Sections 9.1 and 10.2)

Climate change Storm events (Section 3.1.4)

Figure 6.1 Parts of groyne management process covered in Chapter 6

76 CIRIA, C793 6.1 SELECTING A MATERIAL TYPE

6.1.1 Introduction When selecting a material for the construction of groynes, the following issues should be considered and evaluated as part of the option appraisal and development process: „ functional performance and durability of materials and combinations of materials „ material sources and lead-in times „ sustainability, environmental impact and aesthetics „ cost „ ease of construction and maintenance „ health and safety.

The following sub-sections provide general information on these issues, and their relevance to the material selection process. Sections 6.2 to 6.5 provide specific information on the four commonly-used materials for narrow footprint groyne construction. Timber is discussed more comprehensively because of its historic dominant use in the construction of narrow footprint groynes and because guidance and standards on the use of steel and concrete are widely available. In Section 6.2.6, a general discussion on timber groyne fixings is provided, covering the variety of types and materials that can be used, and the effect fixings can have on groyne maintenance and service life.

6.1.2 General performance of materials When designing groynes, it is important to have a general understanding of how specific materials are likely to perform when used in a groyne structure at a specific location, and what the implications are for service life and maintenance needs. Different materials will have different performance levels in terms of initial strength, long-term durability and resilience to climate change impacts. The location of the material in the groyne and the type of beach and level of exposure to wave action will also influence the stressors on the materials and the rate of performance deterioration. It is also important to note that the performance of standard material sub-types, eg alternative mix designs of concrete or timber species, can also vary considerably.

Abrasion is a significant issue in groynes located on coastal beaches, particularly if there is a narrow tidal range that concentrates on a small part of the structure centred below the high-water line and at the beach/groyne interface. Choosing a material with appropriate durability to abrasion loads is a key consideration, and this issue is discussed in detail for each material type in the subsequent sections of this chapter. (See also Sections 6.2.3, 10.2.4 and 10.3.5 for specific discussion of the abrasion of timber.)

Climate change (see Section 3.1.5) may lead to increased sea temperatures affecting some groyne materials (eg timber) and their deterioration agents and to increased storminess causing increased damage (Figure 6.2). These factors will affect the rate at which assets such as groynes are likely to deteriorate, and material vulnerability and future resilience needs should be explicitly considered as part of the material performance assessment (Environment Agency, 2019b).

Groynes in coastal engineering 77 Figure 6.2 Relationship between deterioration processes and climate change factors for a timber groyne (from Environment Agency 2019b)

6.1.3 Use of multiple materials In certain situations, there may be advantages to using many different materials or timber species in groyne construction. However, perceived advantages may be outweighed by site practicalities and buildability. While a combination of materials may deliver more efficient use of high value materials, it is likely to present procurement and construction challenges. Materials may need to be procured from more than one source, which could place a construction project at increased pressure, and building with different timber species, for example, may require more intensive construction management. A policy of using a single timber species obviates these potential risks to the project, providing there is a guaranteed continued and sustainable supply of that species. However, Environment Agency/Defra (2020) policy supports using a range of species (assuming performance is meets design criteria), so it is important that using one species of timber for a single project should not set a precedent for all projects in a programme.

Combinations of materials may also be considered beneficial in terms of scheme constructability and/or maintainability. Intertidal working on sites with narrow tidal ranges and steep beach profiles can present significant construction challenges, and maintenance of vulnerable timber groynes subject to high levels of erosion on shingle beaches can be hazardous. An example of a beneficial combination is a hybrid timber-armourstone groyne with the lower seaward section being encased with armourstone to extend the service life and to significantly extend the maintenance interval at the seaward end (see Box 6.1).

78 CIRIA, C793 Box 6.1 Reinforcement of seaward end of timber groynes with rock armour at Felixstowe, Kent

The tidal range at Felixstowe north beach is small and the beach is made up of a mixture of sand and shingle with shingle being the more prevalent material. The small tidal range on a predominantly shingle beach presents significant challenges in inspecting and maintaining the groynes. The principal hazard is abrasion, and this occurs at the lower third of the groyne. In addition to abrasion, there is significant risk of marine borer attack. The combination of abrasion and marine borer attack causes planks or sheet piles to work loose and break away from the groynes (Figure 6.3). The current strategy, given the limited available resources, is to reinforce the seaward end of every fourth groyne with rock armour (Figure 6.4).

Figure 6.3 Missing sheet piles Figure 6.4 Armourstone reinforcement

6.1.4 Materials sources and lead-in time Sourcing of materials and the lead-in time for procurement are essential parameters when planning and evaluating a scheme. Lead-in times can vary considerably between the different materials (see subsequent sections of this chapter), and design requirements and scheme locations present a risk to the delivery of a project. For all projects that are dependent on critical materials supplies, enquiries with specific suppliers or manufacturers should be made during the design phase to avoid unforeseen programme delays occurring in the lead up to, or during the construction phase.

6.1.5 Cost The financial cost of a groyne system should be considered in whole-life terms including both initial capital expenditure as well as future recurrent expenditure, and the latter will require assessment of deterioration rates (see Section 10.1). Comparisons of the cost of different options should be made to inform the decision- making process. The cost comparison should also include any future major reconstruction or replacement at the end of the serviceable life. For example, it would be expected that a rock groyne would have a considerably longer working life than a timber groyne. Recurrent expenditure should look to account for groyne inspection, maintenance, beach monitoring and nourishment in a holistic way.

As with most coastal engineering works, it is potentially misleading to quote ‘typical’ costs for groyne construction. Costs are inevitably site specific and vary according to the type of construction, conditions of access, local availability of materials and the coastal environment in which the groyne is located. Maintenance costs are even more variable because, in addition to the preceding factors, they are strongly dependant on the size and type of organisation responsible for maintenance, the mobility of the beach and/or the degree to which the beach is stabilised by the groyne system.

The cost of materials for groyne construction is often significantly outweighed by the overall construction cost. However, factors that can influence the cost of materials that should be considered when designing a scheme include: „ changes in price due to, for example, fluctuations in the price of oil or changes in exchange rates between different currencies (see Section 8.1.2) „ transport costs „ pre-ordering of material and stockpiling on site „ amount of processing required for timber, eg additional ‘premium’ costs for planing the surface of longer lengths of timber required to span between multiple piles.

Groynes in coastal engineering 79 6.1.6 Sustainability and environmental impact Materials used in coastal construction undergo five key stages in their life cycle as presented here, and further detail on each of these stages is given in Rogers et al (2010): „ raw material and extraction „ production and processing „ construction use and maintenance „ reuse and recycling „ disposal.

At all these stages a policy of environmental net gain should be pursued, aiming to leave the natural environment in a measurably better state than before construction (see HM Government, 2018). Material sustainability should be considered at the start of a project, as well as at each key stage, and any required works evaluated with sustainability in mind. Sustainability of various material types is discussed in the subsequent sections of this chapter.

The assessment of the wider environmental impact of constructing a single groyne, or whole groyne field, can be a complex process, but one that should be fully considered as part of the process of scheme justification. However, when a decision has been made to construct a groyne it is important to consider both the negative and positive effects that the chosen style of groyne, the materials, or the design details could have on the environment. This information will be needed for any required environmental assessment. For example, a rock groyne may provide good opportunities for the creation of intertidal (eg rock pools) or sub-tidal habitats. While the incorporation of significant intertidal habitats may not be as obvious an objective with timber groyne construction this should still be considered during the design process (see Section 7.9).

All risk management authorities are required to make choices and long-term decisions that result in gains for the environment from reducing carbon by selecting options that reduce the wider carbon costs and/or increase the benefits of flood and coastal risk management projects both through to construction and over their operational life. It is the Environment Agency’s specific aim to promote low carbon solutions and give suppliers an incentive through carbon target setting. They are actively promoting this as part of their sustainability plan and flood and coastal erosion risk management strategy. To support suppliers, the Environment Agency’s carbon planning tool provides a mechanism for assessing carbon over the whole life of built assets such as groynes. Use of this tool will require assessment of deterioration rates (see Section 10.1).

6.1.7 Ease of construction and maintainability There are three key aspects to consider in terms of the ease of construction and maintainability of a groyne: „ opportunities for standardised design in construction to maximise cost efficiencies „ plant, material and workforce access and safety „ workforce skills and training requirements, eg whether specialist contractors or local authority employees are required (any design intricacies and subsequent need for specialists may result in an increase in whole-life costs).

6.1.8 Health and safety Different material types have different associated health and safety risks both during construction and over their design life. These should be evaluated during the design process and managed through the project risk assessment and CDM process (HSE, 2015).

80 CIRIA, C793 6.2 TIMBER

6.2.1 Introduction The marine environment is challenging for all construction materials, but timber suffers remarkably little from the effects of the chloride attack arising from the salt content of seawater compared to concrete and steel. Timber has often been the preferred material for groyne construction (Figure 6.5) because of its favourable strength to weight ratio, relative ease of fabrication and workability. Timber can be easily modified on site to the correct dimensions, where there would be significant delays when working Figure 6.5 Narrow footprint timber groyne on Bournemouth beach, with other materials. Dorset

6.2.2 Functional performance There are many factors that will affect the performance of timber in groyne constructions (Perdok, 2003). These include physical factors, such as structural movement of the timber in service above the intertidal zone, abrasion, and exposure to biological agents of degradation, such as fungal decay and attack by marine borers.

Typically, timber species for use in groynes should: „ have good strength properties and these should be recognised as a specific Strength Class (as defined in BS EN 1912:2012) „ have good natural durability and be resistant to attack by wood decaying fungi „ have good resistance to attack by marine borers „ have good abrasion resistance „ be available in large cross-sections and long lengths.

The main overall service life determining component is the timber pile. Different parts of the groyne will be exposed to different hazards. BS EN 335:2013 provides guidance on the different use classes for timber. The relevant use classes are summarised in Table 6.1 along with a description of where these may be assigned to different parts of a groyne.

Table 6.1 Timber use classes and where they occur on a groyne

Use class Description Groyne timbers Planks located above the beach level and above the intertidal zone. The timber is exposed to 3.2 prolonged wetting conditions The timber is at risk of attack by wet rot type decay fungi and wood boring insects. The timber is in direct contact Planks below the beach level. 4 with the ground or seawall The timber is at risk of attack by wet rot and soft rot type decay. The timber is permanently or Planks within the intertidal zone. 5 regularly submerged in seawater The timber is primarily at risk of attack by marine borers and soft rot fungi.

Details of each of these drivers of timber groyne degradation are described in the following section.

Timber is hygroscopic and will absorb or lose moisture depending on the surrounding environment. Timber for use in groyne construction comprises large cross-sections. The timber is in the green state and will naturally dry out in positions above the intertidal zone. Given the large cross-sections and

Groynes in coastal engineering 81 exposure to the prevailing weather it is unlikely that timber above the intertidal, zone will fully air dry. However, it is reasonable to expect that the surface will shrink (or swell) and this shrinkage may result in the development of fissures that will open after prolonged dry periods. Conversely, they will close in prolonged wet conditions. It is unlikely that shrinkage in service will result in significant gaps developing between groyne planks – gaps are more likely to be attributable to small variations in plank width due to deviations during sawmilling.

Timber will be at risk of attack by fungal decay if the moisture content exceeds 20 per cent for prolonged periods. Given the green state of the timber and its service conditions, it is reasonable to accept that timber for groyne construction will be at permanent risk of decay. So, timber should be naturally durable. It should be noted that good natural durability is not necessarily an indication that the timber is also resistant to marine borer attack. It should also be noted that the outer sapwood of timber, if present, is not durable or resistant to marine borers. Consequently, the resultant absent sapwood can cause localised loss in cross- section. This is usually insignificant unless the fixings have been positioned in the sapwood.

6.2.3 Agents of degradation The performance of timber groynes varies significantly due to construction details, species used, wave action, tidal range and the sediment type. Deterioration of groyne timber is mainly caused by sunlight, mechanical abrasion, and biological attack. This section discusses these issues.

Sunlight The photo-degradation of wood is by ultraviolet (UV) radiation, which is a surface phenomenon. In practical terms it has no effect on strength properties of timber used for groyne construction. Photo- degradation affects a thin surface layer (less than 0.2 mm), but this damaged outer layer is much weaker and may be easily abraded to reveal underlying timber. Photo-degradation is a slow process. Lignin is broken down by sunlight, which causes the surface wood fibres to loosen, and the timber develops a grey appearance and a ‘woolly’ surface texture.

Timber abrasion Abrasion and the resulting wear of timber is the most common and significant agent of mechanical degradation which can take many other forms including sudden and severe impact, deterioration of fixings, undermining, and uplift loading.

On shingle beaches the motion of material is concentrated as bed load, rolling about the interface of the beach with the structure. By contrast sand movement occurs mostly in suspension in the water, cushioning the abrasive effect. As a result, shingle beaches are more abrasive to groynes than sand beaches exposed to similar wave action and tidal range, although well-rounded shingle pebbles are not as damaging as coarse angular material. The resulting life of groynes on shingle beaches may be shorter, for example in the high energy, micro-tidal, shingle beach environment at Milford-on-Sea some groyne components only have a life of about three to five years. By contrast, the timber groynes at Bournemouth are located in a sand environment, where the wave climate is moderate, and the structures are largely buried in sand for most of their life. As a result, the groynes are not regularly susceptible to erosion and have typically achieved a life of 23 to 25 years, often with virtually no maintenance (see Box 10.1).

Loading conditions vary from site to site although the most significant wave loads will be experienced by components facing into the predominant wave direction and subject to regular wave action. A narrow tidal range will focus abrasion on a small part of the structure, centred below the high-water line, and at the beach-structure interface. Even though abrasion rates may be high, structures built at sites where the tidal range is larger may distribute the abrasion along more of the structure length.

82 CIRIA, C793 Results of abrasion Wear of timber piles is often confined to a narrow zone on one or two faces of the piles. The abrasion can be confined to a narrow zone, extending upwards from immediately below the surface of the beach. The damage is often greatest on the seaward face of the piles (Figure 6.6) and on the face parallel with the groyne planks. In instances where significant backwash entrained sediment occurs abrasion may also occur on the landward face of the pile. Abrasion may frequently extend below the surface level at the time of inspection because of previous damage when beach levels were lower.

Piles can also exhibit an abrasion pattern known as ‘necking’, which occurs when piles have been installed or extended above the level of the existing groyne planks to allow further planks to be added. The lack of shelter of the piles by planks from impact of beach Note the extensive wear on the seaward face and more limited wear on the material when the groyne is overtopped landward face, and the related wear of the older plank at the beach surface. exacerbates the aggressive abrasion. Figure 6.6 Abrasion of pile (courtesy New Forest District Council)

In terms of planks on timber groynes, a least three distinct forms of abrasion response may be identified: „ Overtopping of beach material under wave action can cause rapid loss of section of the top planks of groynes on shingle beaches (Figure 6.7). This may not be an issue as top planks are generally easy to replace. „ Gap widening. Any gap that appears between planking will progressively widen as fine sediment passes through, eroding the timber. Eventually even holes may form at weaker locations or locations where the wave energy in more concentrated (Figure 6.8). „ Scalloping. Swash and backwash on the downdrift elevation can cause ‘scalloping’ of planks (Figure 6.9). This may be observed immediately next to their connection to piles on the seaward and landward sections of the groyne planks respectively. Scalloping tends to be worse at the beach/ groyne interface.

The density of the timber can be a good indicator of abrasion resistance. Tropical hardwoods with high density, such as Ekki or Greenheart or lesser used species with comparable density will generally perform best in an abrasive environment. However, the lack of density for softwoods may be offset to some extent by their relative flexibility or ‘springiness’ meaning that particles tend to bounce off rather than gouge into the face of the timber.

Groynes in coastal engineering 83 Figure 6.7 Abrasion of the top planks of a timber groyne at Withernsea, Yorkshire

Figure 6.8 Gaps and holes eroded by abrasion due to sediment Figure 6.9 Scalloping of planks at passing between planks (courtesy New Forest District Council) Bournemouth, Dorset Biological hazards Biological hazards depend on the location of the timber in relation to MHW. Timber below MHW is mainly subject to attack by marine boring animals, which may cause severe damage to a timber structure over a comparatively short space of time. However, marine bacteria and fungi have a comparatively minor role in this zone. Timber exposed above MHW may be attacked by terrestrial wood destroying fungi and insects, with insects to colonising decayed wood.

Marine borers: gribble Marine wood boring Limnoridae, more commonly known as gribble (Figure 6.10), are crustaceans that are found around the UK coastline. They can cause severe damage to groynes, particularly on the less exposed sections of the groynes if the environment is reasonably sheltered. They are ubiquitous, inhabit the surface of the timber and are fully mobile. Attack by gribble tends to be superficial and results in the creation of an extensive network of galleries at or directly below the wood surface (Figure 6.11). However, where structures are exposed to the full force of the sea and where there is a high risk of mechanical abrasion, gribble find it difficult to establish large populations. This is because the abrasive nature Figure 6.10 Scanning electron micrograph of a gribble (limnoria lignorum) (courtesy Simon Cragg) of the environment destroys the galleries. In such instances, gribble tend to be restricted to the sheltered parts of structures, particularly where joints are formed as these can provide suitable refuge for the animals. Box 6.2 describes the links between abrasion and gribble attack.

84 CIRIA, C793 The galleries tend to be 1 mm to 3 mm in diameter with regularly spaced openings to allow respiration. Gribble are very small animals, and only visible to the human eye when timber is inspected closely in situ. They are generally 2 mm to 4 mm in length and are pale grey in appearance. The animals tend to concentrate at the intertidal zone.

Gribble tend to colonise timber more readily on sandy beaches. On shingle beaches, the abrasive effect of shingle tends to smash the gribble galleries immediately below the surface, so in this situation the gribble only colonise more sheltered Figure 6.11 Typical damage caused by gribble on a parts of the groyne structures. Greenheart plank, Felixstowe, Kent

Box 6.2 Abrasion vs. gribble attack at butt joints between groyne planks

Abrasion on groyne planks can cause the opening up of the timber at the butt joints. These increasing gaps can allow the transport of beach sediment, particularly finer sediment, through the groyne. These gaps reduce the efficacy of the groynes as they are no longer able to maintain beach levels. However, combating wear of planks by blocking these gaps may lead to the unintentional consequence of encouraging attack by gribble. Approaches to blocking the gaps to prevent transport of sediment have included fixing plywood sheets or thin timber strips to the downdrift elevation of the groynes. The experience of Bournemouth Borough Council indicates that the addition of plywood sheets has provided a sheltered environment for gribble, which encouraged a population increase with more aggressive attack of the planks. However, at Milford-on-Sea where a similar strategy has been used, there has been no evidence of gribble build up behind the plywood, possibly because gribble find it hard to colonise in the abrasive environment of a shingle beach. However, the abrasive nature of the beach at Milford-on-Sea also makes the use of plywood sheets a short-lived strategy for preventing transport.

Marine borers: shipworm Shipworm found around the southern UK coastline tend to be members of the Teredinidae family. They have a soft worm like body with two shells or valves at the anterior end of the animal which enable them to bore into timber (Figure 6.12). The animal remains in the same tunnel throughout its life. Each tunnel is discrete, and the animals avoid intruding into neighbouring tunnels as they grow and excavate into the timber. Shipworm typically line their excavations with a secretion of calcium carbonate. Even in timbers of limited volume, the shipworm will not emerge from the timber and will continue to bore alongside its neighbours until the Figure 6.12 Example of a shipworm removed from a Douglas fir waling timber is destroyed and breaks apart. attached to the Barmouth Viaduct in the Mawddach estuary at Barmouth, Shipworm can devastate softwood Gwynedd (courtesy TRADA Technology) timbers. Most shipworm damage to tropical hardwoods can be more limited, for example, observations of damage to Greenheart may be limited to the outer 30 mm (Figure 6.13).

Groynes in coastal engineering 85 Figure 6.13 Shipworm damage observed on a Greenheart plank at Felixstowe, Kent

Wood decaying fungi Timber will be vulnerable to attack by wood decaying fungi if the moisture content remains above 20 per cent. Timber in the marine environment will, in most cases, remain wet or be exposed to persistent wetting. Timber sections that are out of contact with the water and, to a lesser extent, in and above the intertidal zone, will be subject to intermittent wetting and may be subject to decay by wet rot fungi. These types of fungi can Figure 6.14 Wet rot decay causing failure of softwood groyne timbers cause significant damage and can result in significant voids developing (Figure 6.14). However, these fungi rarely cause issues where naturally durable tropical hardwoods have been used in groyne construction. They can be a challenge with life-expired preservative treated softwoods and hardwoods with variable natural durability such as oak. Attack by wood destroying insects may well be symptomatic of advanced degradation caused by wet rot type fungi. The softer decayed timber provides a food source and habitat for burrowing insects.

Soft rots are a specific group of fungi, which confine attack to wood with a very high moisture content. Soft rot has a slow progressive action, which attacks the surface layer only, so would not be expected to cause deep pockets of decay within the timber. However, over time, soft rot fungi can significantly reduce the cross-section of members and this type of attack can exacerbate erosion rates in service. The depth of soft rot attack will vary depending on the timber species and its density and hardness. Typically, soft rot will penetrate to a depth of about 5 mm. The rate of penetration is determined by timber durability and density. While soft rot attack is superficial, it can exacerbate mechanical abrasion and facilitate attack by marine borers.

6.2.4 Selection and sourcing of different timber species Selection of timber species is governed by several key considerations which include: „ required strength „ required length and cross-section of piles and planks „ type of beach „ working hazards, eg abrasion, marine borer attack.

86 CIRIA, C793 There are also opportunities associated with a possible hybrid design where different species are used to address location specific hazards on the groynes, eg the use of marine borer resistant timber within the intertidal zone only.

In the late 19th century, groynes were constructed using native woods such as oak, elm and pine. However, with these species there were the fast rates of abrasion especially on gravel beaches and susceptibility to biological deterioration and low densities leading to buoyancy problems. However, when selecting and sourcing appropriate timber there are no definite rules. Each designer will have to consider the location, budget, future maintenance arrangements and sustainability of the species to determine the level of risk and the timber they wish to use.

In recent years, the UK has relied on a narrow range of species with a proven track record, predominantly hardwoods. These are described in Table 6.2. The softwood species in the table may not offer as long a service life but there are examples where they have been used very successfully around the UK, specifically on the south coast of England. There is a case for using softwoods for planking, if a short-term service is required, or if groynes are nearing the end of their service life. Douglas Fir has been used extensively on the south coast for planking groynes, but also for complete groynes and, for example, together with some native oak, it was used to bolster a failed hardwood groyne field to buy time over a 25 to 30 year period before a more sustainable solution could be developed. Designers need to consider cost, design life, sustainability of the species, future maintenance arrangements and estimated durability for the environment when selecting timber for use.

The favoured hardwood species are Greenheart and Ekki. Ekki is distributed across many West African countries. Greenheart originates from Guyana in South America and is only available in commercial volumes from that country. Other species such as Basralocus are available from Guyana, Suriname and French Guiana, but Basralocus is not widely used and has no strength classification. Opepe originates from many West African countries but like Basralocus, it is not widely employed.

Table 6.2 Timber species with an established track record

Strength Commercial name Botanical name Comments class Greenheart Chlorocardium rodiei D70 Proven track record Ekki Lophira alata D70 Proven track record Yellow Balau (Selangan Shore spp. D70 No longer commercially available in large cross-sections batu) Opepe Nauclea diderrichii D50 Readily available but has had limited historic use Jarrah Eucaplyptus marginata D40 No longer commercially available in large cross-sections Comparatively poor resistance to marine borers and Oak Quercus spp. D30 variable natural durability Softwood: comparatively poor resistance to marine borers Pine Pinus spp. C24 and variable natural durability Softwood: comparatively poor resistance to marine borers Douglas Fir Pseudostuga menziesii C24 and variable natural durability

Recognising the combination of conservatism and over-reliance on Ekki and Greenheart, the Environment Agency has sought to promote the use of lesser used species. In particular, Meaden et al (2011) employed a fast-track screening test procedure that benchmarked the performance of forty one lesser used tropical hardwood species against Greenheart and Ekki in a series of laboratory and field trials designed to determine marine borer and abrasion resistance. They also undertook full strength testing on five commercially available Forestry Stewardship Council (FSC) certified species that indicated good marine performance. These are described in Table 6.3. The lower strength classes in comparison with Greenheart and Ekki may not be a significant issue if it can be shown by design that the timber groynes do not require a high strength class timber. For example, king piles typically require high strength but lesser used species with lower strength may often be suitable for groyne planking. In this regard, other species not referenced in Table 6.3 (as they were not selected for strength testing) also demonstrated their potential for use in groyne construction.

Groynes in coastal engineering 87 Table 6.3 Lesser used hardwood species strength classes

Lesser used species Botanical name Origin Strength class

Angelim Vermelho Dinizia excelsa South America D60

Cupiuba Goupia glabra South America D50

Eveuss Klainidoxa gabonensis West Africa D50

Okan Cylicodiscus. gabunensis West Africa D40

Tali Erythrophleum micranthum West Africa D35

Conservatism in the industry is a significant barrier to introducing such lesser used species. This is further exacerbated by the fact that the material cost of timber in coastal engineering is dwarfed by actual construction cost.

There may be advantages of using a combination of different timber species, particularly as part of a sustainable management strategy. This could involve: „ using stronger, resistant species such as Ekki in the middle section of a groyne down to the seaward end, where abrasion and marine borer attack are significant hazards „ using fewer resistant timbers, possibly even softwood, in positions where abrasion and marine borer attack are much less of a threat, for example, in locations above the intertidal range.

The advantages of adopting lesser known species may include lower cost of materials, access to larger volumes and less exposure to seasonality or stock shortfall risks. However, good forward planning is necessary with timber suppliers to ensure forest sources can meet the project’s requirements.

6.2.5 Timber preservation and modified timber Currently available methods for preservation or modification of softwood timber do not appear to offer a solution to timber durability in the marine environment in providing a viable commercial alternative to tropical hardwood timber.

In terms of preservation, timber treated for use in the marine environment may historically have been impregnated with creosote or a formulation containing salts of copper, chromium and arsenic. Ongoing EU legislation has restricted the use of these preservatives in the marine environment and now there are no commercially available preservatives approved for use. Also, pressure impregnation only delivers a protective layer of preservative into the timber. In coastal environments the abrasive nature of the environment will abrade away the treated zone and expose the underlying untreated timber.

In terms of timber modification, this is a process whereby the wood chemistry is altered. This may be carried out by either heat treatment or impregnation with chemical compounds. This differs from preservative treatment because it does not involve the addition of a biocide. Modified wood improves the resistance of timber to attack by decay. In the case of acetylated wood, good marine borer resistance has been reported. However, modified timber tends to be lightweight, porous plantation grown pine. So the lack of strength and lack of abrasion resistance makes these products unsuitable for marine construction. In addition, the modification process is only successful if the timber is fully impregnated. Currently, large cross-section timbers cannot be so modified.

6.2.6 Fixing materials Fixings may comprise stainless steel, galvanised mild steel or non-stainless high tensile carbon steel, or mild steel. These may be plates, coachscrews or nut and bolt sets. Mild steel can be acceptable for brackets, plates etc where there is a sufficient thickness/volume of steel to allow for corrosion and abrasion losses.

88 CIRIA, C793 The type of fixing used to construct timber groynes will have an effect on the maintenance and service life, particularly when exposed to severe abrasion on shingle beaches with a small tidal range. For each type of (steel) fixing that is used, there is a need to consider whether it is best formed of stainless steel or galvanised mild steel. Although more expensive, stainless steel is more resistant to abrasion and will not corrode as quickly as galvanised mild steel but it may develop a degree of brittleness over time. Also, because of the salt content in sea water, the dissociated chloride ions can act as an electrolyte and this can accelerate the corrosion of non-stainless steel and cause localised ‘nail sickness’ in timber. When galvanised fixings are exposed to abrasion and mechanical damage, the galvanised barrier can be quickly lost, and the underlying ferrous material is then vulnerable to rapid corrosion.

Corroded fixings are extremely difficult to remove on account of the expansion of the fixings. Once removed, it is unlikely they can be reused and it is also likely that removal of corroded fixings can cause significant damage to planks which may prevent their reuse. Corrosion of fixings and subsequent expansion can cause structures to lock together. This may be undesirable as timber groynes need to be able to flex structurally when under differential load from the beach. The products of corrosion can cause nuts and bolts to fuse together, which results over time when the strength of the connection increases. However, over the long term, further corrosion weakens the fixings which means the connections are more susceptible to failure in service. The result of this is that the groyne planks become more vulnerable to failure.

Further information on the design and maintenance of fixings can be found in Sections 7.4.4 and 10.3.3 respectively.

6.2.7 Lead-in time Lead-in times for procuring timber can vary significantly and are affected by many factors which include the species required, the cross-sections, lengths and volumes. Schemes that require comparatively long piles and planks may require lead-in times of at least 12 months and may require significant forward ordering of timber so the required volumes may be harvested from the forest concession. Timber suppliers rarely hold significant stocks of material of this size in this country and so will rely upon shipping dates to be able supply clients’ requirements. Other schemes that require shorter, smaller cross- section timbers may require less lead-in time (eg a few weeks) as the timber may be held in stock. There may be periods when large cross-section and long lengths may be available as the exporting sawmill has managed to build up an inventory of common timber sizes.

Lead-in time may influence procurement and price. Pre-ordering large volumes of timber not only allows the supplier to procure the material but enables the end user to fix the price of the order. This removes the risk of unpredictable increases in the costs of timber. It allows certainty when budgeting for timber procurement and protects the end user from significant price fluctuations associated with currency and fuel price volatility. Forward planning is playing an increasing part in maintenance works. Procurement can also be hampered by political instability in exporting countries and market forces where producers may choose to export to end users where the requirements for independent certification demonstrating legal and sustainable forestry practice are less stringent.

While availability of the various sizes of Greenheart and Ekki will be well known to experienced specifiers, when using other lesser used species, care should be taken to avoid assumptions regarding sizes that may be readily available.

6.2.8 Cost The cost of timber plays a role during the specification process, but it is not the principal influence. However, it needs to be considered alongside the combination of proven technical properties, availability and sustainability.

The market prices for species such as Greenheart and Ekki, commonly used for groynes, are subject to significant variation. Many factors will influence price including availability and demand. Other industrial end uses for the timber may also influence demand and price.

Groynes in coastal engineering 89 Costs could be mitigated by producers building an inventory of common sizes and designers working with these sizes. The larger the cross-section and the longer the length required, the more expensive the timber will be because of the difficulties in extracting, transporting and sawmilling long logs. For example, UK groyne piles created from Ekki need to be of large section size which can prove difficult to obtain and does not lead to optimal yield of timber from logs. Some economies may be secured by specifying shorter planks that may span single or double structural bays as opposed to three structural bays. However, these savings may be cancelled out by the requirement for more fixings, especially if stainless steel is used (which is a relatively high cost) and ensuing longer construction times.

At times when there is high demand for Ekki and Greenheart, some savings may be found by specifying lesser used species. However, these savings may not be that significant because the costs associated with logging and sawmilling an alternative dense, tough timber will be similar to those for Greenheart or Ekki.

6.2.9 Sustainability and environmental impact Obtaining sawn timber used in groyne construction involves the following stages: „ forest establishment and cultivation „ saw log harvesting „ transport by road haulage vehicles on forest roads „ sawmilling „ transport by sea and road haulage to site „ use in the construction process.

There is little waste involved in this process for groyne construction. Imported, certified tropical timber (see Section 6.2.10) can be a sustainable construction material providing it originates from a well- managed forest where: „ the annual allowable cut does not exceed growth rates „ harvesting does not do irreparable damage to the forest and surrounding ecosystem „ new trees are planted to replace those that are harvested „ the timber is supported by independent third-party certification (eg FSC) demonstrating its provenance.

Felling single species unsustainably may lead to forests being denuded of those species. Under sustainable forest management strategies, felling is likely to be more selective, with only certain species being taken from multi-species forests, although consideration needs to be given to the damage caused to the remaining forest in recovering those few trees felled per hectare.

Tropical timber can be an environmentally friendly material because it is a natural, recyclable and potentially sustainable material. As the tree grows it locks in carbon dioxide from the atmosphere and stores carbon and well-managed forests can increase the rate of carbon dioxide absorption from the atmosphere as gaps in the canopy above the harvested area stimulate rapid plant growth. Tropical timber has comparatively low embodied carbon, with the main energy costs and carbon emissions involved in delivering timber for a groyne project being those associated with harvesting, sawmilling and transport.

Recent research funded by SCOPAC and Bournemouth Borough Council (SCOPAC, 2020), has been evaluating the potential for reuse of timber from deconstructed groynes. While it is unlikely to be feasible to reuse piles on the same site, evidence suggests that they can be re-sawn to yield groyne planks. Also, planks that have not been exposed to significant wear in service may be re-sawn for reuse on new groynes. Initial results show that it is possible to achieve a high recovery rate. The reuse of timber from life-expired groynes helps reinforce the case for sustainability and the use of tropical timber. Reusing recycled timber for sacrificial pile protection, installed at the same time as groyne maintenance is being undertaken (see Sections 7.6.4 and 10.3.5), is a particularly good reuse example.

90 CIRIA, C793 6.2.10 UK Government timber procurement policy The ability to prove that imported tropical timber for groyne construction is from a legal and sustainable source is a key factor in the specification process. All central government departments, their executive agencies and non-departmental public bodies are required by the UK timber procurement policy to procure timber that is legal and sustainable. See, for example: https://www.gov.uk/government/organisations/environment-agency/about/procurement#timber

An associated EU licensing strategy for timber, called Forest Law Enforcement, Governance Trade (FLEGT), is aimed at ensuring that no illegal timber or timber products can be sold in the EU. It was created as part of the EU’s FLEGT Action Plan (EU, 2003) which now comes under the aegis of the European Union Timber Regulation (EUTR).

Within groyne construction schemes there are two methods of demonstrating legality and sustainability: „ Category A. The timber is supported by a recognised certification scheme such as the FSC or the Programme of Endorsement for Forest Certification (PEFC). Such third-party certification schemes verify suppliers’ claims, enabling them to state that they operate as part of an unbroken ‘chain of custody’ system from managed forest to certified end material. „ Category B. Other evidence of legality and sustainability.

Generally, timber groyne construction schemes that are supported with government funding should comply with the requirements of Category A proof. This is usually FSC certification which is the most common scheme for tropical timbers used for groyne construction. Other schemes for tropical timber exist but the species covered by them are not used for groynes. It should be noted that FSC certification does not automatically comply with the requirements of the EUTR although there is close alignment between the schemes. This means that FSC certification is a robust part of the necessary due diligence to ensure EUTR compliance when procuring tropical hardwoods but does not automatically ensure adherence to UK Government policy.

6.2.11 Health and safety Some hardwood species have sap and other components that are toxic, although this toxicity is often short-lived. While such timbers have a proven track record of being safe on public beaches, handling such timbers may be a risk factor for the workforce. Suitable training and appropriate personal protective equipment (PPE) should be provided to all operatives who come into contact with timber.

The HSE (2003) has published guidance on exposure to what is described as ‘toxic wood’. The following should be avoided: „ Exposure to saw dust as extractives in the dust can cause irritation to the skin (dermatitis), eyes (sore eyes, conjunctivitis) nasal passages (rhinitis, nosebleeds) and can exacerbate asthma. Nasal cancer can develop but this is rare and tends to be triggered by long-term exposure to dust. A dust mask should be worn when machining tropical hardwoods as the risks of inhaling fine particles are well documented. „ Splinters becoming embedded in the skin, particularly from Greenheart, can turn septic and are slow to heal. Bacteria present on the splinter can lead to septicaemia, while fine, sharp hardwood splinters, which are brittle, can penetrate the skin and easily break. These splinters can be more difficult to remove from the subcutaneous layer than softwood splinters. „ Resin latex or other organisms on the timber such as lichens and moulds can cause toxic reactions.

Toxic activity is species specific, so understanding and knowing species is important in establishing what the potential harmful effects may be when producing a risk assessment for working with timber. The reaction to certain species of timber is heavily dependent upon the sensitivity of the person exposed to it.

Groynes in coastal engineering 91 6.3 POLYMER COMPOSITES

6.3.1 Introduction The use of recycled polymers or plastics in groyne construction remains in its infancy at the experimental stage. The advantages of this material in groyne construction are its resilience to biological degradation, the abundance of recycled polymers (and the value to the circular economy of their use) and the relative ease and 0low cost of production. Recycled polymers are being increasingly implemented for various coastal applications, such as alternatives to sheet piling.

However, the use of polymer construction Figure 6.15 Polymer composite breastwork installed at material in the littoral zone has raised Pevensey Bay, East Sussex significant concerns that microplastics may be inadvertently introduced into the ocean when such groynes are abraded. For this reason, the use of polymers in high energy, shingle-dominated abrasive environments is discouraged.

One of the few locations where this technology has been trialled is in Pevensey Bay in East Sussex. Here, the polymer planking has been predominantly used as breastwork in a less dynamic region at the rear of the beach (Figure 6.15).

However, trial planking was installed in the most active intertidal section of one groyne in 2005, where the original timber plank had been dislodged (Figure 6.16) and showed reduced abrasion in comparison with timber.

Figure 6.16 Comparison of Greenheart and polymer abrasion on a groyne 6.2 Functional performance Polymer components of groynes can be produced with up to 100 per cent recycled plastics. Globally, high-density polyethylene (HDPE) is a commonly-recycled plastic. Once recycled it is a versatile material that can be used for many applications, including plastic lumber for groynes. Compared to other plastics, recycled HDPE offers enhanced resistance to environmental parameters including temperature fluctuations. The main characteristics of HDPE relevant to groynes are the following: „ tough material with high impact resistance

92 CIRIA, C793 „ no degradation in salt water „ contains UV stabilisers that have been estimated to provide a working life span of more than 40 years before the product will begin to degrade in sunlight.

It should be noted that recycled HDPE can exhibit lower strength and stiffness than using virgin HDPE. This is mainly due to the variety of sources of the raw material before reproduction. With both post- consumer and post-industrial waste being used, the plastics will have different performance depending on their degradation levels, due to the antecedent storage and reprocessing conditions.

A potential issue given the proposed exposed coastal setting of this material is that it will expand and contract more than timber with changes in temperature (for example, the thermal expansion coefficient of HDPE is about four times that of softwoods such as Douglas Fir) This means that extra consideration should be given to expansion gaps and oversized fixing holes when designing for the coastal environment. Groyne polymer material will usually be selected as being dark in colour, have a large surface area and are occasionally found in hot or sunny locations. These factors will all exacerbate the effects of temperature on the HDPE.

Given the environment in which groynes operate, water absorption and the consequent thickness swelling of polymer groynes are important considerations. (The key difference between HDPE and timber (even if levels of water absorption are similar between the two materials) is that water absorbed into timber is taken into the natural cell structure, which leads to less expansion.) While there is little difference between recycled HDPE and virgin HDPE, a high level of contamination from recycled HDPE with organic material can increase the maximum water absorption.

6.3.3 Agents of degradation There is a lack of research into the performance and subsequent degradation of polymer groynes. However, much like timber, this would be expected to vary significantly due to construction details, polymer type used and location within the groyne structure. Deterioration of polymer groynes is caused by fewer factors than their timber equivalent. However, while polymers are resistant to biological attack, the effect of mechanical abrasion could pose a wider risk than deterioration of the groyne itself.

Immiscibility (insufficient mixing to ensure homogeneity) of the polymer may occur during production. This incapability for the polymers in the plank to mix effectively can reduce durability in a coastal scenario because it subsequently reduces the polymer’s hydrophobic capabilities, making it more susceptible to water absorption and swelling.

Sunlight The mixed presence of prodegradants (carbonyl groups that absorb harmful light) in recycled plastics advances the photo-degradation process (Turku et al, 2017). However, as stated in Section 6.3.2, the added UV stabilisers still ensure a working life of more than 40 years.

Temperature Polymer groynes could be affected by thermo-oxidation or temperature fluctuations, which could lead to gaps appearing in the groynes or unforeseen damage to the structure. The effects of temperature fluctuation could be mitigated by using this material in combination with traditional timber, with the polymer components making up0 the lower, buried sections of the groyne.

Abrasion Although the durability of polymer groynes has been found to be good (see Section 6.3.2), observations of polymer planks on exposed sites where abrasion is a factor indicate that the surface will spall (Figure 6.17). This is proof that polymer particles are being released into the marine environment with concomitant environmental and sustainability implications (see Section 6.3.6).

Groynes in coastal engineering 93 Figure 6.17 Spalling of polymer plank at Withernsea, Yorkshire

Should polymer groynes be seen to have a high risk of environmental damage in a coastal setting, it is suggested that polymer groynes could be successfully implemented in estuarine environments, with a significantly reduced wave climate.

6.3.4 Material sourcing There are an increasing number of manufacturers making polymer composite products from recycled plastics in the UK. However, the lengths of planking required for groyne projects are substantially larger than most off-the-shelf offerings. Special moulds are needed to create these longer lengths which reduces the number of suppliers.

6.3.5 Lead-in time Lead-in times for procuring polymer groyne components can vary significantly, affected by the manufacture rate of the supplier, the volumes required and the transport time. Schemes that require comparatively long sections may require extended lead-in times (and upfront payment) to allow for the design of bespoke moulds. This requires appropriate forward planning to ensure the production of polymer components coincided with the programme.

Other schemes that require shorter, smaller polymer components may need less lead-in time, ie a few weeks, as these may be readily produced on a larger scale and held in stock. Overall, the relatively low use of polymers in groyne construction constrains most orders to bespoke design, incurring longer lead- in times. Lead-in time is hugely variable and as a general rule larger cross-sections, lengths and volumes require longer time – sometimes as long as 12 months.

6.3.6 Cost The cost of polymer components for groyne construction is now at a competitive level with timber. As with timber, some economies may be enjoyed by specifying shorter planks that may span single or double structural bays as opposed to three structural bays. However, these savings may be cancelled out by the requirement for more fixings, especially if stainless steel is used, leading to longer construction times.

6.3.7 Sustainability and environmental impact The sustainability of polymer groynes is an area that requires further research. Calculating average embodied carbon for polymers is highly ambiguous when compared to steel or concrete, due to the variety of sources of the recycled plastic. There are strong arguments for the future application of this material, with benefits including that it is created using recycled plastics so contributing to the circular economy.

94 CIRIA, C793 However, if it is proven that polymer groynes risk polluting the littoral zone with abraded microplastics, its use as a viable construction material for the intertidal zone may begin to fall. One immediate mitigation measure that can be employed is to avoid any on-site fabrication such as drilling or cutting which might result in the production of microplastic waste by-products that will remain on the beach or enter the ocean. The use of this material should focus on off-site fabrication where waste can be contained and recycled. If some on-site fabrication involving drilling or cutting is necessary, measures should be taken to collect and safely dispose of any fine plastic material.

6.3.8 Health and safety Polymers have a relatively low albedo, meaning more sunlight is absorbed than reflected by the material, this is exacerbated by the dark colours often used during pigmentation to give the plastic a more ‘wood- like’ effect. The most significant health and safety risk that results with use of polymer groynes is the temperature that the planks may reach in direct exposure to sunlight, posing risks to the public and making handling a challenge (requiring gloves to be worn).

The potential release of microplastics into the food chain is also a health and safety concern.

6.4 CONCRETE

6.4.1 Introduction Globally, concrete is one of the most widely used materials in construction due to its low cost, availability, formability and excellent structural and durability properties (Morris et al, 2002). However, the environmental conditions affecting maritime structures are more severe than for land-based structures. Concrete used in the coastal environment should be of high quality to resist deterioration and abrasion especially when the beach material is coarse (shingle or cobble). This is not easy to achieve and requires planning, skill and experience. Pre-cast concrete (see Section 6.4.3) may offer advantages as it is easier to deliver better quality control in pre-casting facilities. The specification of maritime concrete in the UK should conform to BS EN 206:2013+A1:2016 and BS 8500-1:2015+A2:2019 and all specification and construction aspects should follow the recommendations given in Dupray et al (2010). Observation of existing structures, concrete seawalls and/or groynes, local to the new groyne, noticing their performance, is often the best indicator as to the future performance of new structures.

6.4.2 Functional performance and agents of degradation Concrete is commonly used for coastal defence in the marine environment. However concrete, like all materials, will face significant abrasive forces when installed in a shingle, or to a lesser extent, sandy beach environment. Abrasion will reduce the thickness of the concrete cross-section, which should be fully considered in the design process. This is most critical for reinforced concrete structures, when abrasion is likely to lead to a direct reduction in the reinforcement cover thickness, and the onset of chloride-induced corrosion. This can be mitigated using larger concrete cover or, alternatively, by using polymer or stainless steel reinforcements. Extensive further information on the design of concrete groynes for the marine environment can be found in BS EN 1992-1-1:2004+A1:2014 and Dupray et al (2010).

6.4.3 Selection and sourcing Types of concrete are largely determined by the water to cement ratio, and cement combinations within the mix design. If steel reinforcement is used, concrete cover over the steel should be provided in accordance with the minima in design codes. This is due to the corrosion associated with chloride ingress. If mass concrete is used to avoid these issues, various measures can be employed to control cracking (Dupray et al, 2010).

Groynes in coastal engineering 95 Pre-cast concrete is made off site and transported for installation. An advantage of this type of concrete is the fast rate of construction assembly and improved quality control, due to a more controlled working environment in contrast to casting concrete in the coastal environment. This can bring overall cost savings and durability improvements compared to in situ concrete. However, these improvements are typically only made when there is a large degree of repeatability in the design, eg the use of pre-cast concrete planking or armour units.

In recent years, class two plastic reinforced fibre has been promoted for coastal concrete applications, due to its reduced carbon content and lower maintenance requirements when compared with traditional steel reinforcement. However, in the abrasive zone, fibres risk being released into the sea as microplastics. If considering use of this material, the level of risk and associated impact should be considered as part of the wider environmental impact of the scheme.

6.4.4 Concrete preservation and treatments Due to the abrasive setting of groynes and the high risk of chloride-induced corrosion use of structural reinforcement should be avoided in groyne structures where possible If reinforcement has to be used, concrete cover over the steel should be provided in accordance with design codes. Surface treatments to concrete, which potentially extend the period of time before the onset of steel reinforcement corrosion, should generally be avoided as the abrasive nature of the coastal environment will soon remove this protective layer.

6.4.5 Lead-in time Of all construction materials, in situ concrete has the shortest lead-in time. However, for larger schemes, it may be necessary to use multiple suppliers. If so, extra care should be taken to ensure uniformity in concrete quality. Where it is possible to make use of it, pre-cast concrete can help to maintain quality, but requires longer lead-in times.

6.4.6 Cost Cost considerations for constructing in concrete need to account for the following: „ all constituent materials „ extra volume as a reserve „ delivery „ additives „ offload/delivery/curing time „ pump hire.

6.4.7 Sustainability and environmental impact

Typically in the UK, about one tonne of CO2 is emitted in making one tonne of Ordinary Portland

Cement (OPC). This process of cooking the CaCO3 to create calcium aluminosilicates is suggested to account for 25 per cent of the carbon footprint associated with construction). However, the carbon impact of OPC can be reduced by combining other cementitious materials, for example, the by-products from the iron and coal power generation industry. These binders, such as ground granulated blast furnace slag (GGBS) and pulverised fuel ash (PFA) have a low embodied carbon content, making a significant difference to the overall carbon factor of concrete and also its overall sustainability. Table 6.4 illustrates differences in embodied CO2.

96 CIRIA, C793 Table 6.4 CO2e of UK cement, additions and cementitious material (from MPA, 2019)

Cement, additions and cementitious material (descriptions of the CO e materials are shown below) 2 Kg CO e/tonne Portland Cement CEM I: cradle to factory gate 8602 2 CEM I (From cradle to leaving the factory gate of the addiction manufacturer) Kg CO e/tonne Ground granulated blastfurnace slag (ggbs) 79.63 2 GGBS Kg CO e/tonne Fly ash (from coal burning power generation) 0.14 2 Fly Ash Kg CO e/tonne Limestone fines 8.05 2 Limestone

Weighted average cement (1). This is the weighted average of all factory- Kg CO e/tonne 820 2 made cements supplied by MPA Cement Member Companies in the UK cement

Weighted Average Cementitious: cradle to factory gate(2). Includes CEM I, Kg CO e/tonne 668 2 II, III, IV cements, ggbs and fly ash supplied in the UK cementitious

Notes 1 MPA Cement members are Breedon, CEMEX, Hanson, Lafarge Cement and Tarmac. Materials imported and sold by companies not manufacturing in the UK are not included.

2 The Weighted Average Cementitious CO2e of the individual cementitious materials, ie CEM I, CEM II, CEM III, CEM IV and additions, weighted

by the relative tonnages of each supplied in the UK. It is a representative number to use to address the CO2e of concrete elements at the design stage where it is not possible to identify or specify a particular cement or equivalent combination.

At the end of the groyne design life, the concrete can be crushed and recycled into aggregate for alternative engineering applications.

6.4.8 Health and safety Most health and safety considerations specific to concrete occur during the construction phase. These include: „ eye, skin and respiratory tract irritation from exposure to cement dust „ inadequate safety guards on equipment „ chemical burns from wet concrete.

Health and safety hazards associated with concrete groynes are often related to old, poorly maintained structures, which can result in exposed, sharp reinforcement bars. Adequate ongoing inspection and maintenance for public safety risk assessments is key to managing risks (see Environment Agency, 2018).

6.5TEEL S

6.5.1 Introduction Steel is only occasionally used as the primary material for groyne construction on beaches. However, large steel sheet-piled structures that are acting as a terminal groyne on a frontage, can be found at sites around the UK. From both an aesthetic and performance-related perspective, other materials may be more favourable. However, steel is frequently used for the fixings for timber and polymer groynes, and as the reinforcement within concrete.

6.5.2 Functional performance The steels commonly used in maritime structures, which includes groynes, are weldable structural steels (BS 6349-1-4:2013). This steel is susceptible to corrosion, the rate of which is increased in a marine environment compared to inland.

Groynes in coastal engineering 97 Stainless steel can have a higher resistance to corrosion so is more suited for use within the marine environment. However, the cost of stainless steel is often prohibitively expensive for use in large structural sections, so its use in the construction of groynes is generally restricted to fixings, such as coach screws or bolts (see Section 6.2.6).

Potential solutions to reducing the risk of corrosion (BS 6349-1-4:2013) include: „ use of a thicker steel section, a higher grade of steel or an increase in pile thickness locally through the addition of steel plates „ use of a high-quality protective coating „ use of an electrical bonding system and cathodic protection „ optimisation of the design to ensure that high bending moments do not occur where corrosion rates are highest.

6.5.3 Agents of degradation The degradation of steel can vary considerably, depending upon the location and conditions prevailing at the location of the structure. This is particularly apparent in coastal waters.

All metals suffer from corrosion in the marine environment. BS 6349-1-4:2013 notes corrosion rates can be typically 0.5 mm/side/year. However, it should be recognised that the corrosion rates for steel can often be greater because of the ‘shot blasting’ effect of sand and shingle that occurs on each tidal cycle. This repeated removal of the corrosion layer culminates in a near-constant exposed fresh surface with no layer of corrosive protection, which would tend to slow degradation rates in a less active marine environment. BS 6349-1-4:2013 suggests that in highly active environments, corrosion rates can increase towards 0.8 mm/side/year. A particularly aggressive form of corrosion is accelerated low water corrosion in steel maritime structures (Breakell et al, 2005), although where steel is used in groynes, the prevailing conditions at low water burial or abrasion may not be conducive for this phenomenon.

6.5.4 Lead-in time Securing delivery of steel requires proper consideration of lead-in times. Even though most steel construction uses standard sections, availability of the desired section in the required quantity may be limited. It is advised to contact potential suppliers to obtain an indication of the likely lead-in time required. Advance purchase of steel materials by clients is often undertaken to mitigate the risks of delays in supply.

6.5.5 Cost While comparing costs of different materials is rather ambiguous, steel is typically more expensive than timber. Cost-related aspects to consider when procuring steel for groynes include: „ galvanisation/treatments „ logistics „ market conditions „ sizes/quantities required.

6.5.6 Sustainability and environmental impact

On average, 1.9 tonnes of CO2 are produced per tonne of steel produced globally, with over 90 per cent of emissions from the steel industry coming from iron production (Kundak et al, 2008). However, steel is the most recycled industrial material in the world, with over 500 Mt recycled annually (World Steel Associated, 2012). The initial production of steel requires non-renewable, finite resources such as iron ore and coke (created by heating coal in oxygen deprived environments) the continued depletion of which is not sustainable. However, unlike other materials, steel is unique as there is no downgrading

98 CIRIA, C793 when it is recycled. So, following initial production it can be used indefinitely, providing any marine preservations or treatments are removed before the recycling process.

Research currently suggests that new steel beams consist of at least 80 per cent recycled steel. When scrap steel is recycled, significant energy and raw materials are saved, equating to about 1400 kg of iron ore, 740 kg of coal, and 120 kg of limestone for every tonne of steel scrap made into new steel. However, due to the longevity of steel products and its durability in a variety of industrial applications, at the time of writing, there is not enough scrap steel to meet the demand for new steel. Sustainable in Steel: https://www.sustainableinsteel.eu/

6.5.7 Health and safety Steel groynes can pose a significant risk on amenity beaches. Corrosion can lead to the creation of sharp edges and uneven surfaces on the groyne. While the largest risk may be an impact injury, from falling into or against the steel, tetanus is also often associated with steel corrosion. Despite popular beliefs, it is not rust itself but the bacteria Clostridium Tetani, which can reside in rust that transmits tetanus. However, for this reason corroded metal structures often resonate poorly with the public and are not advisable on beaches with even when recreational value of the beach is low.

Groynes in coastal engineering 99 7 Structural design of groynes This chapter covers all types of groynes, but gives most attention to narrow footprint groynes explaining the design of each component, including both aspects that relate to structural strength and those that relate to maximising whole-life durability (Figure 7.1). The chapter explains how to determine the structure outline (including the bottom profile) and how to design and select the structural components and details including fixings. Ancillary features and the approach for design for adaptation and for minimising health and safety risks are also discussed. The main sections of this chapter are:

„ Groyne types (Section 7.1) „ Groyne bottom profile/embedment Section( 7.2) „ Outline groyne structure design (Section 7.3) „ Groyne structure design details (Section 7.4) „ Lowest and highest beach profiles Section( 7.5) „ Groyne ancillary features (Section 7.6) „ Designing for maintenance and adaptive management (Section 7.7) „ Health and safety aspects of design (Section 7.8) „ Environmental enhancements (Section 7.9)

Coastal management Functional requirements and context indicators (Section 2.1) (Section 2.1)

Physical coastal environment – hydraulic, geotechnical and morphological constraints (Chapter 3)

Start or continue No Groyne removal to use groynes? (Section 9.4)

Yes

Plan shape design of groyne field (Chapter 4) Evaluate and choose groyne type (Section 2.2) and material (Chapter 6) Design groyne profile (Chapter 5)

Structural design or design review for groynes (outline and detailed) (Chapter 7)

Construction and/or major refurbishment Major (Chapter 8) Minor Groyne repair and Groyne adaptation (Section 9.2) maintenance and adjustments (Section 9.3) (Section 10.3)

Small Beach changes Routine monitoring Large (Section 9.1) (Sections 9.1 and 10.2) Structural deterioration (Section 10.1) Post-incident monitoring (Sections 9.1 and 10.2)

Climate change Storm events (Section 3.1.4)

Figure 7.1 Parts of groyne management process covered in Chapter 7

100 CIRIA, C793 The approach for the structural design of groynes should be based on several principles: 1 Use of existing prototypes. The existing or an existing nearby groyne field provides a full-scale working model for which the as-built drawings can be consulted. This provides the best basis for understanding how the groynes are working in their particular context. From this, both positive and negative lessons can be learnt. 2 Use of comparative frameworks. A large number of groynes have been built and managed in a wide range of situations in the UK (Figure 2.1 and Table 2.1), the experience of which has provided valuable input to this guide. However, it is always useful for design and construction engineers and groyne managers to discuss their situation with counterparts who may have had similar or different experiences to gain more understanding of the function and performance of different types of groyne in different settings. 3 Understanding loading unpredictability relating to operating conditions. Beaches are morphologically dynamic and can be subject to significant variations. As a result, the hydrodynamic loading on groynes can vary considerably. (Some general information on assessing wave conditions may be found in Section 3.1. and information on the specifics of assessing hydrodynamic loadings on narrow footprint groynes in Box 7.2.) 4 Use of simplification. Given the operating conditions, the simplest design is often the most appropriate. It not only facilitates construction and maintenance, but also has the potential to reduce wear and tear. 5 Use of standardisation. With the typical repetitive nature of groynes, it is recommended to maintain standard construction details where possible. This leads to efficient design as well as ease of construction and maintenance.

After the activities to define the groyne layout and profile discussed in the previous chapters, there are seven further iterative stages to the design process that cover the following: 1 Selecting the required groyne type. 2 Selecting the method of groyne embedment for groyne stability. 3 Outline groyne structure design. 4 Structure design details. 5 Ancillary features. 6 Design for maintenance and adaptive management. 7 Design for health and safety risk management.

At each stage in the design process, it is necessary to be mindful of the previous stage(s) and/or subsequent stage(s). ‘Looking back’ may prompt the need to revisit an earlier decision due to new information, and ‘looking forward’ is necessary to consider the impact of aspects of the design process.

7.1 GROYNE TYPE SELECTION

For each type of groyne (see Section 2.2), there are common materials that have historically been used in their construction, ie timber (piled groynes), concrete or masonry (gravity groynes), and rock (bulk groynes). However, it is recognised that alternative material types are available, and the principles outlined in the following sections are broadly transferrable between alternative material types.

In selecting the most appropriate type of groyne the following factors need to be considered: „ The existing situation: current type of on-site groynes and/or the type of groynes at the nearest site performance of existing or nearby groynes beach management policies and practice. „ The required height of the groynes (see Section 5.4). „ Allowances to accommodate future change: sea level rise future lowering of the substrate level due to erosion.

Groynes in coastal engineering 101 „ Public interaction with the groyne field: requirements for access onto all sections of the beach, including over or through the groynes where necessary (see Section 7.6.5) health and safety issues. „ Any positive or negative impacts the groyne design may have to consider with respect to flora, fauna or the environment in general. „ Construction requirements (see Chapter 8) including any need for underwater construction. „ The estimated initial capital and whole-life cost of the groyne.

7.2 GROYNE BOTTOM PROFILE/EMBEDMENT

7.2.1 Piled groynes At their simplest level piled groynes generally comprise king piles driven or fixed into the substrata with planking or panels spanning between the piles. Some groynes have horizontal piles planking installed all the way down to substrate level. However, sometimes the planking stops before the substrate level and this has been the cause of groyne failures (see Section 9.2.1). For this reason, some groynes have additional vertical timber sheet piles or ‘sheeters’ (see also Section 8.3.6) between the king piles below the level of the horizontal planking, driven or installed as panels down to the substrate. The embedment/ fixity of the king piles (together with any ties or struts) provides the stability against failure by overturning (see Box 7.1). The depth of embedment is principally influenced by the lowest likely beach profile or, where the piles are founded in the substrate, by the profile of that substrate. If the substrate is used for the foundation allowance should be made for any erosion of the substrate by designing the embedment to a lowest likely substrate level.

Box 7.1 Rotational failure of groynes under load

Figure 7.2 Overturned groyne due to beach loss and Figure 7.3 Updrift loading of timber groyne causing short piles groyne to deform When longshore transport is predominantly in one direction, reduced structure stability may arise either from large differential weight loadings between updrift and downdrift sides of groynes or when beach levels drop to expose a large area of groyne to severe wave loading. In this situation, failure is likely to occur for one or more of the following reasons:

„ insufficient penetration of the piles „ piles are installed mainly in mobile beach material leading to loss of support and/or buoyant uplift of short piles „ when unexpectedly large wave loads are applied to one side of a structure. Figure 7.2 shows an example structure where it is believed that the piles have been driven primarily through beach material, with only a short section penetrating the substrate. The piles are too short to prevent overturning under high wave loading. The combination of lateral wave forces and the uplift of shallow piles from the structure have caused the structure to collapse, requiring subsequent full reconstruction. Other causes of rotational failure include loss of cross-section of the king piles (necking) at critical locations, perhaps due to gribble attack or abrasion on their lee side. Figure 7.3 shows a structure with a large differential cross-section of sediment from one side to the other. The weight of material is causing the structure to deform, despite the stabilising ties which are visible towards the seaward end of the structure.

102 CIRIA, C793 However, if there is no suitable quantity of available beach material to place either side of a newly- constructed piled groyne then the above situation may prevail until beach nourishment or recycling is undertaken. The construction programming needs to be considered during the design process and/or requirements included within the specification for the construction contract to mitigate the risk of the groyne being left vulnerable to significant wave attack that has not been allowed for in the design.

Given the various complexities and uncertainties, a suitable method for determining the embedment depth is as follows: 1 Use a ‘retaining wall’ calculation (see Gaba et al, 2017) as the initial assessment for pile embedment based on highest/lowest likely beach profiles at various locations along the groyne. The initial design calculations for stability should be based on the groyne acting as a static retaining wall for beach material of the appropriate density and saturation state. The retained height will be the difference between the highest likely beach profile and the lowest likely beach profile (see Section 5.3) or substrate profile (see also Section 3.3.4) at various locations/cross-sections along the length of the groyne. Although the piles will be embedded into potentially mobile beach sediments the beach can be assumed to re-consolidate under natural processes, following the installation of the planking and/or any sheet piles. 2 Check the as-constructed drawings of existing on-site or nearby groynes. Consider any visible modes of failure and similarities/disparities between older structure and the proposed scheme. 3 Depending on the disparity, run some hydrostatic and wave loading calculations using appropriately simplified, realistic scenarios (see Box 7.2). 4 Consider if ties, or less preferably props (see Section 2.2.1), are needed to ensure the stability against overturning. As discussed in Section 2.2.1, ties are to be preferred, being buried in the accreted material on the updrift side of the groyne and not exposed to abrasion. However, the predicted tensile forces in the ties and their connections may be so great that stabilising bracing should be provided instead as struts on the downdrift side and acting in compression. 5 Take a view on the operational context, levels of risk etc that may apply. 6 Using good judgement, identify an appropriate solution based on these considerations.

Box 7.2 Hydrostatic and wave loading scenarios for groynes

Detailed calculations for stability should be based on low beach levels on both sides of the groyne, and the groyne being exposed to wave and differential hydrostatic loadings. Wave and water level conditions at the groyne structures should be determined following the guidance in Section 3.1.1. Hydrostatic and wave forces can be calculated following the guidance in ISO 21650:2007 and in Dupray et al (2010). Depending on water levels, waves may be breaking, and in theory, breaking wave loadings can be as high as 20 KN/sqm. However, impulsive wave forces against the exposed side of a groyne (which can be either side of the groyne depending on the direction of the incoming waves) are often limited because the waves have a low incident angle of attack, ie the wave fronts are nearly perpendicular to the alignment of the groyne. So, even though the angle of incidence may be small, differences in beach levels on either side of the groyne and the effects of diffraction on the more sheltered side may mean that the wave crests and troughs will become out of phase either side of the structure. The result will be significant hydrostatic loadings from the difference in water levels from one side of the groyne to the other. This will be of most concern when significant loss of beach material has occurred and there is a large area of planking exposed on both sides of the groyne, over a significant percentage of the groyne’s length. In practice it may not be appropriate to design the groyne based on the maximum exposed height, maximum wave loading and worst case for hydrostatic loading, for the following reasons:

„ For a groyne to be left at its full exposed height for any length of time represents poor beach management and is an unrealistic worst case. „ When assessing wave forces, the conditions to be used are those prevailing at the point of the structure being considered. In most cases, it may be assumed that wave pressure as well as wave height is depth limited (see Section 3.1.1). „ A breaking wave front only affects part of the groyne at any one time, not the full length and rarely has a significantly ‘steep’ incident angle. „ A breaking wave represents a momentary dynamic load rather than a sustained static load. „ In a dynamic environment there are inherent difficulties in determining the net impact of hydrostatic loadings.

Groynes in coastal engineering 103 The estimate of the embedded lengths of the various components of a piled groyne is achieved by considering the aggregate effects of main elements. For a timber groyne this is likely to consist of its king piles, plus timber sheet piles or permanently buried planking below the lowest likely beach level. Timber sheet piles are often used in the more seaward part of a timber groyne where the installation of planking can be deemed impracticable due to tide levels. The sheet piles are embedded in the beach, their tops are cut off immediately above the lowest likely beach or existing substrate level and then attached to a waling. The use of permanently buried planking to contribute to the lateral stability of the groyne, only applies where any permanently buried wave-cut substrate (see Section 3.2) lies below the required piles’ embedment depth.

Where groynes are constructed on top of or within a wave-cut-platform substrate, the integrity of the substrate needs to be carefully assessed as a suitable foundation before further consideration of the design proceeds. Where there is a ‘hard’ substrate (ie not suitable for piling), options include: „ planting king piles into: pre-drilled sockets in the substrate and plank down to substrate level concrete base cast onto or into substrate and plank down to top of concrete base „ installing king piles and sheet piles or planking down to substrate level and stabilise by means of ties and/or props.

Although king piles provide some resistance to overturning, it is often the ties or props that give most of the resistance.

When a substrate level is used as the founding stratum of a groyne, the lowest planks should be installed tight against the underlying substrate material beneath the beach. Difficulties in applying this principle include the following: „ The nature of the underlying bed material. If the bed is hard, it may not be practical to machine planks to a variable depth to reflect the variation in substrate level. In some cases, the bed material may be sufficiently soft (eg clay) that the material can be excavated to allow the planks to lie along the surface. However, care is required here because clay beds beneath beaches may be subject to further foreshore lowering. „ The depth of mobile material above the bed material is so great that the structure cannot be practicably closed out onto the substrate. In this case the approach should be to set the lowest planks at such a depth in an excavated trench that they are unlikely to be exposed based on the known fluctuations in beach level (see Section 9.1). Figure 7.4 illustrates this approach. In this case a cofferdam was installed at the seaward end of the groyne to facilitate the necessary dewatering of the excavation.

Figure 7.4 Groyne construction at Bournemouth, Dorset (courtesy David Harlow)

104 CIRIA, C793 It needs to be recognised that if the substrate is expected to be exposed on either side of the groyne on a regular basis, then safeguards need to be taken to cater for future change to avoid underrun (see Section 9.2.1) of beach material. Erosion/abrasion/solution of the substrate should be anticipated and allowed for. Some substrates, such as chalk, can suffer more significantly from animals burrowing into the wave-cut platform and to a lesser extent from chemical weathering by loss of some of their constituent minerals by dissolution into the water column as well as, more significantly, by abrasion.

The rate of reduction in substrate level needs to be thoroughly investigated and quantified, including examining any available historic records. To ensure that the groyne can maintain its stability for the design life of its component materials, and to ensure it remains effective as a groyne, the lower profile of the planking or sheet piling may need to be modified, possibly on a number of future occasions, to account for the lowering of the substrate.

7.2.2 Gravity groynes (wide footprint) Any gravity structure needs to be founded on, or more likely within, a sound substratum, eg a wave-cut- platform. As discussed in Section 7.2.1, it needs to be recognised that if the substrate is expected to be exposed on a regular basis, allowance should be made for the anticipated erosion/abrasion of the substrate.

The interface between the substrate and the groyne provides resistance against bearing and sliding.

7.2.3 Bulk groynes (wide footprint) Bulk groynes, which are typically constructed in rock, can be founded on mobile beach material or substrate depending on the relative levels of the lowest likely beach levels and the substrate profiles. The mobile beach materials may need to be fully excavated to provide a firm foundation for the structure.

A significant issue for bulk groynes is the avoidance of the undermining of the structure, whether by scour of mobile beach sediments or loss by abrasion/erosion/solution of the substrate, which may cause movement of the rock and lowering of the groyne profile.

7.3 OUTLINE GROYNE STRUCTURE DESIGN

This step involves producing an outline design for the groyne or groyne field. The outline design should include: „ Type and material for the groyne(s). „ Location of each groyne (Section 4.1.1 addresses planning and layout of groynes). „ Outline elevation of each groyne. This will include both top and bottom profiles for the groyne(s). The selection of appropriate groyne crest levels is described in Section 5.4. Base levels are dependent on embedment design, which is discussed in Section 7.2. „ Typical cross-sections and main elements: These are described in the following sub-sections.

7.3.1 Piled groynes (narrow footprint) Typical cross-sections and main structural elements can include (Figure 7.5): „ king piles „ anchor piles „ ties or props „ sheet piles „ waling(s) „ planking.

Groynes in coastal engineering 105 Sheet piles are the most practicable way of achieving the required bottom profile of the groyne close to the low water line where excavation into saturated sand is usually not possible within the very short low tide tidal window. Sheet piles provide a more efficient way to achieve the desired bottom profile for the groyne into the substrate than use of permanently buried horizontal planking. However, they are more vulnerable to the effects of loss of support due to erosion of beach substrate (see Sections 2.2.1 and 9.2.1)

Waling beams act as longitudinal structural members with the purpose of distributing loads from retained beach material, local wave loading etc along the groyne to the closest tie or strut. Waling beams are also used to ‘help to pull’ the piles into a straighter line, which ensures that the planks sit more ‘fully’ onto the face of the piles to which they are fixed. While walings are not always necessary, in many scenarios they help to maintain the resilience of the structure to local failure.

Planking/panelling acts as the groyne’s ‘cladding’, to hold beach material within a groyne bay. It also provides the local distribution of load between the piles. It is vital that the end joints between planks are staggered between the various horizontal/ sloping rows of planking. This spreads the risk of localised failures due to all plank joints being on a Figure 7.5 Example of a timber groyne section single pile where the bearing of the plank on the pile is only half the width of the pile. In this scenario, all fixings are close to the end of each plank and may fail more easily as the strength of the timber reduces over time.

The plank width is determined by two main considerations, the width and thickness of suitable timber being readily available from suppliers, and the width necessary to have two fixings into the king piles at each pile. If planks are too narrow for a given thickness, they will tend to warp more readily, potentially causing fixing and sand/shingle leakage issues. If the fixings through the planks, into the piles, are too close together they will threaten the longevity of the fixings, as loading may instigate cracking between the drilled holes and cause failure of the planking. Ideally, two adjacent fixings of a plank should avoid being in the same ‘grain-line’ in either the planks or pile, as this can initiate splitting which will form an undesirable weakness in the structure.

King piles provide the main structural support of the groyne, with ties (and props) where needed. Typically, pile spacings are of the order of 2 m for timber groynes. The spacing will depend on a few factors, including: „ The maximum height of the beach material to be retained. This will control the maximum span between the piles that the lowest plank, of the selected/available plank thickness and material, can accommodate. „ The characteristics of the subsoils into which the piles are to be embedded and the maximum height of the beach material to be retained, will influence the length of the piles as well as the spacing. „ The thickness, or diameter of the king pile. This will influence the loading capacity of the king pile and also the spacing required for stability.

106 CIRIA, C793 The optimum balance between spacing of piles is influenced by the thickness of planking/panels, the soils into which the piles are to be driven and the size of the waling(s) to span between tied piles.

Ties, or less usually props (see guidance in Section 2.2.1) are needed when the king piles cannot alone resist the lateral loads being applied to them, due to the required maximum height of the beach material to be retained.

The ties (or props) can be varied in slope angle and length. A typical slope is 1 in 2.5, and a typical length (between main and anchor piles) is 4.5 m. However, the optimum slope angle and length will depend on achieving a structurally efficient arrangement.

Anchor piles (Figure 7.6) provide the anchorage for the ties (or props). For timber groynes, spacings are typically between two and four times that for the king piles depending on their loading. Ideally the pile heads are kept as low as possible but there is an optimum balance between the length/head-head height of the pile, the slope angle and length of the tie, and the height of the connection between the tie and the king pile. The resulting structural configuration is a propped cantilever.

Figure 7.6 Example of timber groyne anchor tie

There are many variants that have been adopted to the solution given in Figure 7.6. Ties may be attached to the king piles or with braces in between piles, and ties and struts/props might be made of tree trunks, square timbers, steel. Examples of variations are given in Figure 7.7.

Groynes in coastal engineering 107 a b

Figure 7.7 Examples of alternative forms of ties and props. Steel tie/prop adopted at Dawlish (a) and short timber prop bolted into supporting pile at Herne Bay, Kent (b) (courtesy John Williams and Uwe Dornbusch)

7.3.2 Gravity groynes (wide footprint) Cross-sections and main elements may comprise: „ foundation section, which provides the interface/connection with the substrate „ wall section, which provides the operational aspect of the groyne.

7.3.3 Bulk groynes (wide footprint) Cross-sections and main elements can include: „ foundation/filter layer „ core „ secondary armour layer „ primary armour layer.

The foundation/filter layer provides an interface between the main structure and the underlying formation, whether beach or substrate. A foundation layer primarily helps to distribute loads where the formation is weak, and a filter layer helps to reduce the loss of fines from a non-cohesive formation. The core provides the main bulk of the groyne, and the armour layers protect the core material from movement or losses as a result of wave action.

7.4 GROYNE STRUCTURE DESIGN DETAILS (PILED NARROW FOOTPRINT GROYNES)

This section describes key considerations for the detailed design of timber piled groynes, some aspects of which link to constructability (see Section 8.3.1). Detailed design should cover all elements, dimensions, materials, surfaces and interfaces.

The principle elements are described here: „ pile/post head rings (see Section 7.4.1)

108 CIRIA, C793 „ pile toes and shoes (see Section 7.4.2) „ joints (see Section 7.4.3) „ fixings (see Section 7.4.4).

7.4.1 Pile/post head rings For driven piles, it is good practice to provide and fit a mild steel pile ring (Figure 7.8) to prevent the top of the pile from splitting during installation/pile driving. To ensure a competent fit, the ring should be slightly tapered so that during piling it achieves a tight fit. On completion of driving it is good practice to leave the ring in place because it helps to prolong the life of the pile by preventing long-term splitting of the timber, due to continual wetting and drying of the end grain. Pile rings are normally circular for rectangular cross-section piles and it is essential that the top of the pile is correctly prepared to allow the ring to be driven further onto the pile without restriction or causing damage to the pile.

Where a (square) ‘post’ is ‘planted’ rather than driven, it is still common practice to fit a ring. Square ‘rings’ can be used which reduce the amount of timber preparation, but they need to be a very tight fit if they are to have a beneficial effect on reducing the rate of degradation of the pile head. As they are not driven into position in the same way as pile rings, nail Figure 7.8 Pile ring fixings may well be required.

7.4.2 Pile toes and shoes Pile toes need to be shaped at least to a blunt point to facilitate driving through beach material and into the substrata. The points need to be symmetrical to reduce the risk of piles ‘wandering-off’ position during driving.

For harder substrata a cast iron symmetrical pyramid- shaped shoe (Figure 7.9) is typically attached to the toe of the pile. It is essential that the shoe is securely fixed to avoid any detachment during driving which would also cause the pile to deviate from position. Where the hardness of the substrata is marginal, it is advisable to fit pile shoes as a precautionary measure.

Figure 7.9 Pile shoe 7.4.3 Joints For all practical purposes there are four main types of joint: 1 Butt joints (Figure 7.10) are the simplest and most common. They involve a simple contact face between two members without any localised shaping of the timber. They can only act in compression. 2 Notched joints (Figure 7.11) are where one member is locally shaped in order to receive a second member. The main application is at the interface between a tie (or prop) and waling beam(s), especially where it is necessary to achieve a ‘neat’ and efficient connection. Otherwise notched joints are generally avoided because they are relatively intricate and slightly weaken the notched member.

Groynes in coastal engineering 109 Figure 7.10 Butt joint Figure 7.11 Notched joint 3 Scarf joints (Figure 7.12) are an in-line connection between two members of the same section size. The purpose of the joints is to maximise the ability of the joint to transfer loading across the connection. Scarf joints are often used on waling beams. 4 Plated/bracketed joints (Figure 7.13) are where galvanised or stainless steel plates or angles are used. They may be used when it is necessary to achieve a higher strength connection than can be achieved by a basic timber and bolting connection. Plates can be used to reinforce scarf joints particularly where significant bending and/or tension loading is anticipated. Otherwise they can be avoided because they are less flexible in accommodating changes that may arise during construction and are more complicated to form during construction. Plated connections between planks away from the locations of the supporting piles are not recommended because they introduce points of weakness into the structure (see Section 10.3.4).

Figure 7.12 Scarf joint Figure 7.13 Plated/bracketed joint 7.4.4 Fixings Both static and dynamic forces acting on planks and fixings should be considered in the design of fixings.

Even on short groynes (eg 20 m long), constructed to a shallow depth, there are likely to be upwards of about 150 fixings per groyne. The number of fixings becomes a significant cost consideration for both materials and labour, particularly in terms of maintenance (see Sections 10.3.3 and 10.3.4). Stainless steel fixings may be more expensive but they offer a longer service life, longer periods between maintenance and can be reused (see Section 6.2.6). They also allow the opportunity to recycle the timber as their removal is less likely to cause damage. There is a risk that timber groynes cannot be dismantled or even repaired without damaging timber components beyond reuse if non-stainless steel materials are specified.

There are two main types of fixing employed with timber groynes: 1 Galvanised or stainless steel bolts (mild steel is not recommended) should normally be used to make main structural connections. They can also be used for planking where high tensile loads are anticipated. To avoid localised damage/weakening of the timber and to help distribute the load

110 CIRIA, C793 transfer from timber to bolt, washers are invariably used under the bolt head and the nut. Where there are high shear loads, large diameter bolts are used for their larger surface area to bear on the timber, their larger cross-sectional steel area and their greater resistance to bending. Whether using coachscrews or bolt and washer connections, localised compression damage to the timber may be offset by suitably-sized washers to distribute the forces. Washers may be square or circular. Experience at some sites suggests square washers perform better than round washers as they are less likely to rotate and pull through the timber when fixings are tightened (Figure 7.14). The use of stainless steel bolts on fine sandy beaches may be less successful than on coarse grained shingle beaches. While the nuts do not corrode, it is often impossible to unscrew them due to sand grains working their way into the threads. The need to cut the bolts to remove them means that there is a significant loss of expensive hardware in these circumstances.

Figure 7.14 Bolts fixed with square washers (courtesy New Forest District Council)

2 Galvanised or stainless steel coach screws should normally be used for fixing planking to the piles, provided that the connection is in compression (see Box 7.3). They are a fast and efficient way of making such connections but require a high standard of workmanship. However, they are not as suitable for main structural connections due to their greater reliance on workmanship, with the risk of pull-out due to failure of the timber and their lack of ‘visibility’ as a simple through- connection. Use of coachscrews has the advantage of allowing the planks to be tightened more fully onto the piles. This minimised gap can also help to reduce the localised risk of gribble attack (see Section 10.3.3).

The service life of coachscrews in a high hazard abrasion zone at the beach/groyne interface can be extended by countersinking the head into the timber so it is better protected from the hazard of abrasion and make further maintenance easier. The example in Box 7.4 exemplifies work undertaken at Milford- on-Sea aimed at extending the service life of connections.

Groynes in coastal engineering 111 Box 7.3 Coachscrews vs. bolts and nut fixings for attaching planking to piles

The choice between bolt and coachscrew will be taken at the design stage although maintenance works may lead to a change of fixing method. It is common for planks to be fixed with one bolt per pile whereas two coachscrews are necessary. Experience suggests that coachscrews, which are installed from the updrift face of the groyne, are to be preferred. Coachscrews do not protrude on the downdrift elevation of a groyne. They contribute to faster and more efficient construction as the installation team only needs to work on the updrift elevation of the groyne. This is desirable on sites with a narrow tidal range as the working time is reduced for planking the groyne. In contrast, Figure 7.15 Example of abrasion stripping the thread of a bolt. The double time and installation costs for nutting reduces the risk of the bolt working loose at Bournemouth, Dorset complicated fixing details associated with bolts and nuts can be much greater. Coachscrews can also provide tighter fixings and can be wound against the timber and this can straighten out any distortion in the planks. They also rarely work loose with time and by remaining buried within the timber, they suffer much less from abrasion so they can be reused. By contrast, the looser fitting of bolted connections, which require slightly oversized holes to be drilled so the bolt can be fed through the hole, can encourage movement in the void as the planks are exposed to wave impact. This movement causes localised erosion in the void, known as socketing, which, as it progresses, allows movement to be magnified and this increases the risk of the plank breaking free from the groyne. The combination of a loose connection and socketing increases the risk of attack by gribble, the widening gap between the pile and plank providing a sheltered refuge which gribble may exploit. Bolts should be considered as an alternative to coachscrews where there is a risk that the direction of drift may permanently reverse, because the coachscrews may be put into tension for long periods of time. If bolts are used in this situation, lengths should be chosen so that very little thread is protruding after tightening. Where hewn piles have been used, there will be a need to have a range of lengths available, as a 230 mm x 230 mm pile may have a cross-sectional tolerance of ± 25 mm. Drilling for bolts (where the original hole is not suitable) should be with a drill to give a tight fit (to avoid wet voids) and done from one direction only (so that the bore is straight). Suitable dimensioned washers should be used. Double nuts (Figure 7.15) should be used to reduce the risk of nuts working loose under wave action, especially where abrasion is likely to strip the thread off the bolt. In terms of materials, stainless steel bolts are to be preferred as mild steel and galvanised bolts will corrode and eventually loosen the connection.

Box 7.4 Modification of fixing detail in an abrasive environment at Milford-on-Sea, Hampshire

a b

Figure 7.16 Coachscrew details, proud of surface (a) and countersunk (b)

Failure of fixings is commonly caused by abrasion. This is more of a hazard on shingle beaches with a small tidal range. Design detail can influence the life of a groyne and affect the frequency and intensity of maintenance. The length of intervals between maintenance cycles and service life may be extended by improving detailed design. Countersinking coachscrews into the timber reduces exposure to abrasion. Figure 7.16 compares the performance of stainless steel coachscrews after about five years in service. The wear on the countersunk coachscrew in considerably less than the coachscrew proud of the timber surface. Although there is an additional cost for the initial installation, the increased lifespan of these fixings may be significant, reducing life cycle costs.

112 CIRIA, C793 7.5 GROYNE STRUCTURE DETAILS (WIDE FOOTPRINT GROYNES)

This section describes key detailed design considerations for gravity and bulk groynes, including the most commonly used material for each groyne type, ie gravity concrete groynes and bulk rock groynes. However, many of the considerations can be applied when using other materials in the construction of these groyne types. As for piled groynes, the detailed design should include all: „ elements „ dimensions „ materials „ surfaces „ interfaces.

7.5.1 Gravity concrete groynes (wide footprint) Guidance on the design of concrete structures is covered in several standard and guides including BS 6349-1-1:2013, BS EN 1992-1-1:2004+A1:2014, as well as BS 8500-1:2015+A2:2019.

The detailed design can include: „ concrete „ reinforcement „ ‘anchorage’ dowels „ joints.

As mentioned in Section 6.4, it should be noted that concrete structures located within a shingle/cobble beach area can be vulnerable to abrasion of the concrete. Observation of existing structures, concrete seawalls and/or groynes, local to the new groyne, noting their performance is often the best indicator to future performance of new structures, unless something has changed.

Concrete Should the structure be formed from reinforced concrete, then the cover to the reinforcing steel needs careful consideration. The specified compressive strength and mix design of the concrete is also vital.

Reinforcement This should follow standard practice for steel reinforcement in the marine environment (see Section 6.4). Consideration should be given to the potential use of fibre reinforced mass concrete as an alternative to avoid the risks associated with corrosion of reinforcement, providing the risk of release of microplastics from the fibre to the marine environment by abrasion is managed.

Anchorage dowels These may be needed at construction joints and/or as additional anchorage if the natural ‘roughness’ of the founding-surface is assessed to be poor. The use of stainless steel is advisable for use within and at the base of the groyne structure.

Joints These should follow common practice for construction and contraction joints, eg construction joints at 5 m to 6 m spacing with expansion joints at 15 m to 18 m spacing. Keyway joints should be included in all joints to help minimise the risk of water and beach material passing through the structure.

Groynes in coastal engineering 113 7.5.2 Bulk rock groynes (wide footprint) Detailed guidance on the design of rock structures is available in CIRIA, CUR, CETMEF (2007), but considerations specific to groynes include the detailing of geotextile layers and packing densities.

Geotextile layer Careful consideration is required as to whether geotextiles are required at the interface between the foundation/filter layer and the formation where it is particularly weak or susceptible to erosion.

Packing density On an amenity beach it may be appropriate to place the rock at a closer packing density to reduce the size of voids (see Section 2.2.1). The effect on sediment trapping efficiency of different packing densities appears to be minimal. However, the tighter packing has the benefit of somewhat reducing the health and safety risk of entrapment. In addition, it slightly increases the armour stability although it is also thought to slightly reduce the hydraulic performance with respect to wave absorption. (A full discussion of the issues associated with packing density of rock may be found in Sections 3.5.1 and 5.2.2.2 of CIRIA, CUR, CETMEF (2007).)

7.6 GROYNE ANCILLARY FEATURES

7.6.1 Interface with coastline or seawall Often an additional arrangement is required at the root of the groyne to manage the higher concentration of wave loading and/or the nature of the existing or new structure which will form the root of the groyne. This concentration of wave loading due to the containment effect between the groyne and shore- parallel structure (eg seawall) at the head of the beach is discussed at length in Dornbusch (2010), which higlights the potential for damage to the groyne and seawall or cliff due to the high loads and resulting abrasive effects. Maximum erosion was found between 0.5 m to 1.0 m away from the groyne and decreases with distance from it. Erosion is limited on wide beaches, where waves do not frequently reach the groyne interface. Similarly, where there is no beach in front of the groyne, there is an absence of abrasive material to erode the groyne interface. The effect of wave loading is most prevalent where there is a small beach width, located between MHW and MHW. To avoid these issuess, Dornbusch (2010) recommends adopting a “substantial beach with crest levels exceeding storm wave run-up fronting the seawall or to have no beach at all”. When the structure of the groyne root fails, then the retention of any material in the bay may be negligible and any benefit of the remaining groyne structure will be lost via landward outflanking (see Section 9.2.2).

Where the shore-parallel structure is a hard structure such as a seawall, any extra loading at the interface can be compensated by a robust connection between the groyne and seawall and a strengthened inner- end length of groyne. The groyne strengthening may involve piles at closer centres, the addition of ties or props, or an alternative construction in concrete. A bridging structure may also be required across the apron of any concrete seawall. Any connection between a timber groyne and a seawall needs to be robust, to ensure that there is no movement in the root of the groyne and the start of the planking. The first vertical timber component may be a driven pile, or a timber ‘planted’ onto the concrete seawall. Ragged bolts are used for this purpose – headless bolts with the end of the shaft deformed to enable anchoring into the concrete seawall using cementitious mortar or a chemical anchor system. The timber component is appropriately drilled and fixed over the bolt(s) once firmly fixed into the concrete and then a washer and nut run on and tightened, to pull the timber onto the seawall. Care needs to be taken to ensure the bolts are fixed square into the concrete so that the timber may be slid over the full length of the bolt.

Where the root of the groyne is formed by a sand dune or cliff face, the root of each groyne may be strengthened by the use of a shore-parallel structure that allows ongoing erosion of the cliff of dune without compromising the performance of the groyne. A frequently adopted solution to this involves

114 CIRIA, C793 use of rock armour, extending some distance along the groyne structure and the dune/cliff face on both sides of the groyne (as usually waves can and do attack from both directions). The downdrift side of the groyne will need greater lengths of rock protection, as the beach level is usually lower on this side of any groyne, so it is more vulnerable to failure.

7.6.2 Head of groyne The head of the groyne is often near to the low water line. In certain cases, it can be below the mean low water springs (MLWS) tidal level. Near the end of the groyne there can be complex local current flows often due to wave fronts hitting the groyne at an oblique angle. These local currents can cause a scour hole to form at the end of the groyne. This localised lowering of the seabed can threaten the stability of the groyne. Mitigation measures for this risk may include provision of anti-scour armouring or, in the case of piled groynes, ensuring that the depth of penetration of piles and the associated structural calculations takes account of the likely depth of scour.

Another cause for the destabilisation of the head of a groyne can be the lowering of the local offshore bathymetry. For example, this can occur when an offshore sandbank feature starts to migrate shoreward and the intervening channel is forced to migrate landward causing change in the shallow sub-tidal zone.

7.6.3 Sediment bypassing structures To reduce the differential load of sediment against the groyne and to reduce the risk of overturning, it may be necessary to introduce structural measures that can be operated to allow or encourage sediment to bypass the groynes from time to time. Different solutions have been adopted to achieve this. Figure 7.17 shows a groyne in Sheringham, Norfolk where a small gap, manually operated, can be opened to allow shingle to pass through the groyne when differences in beach levels across the groyne exceed a safe threshold.

a b

Figure 7.17 Timber groyne showing the manually-operated gap, when beach levels are low (a) and high (b) at Sheringham, Norfolk (courtesy HR Wallingford and Uwe Dornbusch)

7.6.4 Pile abrasion protection To help maintain the intended cross-section of the pile throughout its service life, especially in abrasive shingle environments, sacrificial pads of timber can be fixed to the pile and replaced when worn. They are usually only necessary for a relatively small number of piles in any one groyne, but it may not be evident which piles require these pads until inspections have identified a problem. Some minor wear to the pile can be permitted, so that after, for example, a year to 18 months, the zone of greatest wear can be identified, and the pads fixed. They may be required to one or two sides of the pile. In cases where the wear has not been checked at an early date, three sided ‘boxes’ may be necessary to provide protection to the piles. In such cases it may be necessary to dispense with the sacrificial pad concept and fix a permanent strengthening piece to accept new plank fixings, which may have been compromised by the abrasion of the pile. Off-cuts of planking may be useful for forming these sacrificial pads.

Groynes in coastal engineering 115 Sacrificial elements may not be necessary in all cases. For example, where it is intended that the groyne is to promote a beach that should continuously gain in profile, the abrasion zone will tend to move with the growth of the beach (upwards or seawards) and so some degree of wear may be acceptable as long as the active wear zone will move over time.

Maintenance of sacrificial protection is discussed in more detail in Section 10.3.5.

7.6.5 Vehicle and pedestrian access For beach and groyne maintenance purposes vehicle access across/through the groyne is often required, but the temptation to leave a permanent gap in the groyne should be resisted. For timber and concrete groynes this can be achieved by designing and constructing a removable panel forming one of the structural bays of the groyne structure. Such a ‘plant bay’ may be located towards (but not exactly) the inner end of a groyne, because there can be higher wave loadings at this position. For rock groynes, access may be provided by heavy duty ramps at the inner end that allow ‘up and over’ access.

By locating the plant bay near the inner end of the groyne it allows access along the beach early within the tidal cycle, which helps to improve maintenance efficiencies. A plant bay through the landward end of the groyne also helps to avoid the need to operate heavy machinery on the lower foreshore to gain access along the beach, which can be damaging to the lower foreshore.

For amenity and safety purposes, pedestrian access across the Figure 7.18 Steps over groyne which also act as an access to the promenade groyne is often required. For behind at Eastbourne, Kent (courtesy Uwe Dornbusch) timber and concrete groynes ‘up and over’ steps may well be adopted (Figure 7.18). For rock groynes, a light duty ramp or steps may be the preferred approach.

Navigation markers Reference to the local harbour authority is recommended when any lengthening or realigning of a groyne in a groyne field is to be made to ensure that any effect on local navigation aspects can be discussed and agreed. These discussions will also enable agreement of what number, location and form of marker beacon(s) are required.

Navigation marker beacons are located at the head of the groyne to help, for example, vessel skippers, surfers, kayakers, paddle boarders, jet skies etc, know exactly where the end of the structure is and its alignment shore-wards. So, risk of damage to vessels and injury to people can be significantly mitigated.

Navigation marker beacons can be independently piled structures (Figure 7.19) or, in the case of concrete and rock groynes, post structures ‘planted’ into the concrete or rock. Where possible cantilever structures are preferable to braced structures for reasons of health and safety and wear and tear.

7.7 DESIGNING FOR MAINTENANCE AND ADAPTIVE MANAGEMENT

Timber groynes require maintenance and repair. The main challenges are identifying suitable levels of repair, when to carry out these repairs and ensuring that they are effective in minimising the

116 CIRIA, C793 performance impact caused by the numerous agents of degradation. It is important to identify what options are available within the set budget and the practicalities of undertaking maintenance during tidal working. There is a balance between designing for long service life and designing for comparative ease of maintenance and sometimes it is the anticipation of repair and maintenance that can influence design. These issues are discussed more comprehensively in Chapter 10.

As discussed in Chapter 9, to adapt to a changing beach, modification of groynes and groyne fields may also be required. This may involve modifications to the profile and length of individual groynes and even sometimes changes to the overall layout and spacing of the groyne fields. The design will have a target beach plan shape and from this a target groyne profile to achieve this.

To enable eventual adaptation of the beach longshore crest alignment and associated cross-shore profile (see Section 9.3), piles may be left higher than the plank fixed to them in the first instance. This is to allow for gradually ‘planking up’ of the profile over time. This should be done slowly, not only to ensure that the available littoral drift (sediment budget) is not unduly affected, but also so that the exposed face of planking does not present a situation where it is highly reflective (of wave energy). With the planking playing a significant supporting role in the strength of a groyne, it is advisable to fix planks that are as long as practicable, given the method being used to install them (see Section 8.3.5). This can be best achieved when the planking is fixed at, or parallel to, the natural beach slope. In this way fewer, longer lengths of plank can be fixed, rather than a greater number of shorter lengths required for a single slope planking system, to achieve the same degree of profile change.

If it is known that incident waves in storms will run up the groyne and overtop the defence line, this effect can be mitigated by the placement of rock next to the groyne, although management of the top height of the planks is preferable in the first instance.

Improvements in design may be incorporated based on the local experience of the asset manager. In some cases, local improvements may be expanded to other locations if it can be shown that similar conditions and hazards are present. For example, the incorporation of sacrificial pile protection at Milford-on-Sea has significantly increased the service life of the timber piles. Timber piles are the most expensive components to procure and install so any transferable strategy that extends service life is desirable. Improvements in design should be supported by empirical evidence, but potential improvements identified at the design stage and incorporated during construction could have unintentional consequences, such as creating further maintenance issues (see Box 7.5).

7.8 HEALTH AND SAFETY ASPECTS OF DESIGN

In addition to the health and safety aspects associated with different groyne materials discussed in Chapter 6, there are several other aspects to be considered as part of the design process. These are described in the following sections.

Interrupted access along a beach The presence of groynes can interfere with free access along the beach. At worst they can cause a complete obstruction, and where there is a significant ‘step-down’ in beach levels across a groyne, cause a health and safety hazard, particularly if people are tempted to climb over the groyne. Inner-end steps or ramps may be necessary to manage this issue.

Navigation hazard Groynes can be a navigation hazard especially when (partially) submerged at high tide. Outer-end marker beacons may be necessary to manage with this issue.

Groynes in coastal engineering 117 Scour holes Narrow footprint groynes are likely to develop scour holes in the foreshore at their outer ends (Figure 7.19), due to local wave reflections, currents etc. These scour holes can be a hazard to beach users, as they may fall unexpectedly into deep water. The scour holes may also cause the groyne structure to become unstable and unexpectedly collapse.

Figure 7.19 Scour hole at head of groyne also showing navigation marker at Dawlish Warren, Devon (courtesy Uwe Dornbusch)

Wave overtopping Localised wave overtopping can occur at the root of a groyne, where it connects to the shoreline, especially where connecting to a vertical seawall. Low beach conditions may lead to a concentration of wave energy at the groyne/seawall interface, which can result in a vertical plume of water during wave overtopping events (see Section 7.6.1). When beach conditions are high, the groyne/seawall interface can also be the focus of overwashing and sediment transport onto promenades (Figure 7.20).

Figure 7.20 Coastal defences showing potential for overtopping of shingle onto promenade at junction with nearest groyne at Shoreham Port, West Sussex (courtesy Uwe Dornbusch)

Form of groyne structure Although climbing on groynes is often prohibited, beach users may not follow these instructions. As discussed in Section 7.5.2, the arrangement of the rock in a bulk groyne can present a hazard for beach users if it is loosely packed, although this has benefits for dissipating wave energy. With narrow footprint timber groynes, protruding and/or rusty fixings may also present a hazard.

118 CIRIA, C793 Box 7.5 Innovative plank fixing detail at Dawlish Warren, Devon

Figure 7.21 Coachscrew fixings Figure 7.22 Innovative plate connection

The groyne construction programme at Dawlish Warren in 2017 saw a significant change in the plank fixing strategy. The previous method of fixing used pairs of coachscrews at each pile interval (Figure 7.21). This method of fixing presented maintenance difficulties as replacing groyne planks required additional holes to be drilled into the piles, resulting in comparatively loose fixings if the holes overlapped. One strategy to simplify fixing has been to use a single coachscrew and plate connection to fix groyne planks at butt joints (Figure 7.22). Instead of having to loosen four coachscrews at each butt joint, maintenance staff now need to loosen one coachscrew and to remove the cover plate. The hole in the plate has a diameter of 50 mm to allow for adjustment and a washer holds the coachscrew tight against the plate, which has barbed edges so it can ‘bite’ into the timber. The adoption of the plate detail delivers faster construction time, uses less material and makes it easier to replace planks, as only one fixing per pile interval is required. However, the plate covers an open butt joint and although gribble attack at Dawlish Warren is very light, this may provide a sheltered environment for gribble and may result in the underlying pile section being more vulnerable to attack. The unintended provision of a sheltered environment has been noted previously at Bournemouth where gribble attacked timber behind plywood sheets that had been fixed to the prevailing downdrift elevation of groynes preventing fine beach material passing through the gaps between groyne planks. This may be significant over the long term as attack could lead to socketing and subsequent loosening of the single coachscrew holding the plate against the planks. The long-term effectiveness of this new fixing strategy has yet to be evaluated. Should the plate eventually disengage from the planks, displacement and possible loss of planks will arise.

7.9 ENVIRONMENTAL ENHANCEMENTS

It is recognised that there are considerable environmental, ecological and social benefits from ecological enhancements (‘greening the grey’), with green concrete being incorporated into some rock groynes and artificial rock pools being incorporated into some timber groynes (Figure 7.23). Experience to date is limited but suggests that use of artificial rock pools on narrow footprint groynes may be more appropriate for less dynamic and abrasive environments, eg on sandy beaches rather than shingle. At the time of writing, the concept is still being trialled and reference should be made Figure 7.23 Artificial rock pools attached to a timber and concrete to the evolving research and guidance groyne (courtesy Nigel George, Artecology) on the subject (see Naylor et al, 2017).

Groynes in coastal engineering 119 8 Groyne construction

This chapter gives an overview of the planning and management of the construction of narrow footprint groynes (Figure 8.1). It discusses obtaining the materials, investigating the site working conditions, methods of construction of the various components, tolerances and accuracy, and managing construction risks. In addition, repair methods are discussed (see also Chapter 9). Specialist equipment plant and labour required for timber groynes construction are described, and health and safety issues for construction are discussed. The main sections of this chapter are:

„ Construction risk management (Section 8.1) „ Stockpiling, handling and inspecting timber (Section 8.2) „ Construction of timber groyne elements (Section 8.3) „ Plant, equipment and labour (Section 8.4) „ Additional health and safety issues (Section 8.5)

Coastal management Functional requirements and context indicators (Section 2.1) (Section 2.1)

Physical coastal environment – hydraulic, geotechnical and morphological constraints (Chapter 3)

Start or continue to No Groyne removal use groynes? (Section 9.4)

Yes

Plan shape design of groyne Evaluate and choose groyne field (Chapter 4) type (Section 2.2) and material (Chapter 6)

Design groyne profile (Chapter 5)

Structural design or design review for groynes (outline and detailed) (Chapter 7)

Construction and/or major refurbishment (Chapter 8) Major Groyne repair and Minor maintenance Groyne adaptation (Section 9.2) (Section 10.3) and adjustments (Section 9.3)

Small Beach changes Routine monitoring Large (Section 9.1) (Sections 9.1 and 10.2) Structural deterioration (Section 10.1) Post-incident monitoring (Sections 9.1 and 10.2)

Climate change Storm events (Section 3.1.4)

Figure 8.1 Parts of groyne management process covered in Chapter 8

120 CIRIA, C793 8.1 CONSTRUCTION RISK MANAGEMENT

8.1.1 Introduction Working near water, particularly on the coast is seldom easy and restrictions including limitations on physical access, possible working times, tidal levels, storms and variable ground conditions, location of coastal properties, and the specific construction materials can all combine to increase risks. The use of timber has many advantages because it is relatively easy to work with and it can be prefabricated to a greater or lesser extent depending on the application. Consideration of constructability at the design stage is a fundamental step in subsequent construction risk management.

Many of the most significant risks associated with timber construction on the coast are described in existing guidance documents such as Simm and Cruickshank (1998), which may be referenced by designers for more detail.

The next sub-sections will discuss specific issues for construction risk management including: „ material procurement (including currency risk) „ wastage of timber „ tolerances „ site constraints „ ground conditions.

Health and safety risks including working with hardwoods are discussed in Section 8.5.

8.1.2 Planning for material procurement Material procurement for groynes needs to be carefully planned due to the long lead-in times required for timber, rock and fixings. Although groynes can be constructed from concrete, masonry, rock or timber, timber (see Section 6.2) demands the longest lead-in time. These long lead-in times need to be considered by all parties to the contract, as it will determine the earliest start date on site from date of contract award.

The responsibility for the procurement of timber may be separated from the main contract to mitigate any risks of delays and allow the lead-in time to run consecutively with the contract procurement process.

The timber will need to be procured following UK Government timber procurement policy (see Section 6.2.10). The most economical way of purchasing timber is to place an order with an accredited supplier well in advance of starting the work on site. To remove the risk of delays, in some cases the client procures and provides the timber to the contractor at no cost. This is because the client is usually in a position to order the timber once the design has been completed, and before works begin. Lead-in times for timber extend up to 26 to 30 weeks or, in cases of long piles or planks, up to 12 months (see also Section 6.2.7).

However, if the procurement of timber is to be included within the main contract, enough time should be allowed within the tender period for an appropriate source of timber to be identified. The construction programme should then include sufficient lead-in time for procurement activities (see also Section 6.1.4). This will enable the contractor to order, produce the required quantity of timber sections and transport these to the UK before construction starts.

In cases where a specific type of timber is being procured overseas, such as tropical hardwood, a currency risk and taxes will need to be considered. This is because the supplier will offer the timber in sterling based on a fixed currency conversion with US dollars, Euros or local currency. Procurement times will also affect costs and cost risk management needs planning and mitigating as part of the material procurement process.

Groynes in coastal engineering 121 Fixings for timber groynes will typically be galvanised or stainless steel, bolts and/or coachscrews. Typical lead-in times to procure these items will differ between galvanised and stainless steel fixings. Stainless steel is likely to take in the order of three to four months and galvanised would be about one to two months.

The fabrication of the pile rings and shoes will also require a long lead-in time and can require two to three months from the date of the order.

In the case of rock material, it is important to consider that rock sources are limited in the UK and many coastal defences are constructed from rock delivered by sea (eg from Europe). This means, in most cases, that the rock needs to be ordered well in advance of the works to organise the procurement of enough rock of the required grading, its shipping and any trans-shipment barges. Delivery locations and fisheries liaison (to ensure appropriate mitigation of any ecological risks associated with delivery and placement) also need to be organised well in advance. Typical lead-in times for armourstone could be two to four months.

8.1.3 Minimising waste Most timber groynes are constructed from tropical hardwoods that are a valuable and limited resource, so it is important that waste is minimised. Waste can be avoided by scheduling the timber components in the form of a cutting list, which provides the quantity, length, number and section sizes of the required timber components. Planking should ideally be ordered in lengths, which are multiples of the pile centres plus a small percentage of cutting waste.

The cutting schedule produced before ordering the timber will include cutting, squaring and dimensional waste. This can be simplified if information on all elements of the structure such as the number of the different lengths of planking is included within the contract documents.

Short contract periods/lead-in times should be avoided as these will force the use of stock timber and significantly increase both cost and waste.

8.1.4 Tolerances Timber for groynes may be ordered as ‘sawn’, but this can mean a tolerance of +4 mm/-2 mm, which may not comply with some groyne-specific specifications. So, planing may be necessary to achieve the required tolerances.

Dimensional tolerances for timber members and constructional tolerances for installing members have to be achievable. Unless for a specific purpose (such as retention of beach material when this is fine sand), overly severe tolerances will not contribute to the integrity or functionality of the structure and may result in significant inefficiencies during construction. Avoiding unnecessary severe tolerances at a recent project at Herne Bay achieved a 10 per cent price reduction in materials. The use of alternative structural configurations should also be considered where tolerances are critical, eg planted posts may be used instead of driven piles for access steps where more stringent tolerances are required. Similarly, the detailing of joints and connections should be considered carefully, avoiding the need for long small diameter bolt holes where there is a risk of deflection of the drill bit.

Piling tolerances need to be carefully considered when specifying groynes. As discussed in Sections 8.3.2 and 8.3.3, the tolerances on position and verticality required for producing a good line of king piles on a groyne are likely to be more stringent than the tolerances given in ICE (2016). It is important to understand why the tolerances are severe and the repercussions if these tolerances are not achieved.

122 CIRIA, C793 8.1.5 Planning for site constraints Seasonal working constraints When working on amenity beaches within a coastal resort, it is likely that the works will have to be programmed over the winter months to prevent any disruption during the tourist season (April to September). This is likely to have many ramifications in terms of risks to the project, such as from weather conditions, daylight hours, rock deliveries by sea, over wintering birds, seal pups etc. However, on remote beaches that are not amenity beaches, it may be possible to plan the works to be carried out in the summer months. Summer working reduces the risk of adverse weather conditions and mitigates the risk of delays if rock is being delivered by sea. Seasonal working constraints will need full consideration within any environmental and health and safety risk management planning for the construction risk management process.

Planning constraints and legal obligation To reduce risks and disturbance to the public and local residents, planning constraints are generally specified for construction contracts on public beaches as follows: „ noise restrictions, which would include a limit on the decibel (dB) rating linked to the time of day (eg between 19:00 hrs and 07:00 hrs the maximum limit is 45dB) „ working hours, which could be limited to 07:00 hrs to 19:00 hrs Monday to Friday and Saturday to 07:00 hrs to 13:00hrs (There is often a provision that permission to work outside of these hours will not be unreasonably withheld if the nature of the work requires it, eg when working on spring tides on the south coast of England, as low water would be around 06:00 hrs in the morning and 18:00 hrs in the evening.) „ vibration limits „ traffic and delivery restrictions.

However, seasonal, weather and sea-state constraints may dictate significantly reduced available working hours within these periods. In addition, the need for working around low water will inevitably mean working tidal shifts out of normal working hours. This will require detailed planning in terms of environmental requirements and safe, health and environment (SHE). Consideration of prefabrication of some components could improve productivity and minimise access risks.

Consideration should always be given to the location of houses and properties to the beach on which groynes are being installed and efforts made to mitigate stress to residents.

Weather/sea-state constraints Wind conditions can cause delays. High winds prevent safe lifting operations and affect the sea state creating larger waves and modifying water levels. An ‘onshore’ wind can cause an increase in water levels and so can reduce the safe working time around low water, causing delay. However, an ‘offshore’ wind has the opposite effect as it ‘pushes the tide out’ and holds it off the beach. On the south coast of England, for example, the prevailing wind is south westerly, so the wind tends to have a more detrimental effect. Surges, due to low atmospheric pressure, can significantly increase the sea level above its predicted value and reduce working time around low water. High pressure has the opposite effect and can help extend working times at low water, particularly during the summer months.

Construction of groynes is highly dependable on weather conditions, so it is extremely important to continuously monitor and scrutinise weather forecasts. This should be considered from both the programme point of view, so that certain construction activities can be adjusted where necessary, but also for any necessary arrangements to be undertaken in relation to safety and security on site (see Section 8.5).

Groynes in coastal engineering 123 Tidal working constraints Most groyne construction contracts require the groynes to be constructed within the tidal zone between high water and low water. This requires detailed planning and monitoring of the sea state and weather conditions.

Tides in the UK and Ireland are generally semi-diurnal (‘about’ two high tides a day) with a period of about 12.4 hours. However, the heights of the tide can vary significantly from one site to another (Figure 8.2). This means that construction operations need to be planned carefully to take account of the tide times, heights, surges (positive and negative). Accessibility at low tide during daylight will affect duration of construction, costs, health and safety and construction methods. In some cases, this element can restrict the design groyne length. Working times tend to be a few hours either side of low water. The working period (start and finish) will change daily to match the tide times. Working hours should be carefully planned to maximise the benefits of working around low water and take account of the time required to bring construction plant on and off the beach.

Figure 8.2 Typical tidal cycle at three locations in the UK. Note the differences in vertical scales (courtesy HR Wallingford)

It is important to identify which operations will require access to the lower foreshore, so these can be planned for appropriate tides. In many parts of the UK, low spring tides tend to occur in the early morning or evening, which is often not ideal for working hours and creates issues such as lack of light while working in the winter. Lowest tides of the year in the UK are likely to coincide with the vernal and autumnal equinoxes in March and September, whereas solstice spring tides generally recede the least. Figure 8.3 shows an example of tidal working, with a rising tide.

Figure 8.3 Tidal working with a rising tide (courtesy Mackley Construction)

124 CIRIA, C793 8.1.6 Planning for ground conditions As discussed in Section 3.3, a key requirement for construction planning and risk management is the need to establish the underlying ground conditions at the groyne location, especially the soil or rock substrate beneath the beach ideally by means of boreholes and SPTs. The results from these will aid the contractor in determining: „ the type and size of piling equipment required to install king piles, anchor piles and/or sheet piles via driveability analysis „ any temporary works arrangements necessary to enable safe access and egress for construction plant (haul roads, platforms etc).

If the SPT N-values are very high (greater than 40), it is possible that the pile positions will need to be pre-augered to ‘loosen’ the ground to allow the piles to be driven without the risk of refusal or damage (see BS EN 1997- 1:2004+A1:2013). Figure 8.4 shows an example of the pre- augering process. Figure 8.4 Pre-augering (courtesy Mackley Construction) As explained in Section 3.3, there will be a need to excavate trial holes or drill boreholes to the depth of the proposed sheet piles to determine the soil type (cohesive/non-cohesive) and where the soil types may change. This is important as it is not advisable to disturb cohesive material. It is extremely difficult to backfill ‘clay’ in tidal conditions to get the required compaction/consolidation so there is a high risk it will soon scour. It is suggested to excavate down to the top of any cohesive soil type to avoid disturbance when planking. When installing sheet piles into clay, these would need to be impact driven.

In non-cohesive soil types such as sand, it is possible to excavate and backfill without the risk of under consolidation. However, some compacted sands within a tidal environment could remain ‘soft’ or in a ‘quick’ condition for a period of time after backfilling until it consolidates. This can take up to 10 to 14 days and it is extremely important that these backfilled areas are fenced, to prevent unauthorised access by plant or persons, as the ground will be very soft and unstable.

8.2 STOCKPILING, HANDLING AND INSPECTING TIMBER

8.2.1 Storage and stockpiling A good stockpiling procedure is required to prevent deterioration of timber between delivery to site and incorporation in the works, which can lead to expensive and unnecessary waste. Some aspects of good practice are: „ An area should be chosen that is clean and level. Stacking on uneven ground should be avoided. „ Each type and section size should be stacked in separate clearly marked stockpiles. „ Worked timber (such as piles which have been ‘topped and tailed’) should have its own stockpile. „ Salvaged timber should be stockpiled separately, with timber available for reuse separate from un– inspected timber. „ All timber should be stockpiled on bearers to keep it well above the ground and to allow airflow. Bearers should be positioned relative to one another such that they are spaced to carry the section of timber in the stack without any of the supported timbers warping or bending. See Figure 8.5.

Groynes in coastal engineering 125 Figure 8.5 Stockpiling (courtesy Mackley Construction)

„ Timber should not normally be stacked more than 2 m high. „ Stockpiles should be protected against the weather, both sunshine and rain. Even timber Figure 8.6 Timber storage for construction (courtesy scheduled to be fixed into a wet environment Mackley Construction) can warp and twist without the stability afforded by the fixings. However, it is not advisable simply to throw a tarpaulin over the stack of timber as there should be an air gap at the top of the stack to allow ventilation.

If the timber needs to be stored at an interim holding location, before it is transfer to site, then the same good practice principles should be adhered to.

Figure 8.6 shows an example of a timber storage area for construction.

8.2.2 Handling timber The penalties for bad handling range from breakage and premature failure of the structure to the potential for serious injury to operatives. None of these are desirable and all are avoidable. Bundles and wrapping should not be undone until the timber is about to be used.

There is no completely risk-free approach to lifting and manoeuvring timber. Care should be taken not to over-stress the material. Lifting at one or two closely-spaced points can cause stress damage to the fibres of the timber. This is hard to detect visually, but seriously weakens it, leading to premature failure in service.

Planking and piling is often delivered in ‘bundles or packs’ and can be off-loaded using forklifts or telehandlers. Forklifts should be used with particular care, as most site machinery have their forks more closely spaced than timber yard machines and they often bounce across an uneven surface, which exacerbates the issue. Using a site forklift can stress timber fibres.

Once the supplied timber packs are opened and the planks/ piles are being handled individually, great care is required to prevent damage to the edges of the timber pieces. The use of chain slings should be avoided as they are liable to cause splinters which are a health hazard. Chains can also damage the edges, so planks no longer provide a full and tight fit to adjacent members. In extreme cases they can cause high point loadings, leading to similar stress damage. Figure 8.7 Hoisting (courtesy Mackley Fabric slings represent the best option but should still be Construction)

126 CIRIA, C793 used with care. They should be double or chocked as they are liable to slip, particularly in wet weather. Control lines should be fixed to the ends of the load to minimise swinging, which is what may lead to slippage in slings. Figure 8.7 shows an example of hoisting practice.

8.2.3 Pre-construction inspection and checking The inspection process is critical to the effective management of timber on site and records should be kept, particularly if the timber is supplied by the client. It is good practice to agree an inspection and checking procedure at the start of a contract to ensure that there is no misunderstanding between the various parties.

Upon delivery of the timber to site it is important to ensure that certain procedures for receiving goods are followed, such as: „ Before unloading, the delivery ticket should be checked to make sure the delivery has come to the right site with the correct load. „ All certification documents (see Section 6.2.10) should be checked, ensuring that the consignment markings correspond to those in the certification document. Certified timber without the correct documentation and markings should be rejected. „ The consignment should be checked for signs of poor handling such as splinters, bruising or breakage. „ Compliance with the specification and/or grading should also be checked (splits or shakes, sapwood and twist or warp are generally an indicator of a poor quality consignment).

Suppliers will normally replace timbers demonstrated to be substandard. However, the cost of off-loading, inspection, storage and subsequent loading for return can cause disputes between the timber supplier and principal contractor.

8.3 CONSTRUCTION OF TIMBER GROYNE ELEMENTS

8.3.1 Preparing king piles/anchor piles As discussed in Section 7.4.1, the piles need to be ‘topped’ with a tapered pile ring (Figures 7.8 and Figure 8.15) to prevent the pile from splitting when it is being driven using an impact hammer. The top of the pile is fashioned into a circle and the ring fitted tight. At the same time the shoulders of the pile are chamfered such that if the ring is forced down the pile the pile ring does not ‘bite’ into the pile and split it. If the pile is being driven by a vibrating hammer, it is safer to remove the ring and re-fit it after installation.

The toe of a timber pile should have a cast iron ‘shoe’ fitted (Figure 7.9). Pile shoes come in various weights and sizes. The most important aspect of fixing pile shoes is to ensure the shoe is concentric and firmly attached to the end of the timber pile. The contact area between shoe and timber should be sufficient to avoid over-stressing during the driving operation. Careful fitting of the pile shoes helps the pile to be driven accurately and prevents it from ‘wandering’. If the shoe is not fitted parallel/concentrically, when the pile is driven, it will follow the alignment of the shoe and possibly run out of position.

8.3.2 Installing king piles Optimising the method and plant used for piling can have a significant effect on the speed and cost of their installation.

The position of the piles can be established in one of two ways: „ using a timber gate and positioning the piles within the gate to their required centres „ setting out the individual pile positions using a global positioning system (GPS) and checking the positioning as the pile is driven.

Groynes in coastal engineering 127 In most circumstances, timber piles are driven using a piling rig. Timber piles absorb more energy and require larger hammers than equivalent concrete or steel piles. However, they are relatively easy to handle and can be driven by a range of equipment. The energy to drive the piles is generally produced either by an impact hammer (Figure 8.8), which performs well in cohesive material, or via a machine- mounted hydraulic vibrating hammer, eg a ‘Movax’ machine (Figure 8.9), which performs well in non- cohesive soils (sands and gravels).

Figure 8.8 Installing a king pile by driving with an Figure 8.9 Installing a king pile using a hydraulic impact hammer (courtesy Mackley Construction) vibrating hammer (courtesy Mackley Construction)

There is no simple solution to choosing the ideal plant. As discussed, an alternative hammer type may need to be trialled, so having a versatile piling rig can be more important than matching it to the first- choice hammer. Although a site investigation program should identify the critical factors that can affect the construction, they cannot remove all uncertainties and there is always the possibility of encountering something unexpected. For instance, on a mixed sediment beach, it is not unusual to encounter layers of cobbles deep below the surface. These may be enough to cause piles to go offline and split because the shoe will not penetrate the layer. So it may be necessary to dig down and remove the cobbles before restarting piling. In this case, having a longer reach excavator on site may prove better in the long run rather than one that is less capable.

Using a gate to control the positioning of the piles and the orientation of the planking face helps to avoid the piles ‘twisting’ out of alignment.

It is extremely important to ensure the planking face of the king piles is perpendicular to the centre line of the groyne. This is to ensure that there are no gaps between the face of the pile and the plank where it is fixed to the king pile. If a pile is twisted and the plank does not bear evenly across the face of the pile, shingle can get trapped between the pile and the plank and act as a ‘wedge’, gradually forcing the plank from the pile over time.

As discussed in Section 8.1.6, it may be necessary to pre-auger the piles if the ground conditions are very hard and the SPT N-values high (greater than about 40 to 50). High N-values can occur in both cohesive and non-cohesive soils and determining the ideal hammer type may have to be decided by ‘trial and error’. A common approach is to pre-auger the holes to a depth of about 0.5 m higher than the final desired toe level, with a final drive of about 0.5 m to ‘refusal’, ie no further penetration possible with the piling hammer. Another approach with pre-augered holes where driving conditions are difficult is to create 3 m deep, 0.6 m diameter sockets and then concrete the piles into these holes.

8.3.3 Pile verticality and centre spacing Pile verticality is critical. Even if the verticality complies with the tolerance in ICE (2016), this is normally still not sufficient as even very small deviations can affect the quality and alignment of the resulting planked face. If the pile is not driven accurately in the vertical plane, the toe of the pile could be out of alignment along the planking face. This will affect the lower planking alignment and it may be very

128 CIRIA, C793 difficult to fix the lower planks and get a tight fit between the planking face and pile face. Also, the planks may not be able to be fitted around a pile which encroaches on the planking face. This would then require remedial action.

The position and verticality of the piles down the line of the groyne is equally important. If the piles are installed such that the gap between the pile centres is too great, then the planking, which is purchased pre-cut (including some tolerance of about three per cent), will be too short. As the timber is pre- ordered, there is no easy remedy.

8.3.4 Walings and ties Walings Most groynes will require walings to be fixed to the piles on the leeward side of the groyne in order to transfer loads along the complete length of the groyne. Piles that have twisted on driving (within tolerance) should be notched to accept the walings.

Walings can be installed either at high level (upper walings), sometimes to facilitate the incorporation of connections to ties, or at low level (lower walings) to incorporate the fixing of the sheet piles (Figure 7.5). When fixing at high level it is important to plan the access arrangements for the operatives installing the waling.

Upper walings should normally be fixed before the planking as the bolt heads on the planking face will need to be rebated into the pile to allow the planks to be fixed flush to the rile face. Lower walings would require the bolt heads fixing the waling to the piles to be rebated to allow the sheet piles to be fixed flush to the lower waling.

Each length of waling should be connected to the next via a scarf joint or half joint and the joint should be strengthened by fixing galvanised metal scarf plates either side of the waling by bolting through the waling and plate. These joints should normally coincide with a pile where possible. The section size for timber walings can vary depending on the design criteria, but a common size for an upper waling would be 230 mm wide by 150 mm deep and a lower waling 150 mm square. Further information on timber jointing can be found in Crossman and Simm (2004).

Ties As described in Section 7.3.1, in some cases props or ties (Figure 7.6) will be required to provide stability to the groyne, as also illustrated by the tie heads above the upper waling in Figure 8.10. Ties can be constructed from standard square section timber or even logs can be used. The importance is to ensure a secure connection between the groyne and the anchor piles. The connection to the groyne requires the tie to be notched onto the upper waling and an anchor waling fixed above the tie, spanning two piles. The tie is then fixed by bolts fixed vertically through both walings. The Figure 8.10 Tie heads above upper waling (courtesy walings are spaced off the planking using timber Mackley Construction) spacer blocks.

The tie at the anchor pile will be bolted through either one or two anchor piles depending on the design. Extensive abrasion will occur at the pile end of the tie and the use of double piles should be considered to protect against this occurrence. See also Section 6.2.3 for the effects of abrasion on timber piles.

Groynes in coastal engineering 129 Bolted connections between walings, ties and piles Holes for bolted connections should be bored from one side only (to ensure a true hole) and so longer than normal augers will be required. The diameter of the hole should be such that the bolt needs to be struck through forming a tight fit in the hole.

8.3.5 Installing planking Planking is fixed to the updrift side of the piles and will either be bolted, or coach screwed to the piles. As recommended in Section 7.4.4, coachscrews are to be preferred and enable more robust approach to maintenance (see Section 10.3.3). Holes for coachscrews should be double drilled – first, part way for the shank to avoid splitting of the plank and then again with a smaller size drill/auger full depth for the threaded portion of the coach screw to ‘bite’ into the receiving pile. Coach screws may be started off by being struck in, but they should never be hammered all the way – this is a sign of poor workmanship and will lead to early loss of planks. The thread is provided for a purpose and the coach screw should be wound in and not hammered into place.

Planking should be cut to length giving a tight fit to the previous and succeeding elements. Waste will be an issue where there are a range of dimensions between centres of piles for various groynes or sections of groynes. This supports the benefits of standardising the dimensions between pile centres and avoiding the need to hold stocks of many different planking lengths. If piles have twisted on driving then it will be beneficial to the overall life of the structure to trim protruding edges, so that meeting faces between planks are as tight as practicable.

Planks may be fitted either starting with the lower planks and working up or starting from the upper planks and working down (Figure 8.11). The joints on the piles between planks fitted above one another should be staggered to provide a strong ‘spine’ to the groyne. Straight joints down the planking should be avoided unless there is a change of gradient in the planking. Planks should be fixed with minimal gaps between planks. This can be Figure 8.11 Planking installation (courtesy Mackley Construction achieved by clamping the plank to the pile and then compressing it down onto the next one below using an attachment on the excavator which slots over the plank (or in the case of working top to bottom, pulling the plank up via strops using an adapted forklift or equivalent machine). The plank is then fixed with either bolts or coach screws while being compressed.

Ensuring there are minimal gaps (maximum of about 3 mm) between planks on shingle beaches is important. As well as reducing material transmission, this will: „ produce a more rigid and robust structure „ prevent abrasion of the plank edges „ reduce the risk of rot.

On sand beaches it is even more critical to ensure that there is little or no material transmission through the structure via gaps in the planking. This typically requires any residual gaps to be less than 0.1 mm. This can only be achieved by planning the timber and regularising the planking such that the cross- sectional dimensions comply with a very tight tolerance and by fixing the planks as previously described. There is an additional cost premium to provide this quality of planking workmanship. Planks also need to be straight and true to avoid any gaps in the vertical dimension.

130 CIRIA, C793 Where practicable, joints in planking should be configured so that when the fixings are tightened, the components are drawn together tightly in as many axes as possible. This can be done by shaping the components of their meeting faces and/or by using fixings made at a slight angle. Angling the coach screws also ensures that the thread of the screw penetrates more into the centre of the pile, not only for strength but also for giving the thread added protection should the pile be abraded in its service life.

Cutting or trimming planks (and other members) in their intended final place is to be avoided. Components should be marked and moved for cutting, which avoids the risk of over-cut and leaving cuts or grooves in the piles to which the plank is to be fixed. Dampness in these cuts will lead to rot or premature ageing.

8.3.6 Installing sheet piles Timber sheet piles are normally of the same cross- section as planks and are installed vertically. Average lengths of sheet piles are usually between 1.2 m and 3 m long. Vertical sheet piles should be tight fitted, with a shaped ‘toe’ to encourage each timber to be driven/pushed against the preceding timber. The toes of the sheet piles are shaped to aid driving by being cut at an angle of about 45 degrees and chamfered to provide a pointed edge. The angle of the cut allows the driven sheet pile to move towards the adjacent sheet pile and ensures a ‘tight joint’. Sheet piles may be driven into a cohesive substrate but can be ‘dug in’ when the substrate type is non- cohesive. For some substrates, it may be necessary to fashion a pressed steel shoe which is fitted around the toe of the sheet pile and fixed with nails. Installation of sheet piles is shown in Figure 8.12. Figure 8.12 Sheet piles (courtesy Mackley Construction)

8.4 PLANT, EQUIPMENT AND LABOUR

8.4.1 Plant The most common types of plant used in groyne construction include the 360 degrees tracked excavators. These can vary in size from small five tonne machines for minor maintenance to 70 to 80 tonne machines for rock placing and/ or excavation of the beach. Piling rigs are often used because they offer increased productivity and versatility compared with pile hammers mounted on traditional excavators. Piling rigs can also vary in size and stability.

The fixed leader rigs weigh about 45 tonnes and Figure 8.13 Machines working on a beach in the south the mounted rigs are often mounted on 35 tonne coast (courtesy Mackley Construction) excavators. Wheeled loading shovels and telehandlers are also used. Figure 8.13 shows different type of equipment and machinery used during piling works.

Many contractors use purpose-made equipment (eg piling rigs) for working on the beach, as they will be designed to be as productive as possible. These specialist pieces of plant include reduced swing excavators which minimise the risk of an operative being struck by the excavator counterweight.

Groynes in coastal engineering 131 The drilling and boring equipment (Figure 8.14) used in timber groyne construction is mainly air powered.

Groyne construction will involve the need for plant, equipment and operatives to access the tidal foreshore. So it is important to determine the safe bearing capacity of the foreshore before plant accesses the beach (see Section 3.1). There is always a risk that any of these pieces of equipment could break down on the beach and will need to Figure 8.14 Boring (courtesy Mackley Construction) be recovered. It is imperative that a recovery procedure/plan is available and adhered to. The essence of a recovery is to remove the machine from the tidal zone before the sea claims it. The recovery procedure should be briefed to the site team and a controlled recovery carried out as an exercise to ensure all members of the team understand their responsibility. Plant should be regularly maintained and operated in accordance with safe working practices (see also Section 8.1).

8.4.2 Portable equipment Chainsaws Cutting and shaping of timber is often undertaken using chainsaws. These are available in many sizes and with a variety of types of chain. Care should be taken to ensure that the saw used is appropriate for the type of work required. Chains will need regular sharpening, particularly when used on tropical hardwoods. PPE should be worn by operatives when using a chainsaw, this includes a helmet, mesh visor, ear defenders, chain mail leggings, bib, jacket and gauntlets, and steel lined safety boots.

Hand tools Tools such as the adze have been used for centuries and are still widely used for shaping timber, including the preparation of the top of the pile in order to fit the pile ring (Figure 8.15). The use of the adze requires considerable skill and training so supervising novices by experienced operatives is essential. Suitable PPE should be worn. Figure 8.15 Hand tools (courtesy Mackley Construction) Augers and wrenches These can either be electric, hydraulic (driven by diesel power packs) or air driven. Generally, tools powered by compressed air are safer to operate in a wet environment and are more powerful. The main disadvantages are the weight of the air tools and the tendency to ‘freeze’ up when the temperatures are low (below about 2oC to 3oC). The freezing can be remedied by minimizing the amount of moisture in the hoses and lubricating constantly with oil and anti-freeze, fed through the hoses. The reduction in moisture helps to limit further rusting of the air vanes within an air auger or wrench.

The weights of the air augers and wrenches should be calculated, and consideration given to using lighter models when possible. This will form part of the RAMS developed by the contractor and will include a procedure for monitoring of hand-arm vibration syndrome (HAVS) exposure for the operatives. This is to ensure the maximum exposure time is not exceeded when using either augers or wrenches to tighten the bolts or coach screws.

Regular weekly maintenance should be carried out on all tools and compressors.

132 CIRIA, C793 8.4.3 Labour Trained specialist operatives, craftsmen and site workers are required for the construction of timber groynes.

Plant operators should be able to maintain and manage their machines without being stranded on the beach. They should have the ability to construct and maintain ‘bunds’ safely when working at the outer end of the groyne and competent in using the excavator as a lifting device/crane and know the machine’s limitations when lifting.

Timber workers should be trained and prove their competency in the safe use of chain saws, air augers and adze, and in slinging and signalling.

Operatives involved in the use and care of chain saws should be trained under supervision.

Other aspects of health and safety are discussed in Section 8.5 and the need for planning working hours around tidal conditions is referenced in Section 8.1.5.

8.5 ADDITIONAL HEALTH AND SAFETY ISSUES

8.5.1 Introduction Health and safety risks for all phases of groyne preparation, implementation, operation and decommissioning should be identified at early planning stages, and appropriate risk management strategies incorporated into the design, construction management and maintenance plan. Health and safety risk management issues in construction have been highlighted through this chapter, however, some additional issues specific to groyne construction are discussed in the following sections.

8.5.2 Lighting When working out of hours (ie early mornings and late evenings) good lighting to permit safe working will be required at the workface. This can be provided by lights on the machines together with portable lighting towers.

8.5.3 Access routes on and off the foreshore Safe access onto the beach for plant, equipment and operatives is essential. This may be achieved by using existing ramps or temporary ramps may need to be constructed. It is very important for the plant to have a safe refuge above high water within a short distance from the work face. It may be necessary to construct access tracks for larger plant which will need to be maintained daily.

8.5.4 Segregating the public from the works Beaches attract people and it is imperative that the public (including dogs) are separated from the works by a hoarding or fencing. This is not easy to do within the inter-tidal zone, as fences will be damaged by the rising tide and they could become a hazard themselves. Few beach working sites can be completely closed off and this is especially difficult in tidal areas. So suitable signage, portable fencing (Figure 8.16) may be required and/or marshals may be necessary to keep the public away from the works. These issues may be mitigated if the work is undertaken during unseasonable/unsociable hours, ie avoiding busy summer periods.

Figure 8.17 shows an example of working on an amenity beach and related warning sign.

Groynes in coastal engineering 133 Figure 8.16 Examples of signage and portable fencing Figure 8.17 Plant on a public beach with warning sign (courtesy Mackley Construction) (courtesy Mackley Construction) 8.5.5 Lifting operations There will be numerous lifting operations when constructing a groyne, and piles, planks, walings etc all need to be lifted into position. A lift plan should be produced by the appointed person and supervised by the lift supervisor. The operatives will require training as slinger/signallers. Chains, strops and slings will need to be checked and ensure that the required six-month inspections are clearly displayed.

8.5.6 Deep excavations To gain access to the lower sections of the groyne to install planks, deep excavations within the beach will be required (Figure 8.18). To ensure the beach is left safe at the end of each shift, all excavations should be backfilled to avoid deep, water filled holes that are a hazard to anyone gaining entry to the site. (The exception is where a full cofferdam or temporary bund has been provided, see Figure 10.21.) When excavating in sand, it is prudent to not only backfill the excavations but also cordon the area off to avoid any pedestrian or operative getting stuck in the Figure 8.18 Example of deep excavation (courtesy soft sand. Sand takes on a ‘quick’ condition until it Mackley Construction) has consolidated sufficiently via the tides covering it over a period of a week or more.

8.5.7 Risk of drowning Although this appears to be very unlikely on a flat beach, the risk can materialise when the operatives are working at low level (below sea level) in an excavation and the excavation which becomes inundated by the sea. This could occur when working at depth at the outer end of a groyne. It is imperative that sufficient plant is used to maintain a watertight bund, the excavation (if open) has the sides battered at an angle several degrees less than the natural angle of repose of the beach material, and there is a detailed plan for maintaining a dry excavation. Finally, the supervisor of the shift should monitor the sea conditions and not leave it too late to complete the shift and the backfilling.

8.5.8 Traffic management plan Access for deliveries will need to be planned carefully and areas provided for stockpiling timber. HGVs will require a planned route to avoid congestion. Where possible pedestrians/operatives and vehicles should be segregated to reduce the risk of vehicles hitting any pedestrians. Measures should be taken to minimise the risk of collision between an item of plant and operatives as the working space around a groyne is minimal.

134 CIRIA, C793 8.5.9 Working with hardwoods When tropical hardwoods are being used, operatives should be briefed on the dangers of dust and splinters. It is important to use good quality PPE (working gloves, overalls and dust masks) to mitigate this risk. For health and safety aspects of working with hardwoods see Section 6.2.11.

8.5.10 Risks of buried services In-line with CDM 2015, the contractor should ensure that pre-construction information includes historical details such as drawings showing existing services or potential presence of unexploded ordnance. If absent, these records should be obtained from the client.

Areas such as North Norfolk or other parts of East Anglian coast where existing offshore wind farms are linked to an onshore substation, power and telecom cabling can be present. For example, the gas terminals at Bacton are linked with offshore platforms, so both live or disused transfer pipes are likely to be present. The location of such services can restrict access for plant and installation methodology.

Groynes in coastal engineering 135 9 Monitoring and managing beaches with groynes

This chapter describes the need for monitoring beaches (to support both effective initial groyne design and ongoing groyne and linked beach-groyne system management strategies) (Figure 9.1). It discusses approaches to managing scenarios where the function of the groyne is jeopardised because of physical beach and/or cliff processes, the use of ongoing groyne adaptation to improve beach performance, and considerations for groyne removal. The main sections of this chapter are:

„ Beach monitoring (Section 9.1) „ Managing functional failure of timber groynes (Section 9.2) „ Adaptive management of groyne profiles Section( 9.3) „ Groyne removal (Section 9.4)

Coastal management Functional requirements and context indicators (Section 2.1) (Section 2.1)

Physical coastal environment – hydraulic, geotechnical and morphological constraints (Chapter 3)

Start or continue to No Groyne removal use groynes? (Section 9.4)

Yes

Plan shape design of groyne Evaluate and choose groyne field (Chapter 4) type (Section 2.2) and material (Chapter 6)

Design groyne profile (Chapter 5)

Structural design or design review for groynes (outline and detailed) (Chapter 7)

Construction and/or major refurbishment Major (Chapter 8) Minor Groyne repair and maintenance (Section 10.3)

Groyne adaptation (Section 9.2) and adjustments (Section 9.3) Small

Beach changes Routine monitoring Large (Section 9.1) (Sections 9.1 and 10.2) Structural deterioration (Section 10.1) Post-incident monitoring (Sections 9.1 and 10.2)

Climate change Storm events (Section 3.1.4)

Figure 9.1 Parts of groyne management process covered in Chapter 9

136 CIRIA, C793 9.1 BEACH MONITORING

As discussed in Rogers et al (2010), beach management works require a range of monitoring information from both local and regional/national monitoring programmes (see Box 9.1) to support both their initial development and to inform and direct their ongoing management. Different spatial and temporal scales of schemes will require different types, levels, and extents of information.

Beach profile surveys provide invaluable information on beach changes and can be analysed using software such as Shoreline and Nearshore Database System (SANDS) to derive maximum and minimum levels These surveys may be taken annually and/or seasonally to provide information on the seasonal variability as well as on the long-term trend. The seaward extent of the profiles survey is important, particularly when planning a groyne system and should extend below the low tide mark, to evaluate the likely nature and the extent of long-term changes. Regular surveys should also be supplemented by post- storm surveys. These surveys provide useful information on changes undergone by the beach during an extreme event, even though they may not be able to reflect the full extent of the temporary changes that may have taken place during the storm. When groynes are present along a coastline, beach profile surveys will be carried out at specific locations within a groyne bay and at set time intervals (eg every six months). So they may not capture the plan shape evolution of the bay. Where possible, profile data should be supplemented by 3D laser scanning surveys from roving vehicles or airborne light detection and ranging (LiDAR) enabling the build up of DTMs that can give a comprehensive picture of beach behaviour. New techniques and improved computing power have supported this shift from profile-based analysis (although extraction of profiles is still possible and valuable in some situations). DTMs can also be used to create profiles for comparison with older survey data, although may well not extend below low water. Beach profile data in England is currently available from the CCO (see Box 9.1).

Bathymetric monitoring has also moved from single point echo-sounding (on profiles) to multi-beam swath techniques, again enabling the building of a DTM model, which provides a detailed undersea picture of the bed conditions. Echo-sounding and multi-beam surveys in shallow water are, however, expensive if carried out using conventional survey boats and for this reason, small remote-controlled survey craft (eg ARCboat) are now being used.

Box 9.1 Evolution of the national coastal and beach monitoring programme

Beaches around the UK coast have been monitored (ie measured) for many years although this has often been ad hoc. A common approach was to record the beach height (above a datum, preferably OD, but sometimes an arbitrary local datum) along a repeatable, but imaginary, line roughly perpendicular to the general beach line. Alternatively, the locations of linear plan shapes (eg crest, MHW, mid-tide, MLW) were plotted. Several areas (eg Anglia, Sussex) and some individual local authority areas (eg Bournemouth) started more structured programmes of beach profile monitoring in the 1970s. Gradually, the foresight of grant awarding bodies (ie the Ministry of Agriculture, Fisheries and Food (MAFF), Defra and latterly the Environment Agency) led to the larger capital schemes being monitored post-construction, to assess scheme performance. A common approach was to take measurements along roughly orthogonal beach profiles. This was done either by stereo photogrammetric analysis from airborne photography or by traditional level/theodolite surveys. In the late 1990s, the coastal groups began to develop a system of region wide coastal monitoring programmes. These grew to become the national network of regional coastal monitoring programmes, which covers the whole coastline of England and is now approaching its third five-year phase. More information can be found at: www.channelcoast.org In the 1970s most surveys were undertaken annually and looked at profiles ranging in spacing from a few tens of metres (to suit groyne bays) up to one or two kilometres where the coastline was largely un-groyned. As increased funding became available, the following improvements were put in place:

„ smaller spacings between profile lines to give more insight to the behaviour of the beach in plan shape „ lower flying height for airborne surveys to deliver increased accuracy „ more than one survey annually. The advent of the regional monitoring programmes, which became the national programme, led to a re-examination of local monitoring programmes in terms of need and worth. The English coastal project now assesses the need to monitor beaches (and a range of other parameters) using a risk-based approach.

Groynes in coastal engineering 137 Shoreline positions extracted from topographic (including LiDAR) and bathymetric surveys, Ordnance Survey maps and/or aerial/satellite images can be used to supplement the beach profile survey data and to support the validation of plan shape models. More information on beach surveys and analysis of plan shape changes can be found in Rogers et al (2010) and from the CCO. To understand what a groyne or groyne field may achieve in terms of beach control, evidence of what has happened in the past or to similar systems is often more valuable than modelling, and beach monitoring data is essential.

While understanding the nature and variability of the bathymetry and topography of a particular beach is important when designing a scheme, it may also be important when trying to understand what may be causing certain anomalies in a maintained coastline and how these can be addressed through proactive maintenance. In this regard it is important to examine the monitored beach volumes downdrift of the groyne field as well as within the groyne bays themselves to check trends in the movement of material and assess any potential detrimental effects.

Where the movement of the beach may influence groyne stability or functionality, coastal state indicators and critical thresholds may be identified (using knowledge gained from modelling, local knowledge, previous monitoring etc). Further targeted monitoring will be required to determine any progression of the indicators towards the thresholds and to highlight when actions need to be taken. For more information on managing coasts using coastal state indicators, see Blanco et al (2019).

9.2 MANAGING FUNCTIONAL FAILURE OF TIMBER GROYNES

9.2.1 Managing underrun of beach material When the beach level falls to such an extent that it exposes the base of the lowest row of planks, a gap forms beneath the base of the groyne and beach material starts to underrun the groyne (Figure 9.2). In this instance, the groyne loses all effectiveness at retaining sediment, with subsequent scour of the beach likely to be initiated. The formation of this gap may result from shortcomings in the original groyne design (including failure to anticipate the range of beach movement), poor quality construction, lack of maintenance or from excessive beach movement. Where groynes are constructed on sites that are subsequently nourished with sediment, beach loss and underrun is generally not an issue for some years.

If a groyne is to continue to be effective, closure between the lowest beach levels, seabed and groyne planks should be maintained (either by closing the planks onto the substrate or by extending the groyne planks some way beneath the typical beach levels) (see Section 7.2). The inefficiency of the groyne in the example in Figure 9.2 is demonstrated by the identical beach levels on either side of the structure, the general absence of sediment and the gaps beneath the planks. One of the benefits of rock groynes relative to timber Figure 9.2 Beach lowering resulting in gaps beneath the groynes (courtesy New Forest District Council) groynes is that if provided the erosion is not too severe, then underrun should not occur, as the rock will settle naturally to a new level maintaining permanent closure between the substrate, the beach and the structure.

As shown in Figure 9.3, the groyne is no longer effective because of loss of sediment and underrun.

A further complication arises in this example – the Figure 9.3 Falling beach levels have rendered this groyne erosion of the beach at the seaward end of the ineffective (courtesy New Forest District Council)

138 CIRIA, C793 groyne now means that beach levels at the end of the groyne are almost permanently immersed, limiting access for maintenance to very low spring tides.

When maintaining existing groynes to avoid the underrun (see Section 7.2), three pieces of information are required: „ Regularly measured beach profiles that provide an indication of the maximum and minimum extent of the beach levels. „ As-built drawings that identify the levels of the lowest parts of the groynes. These may not be available, or modifications made to structures during maintenance may not have been recorded in a systematic manner. In this situation, groyne managers may be unaware of how likely groyne undermining may be, but regular checking of beach profiles relative to the groyne geometry may eventually assist in establishing the unknown base levels. „ Details of substrate levels beneath the beach. These may be determined by site investigation, during construction or over a period of time as different maintenance activities are undertaken. Another source of information is pile driving records that have recorded a change of resistance when the piles encounter the bed.

Figure 9.4 shows an example of clay substrate levels established by excavation of small areas during previous maintenance activities and annotated onto an as-built drawing, together with data from a recent beach survey.

Figure 9.4 As-built groyne elevation showing (in red lines) clay substrate (a) and recent beach profile (b) (courtesy New Forest District Council)

Figure 9.5 gives an example of beach profile surveys in the vicinity of a groyne. The line labelled ‘recent eroded profile’ suggests that the current beach elevation is at the bottom of the beach profile envelope. This information may trigger an alert based on the agreed coastal state indicators (see Blanco et al, 2019) as the bottom of the groynes may well have been set at the lowest beach profile line. Care is needed to make sure that the beach profile data being used for any assessment is reflecting conditions at the groyne itself rather than another position in the groyne bay.

Figure 9.5 Beach profile envelope within groyne compartment, showing substrate (bedrock) profile and recent beach profile

Groynes in coastal engineering 139 In situations where undermining has rendered the structure ineffective, additional planks may be added to the bottom of the structure to close the gap with the seabed (Figure 9.6). Additional excavation well below the current surface is advisable to avoid repetition of the issue.

One of the challenges of adding planks to the lower part of the structure is the safety of contractors working in a zone where water might be naturally present over part of the tidal cycle. One approach that has been used to aid the safety and speed of installation of either new construction, or maintenance over the lower part of the structure is the use of prefabricated ‘sheet pile panels’. These may enable more rapid fixing in the difficult lower intertidal zone (Figure 9.7). However, this approach is no longer recommended because of the risk of so-called ‘cat-flap’ failures, where the sheet pile panels become detached from the piles and lateral forces push the sheet piles to one side of the structure while remaining hinged at their top.

Figure 9.6 Additional planks being added to the base Figure 9.7 Sheet pile panel installation on a timber groyne of a structure, following undermining (courtesy New Forest (courtesy David Harlow) District Council) 9.2.2 Managing landward outflanking Although groyne fields are generally constructed in combination with seawalls, they may also be constructed to control the flow of sediment along duned, cliffed or barrier beach frontages. In both cases, erosion may occur at the landward end of the structures and outflanking may occur because of damage or removal of part of the groyne – potentially isolating the end of the structure (Figures 9.8 and 9.9). Under these circumstances when water levels and wave run-up reach the upper part of the beach, material may be able to pass downdrift unhindered by the groyne and the system then becomes much less effective at reducing longshore transport rates. It is very difficult to tie the groyne structure in at its landward end in these situations, so it should be recognised that considerable maintenance may be necessary to ensure that the groyne system continues to perform as required.

Figure 9.8 Stranded groynes on a barrier beach frontage at Figure 9.9 Stranded groynes on a soft Medmerry, West Sussex eroding cliff frontage, Colwell Bay (courtesy Isle of Wight Council Solutions to this include: „ creating landward extensions of the groyne

140 CIRIA, C793 „ limiting localised erosion (exacerbated by reflection off the root of the groyne by linking the groyne to a breastwork or by providing armourstone that can be flexible because it can be added to or be rearranged if required) „ providing local beach nourishment at the landward end of the groyne.

9.3 ADAPTIVE MANAGEMENT OF GROYNE PROFILES

The groyne crest profile should be set in relation to the beach profile as described in Section 5.4 and reflecting the likely maximum and minimum beach profiles discussed in Section 5.3. However, designing groynes to accurately reflect beach behaviour is highly uncertain and adjustments may have to be considered to maximise their value and minimise long-term maintenance needs. In instances where groynes are repeatedly failing, a change in the management approach may need to be considered, for example, where this is possible, adopting an adaptive rollback approach (see Section 2.3.3).

It is possible to adjust the level of the groyne planks relative to the beach, either by removing or adding planks to the structure, during maintenance. This activity is restricted by the elevation of the pile tops, relative to the groyne planks. In forming an opinion as to the need for adding or removing planks, the effect of the resulting change of planking on beach profiles and volumes should be assessed, not only on the immediate (updrift) groyne bay but also on downdrift bays and possibly beaches.

If it is the intention to ‘grow’ the beach by adding planks to trap additional material from the longshore transport (see Box 9.2), this should be done at a pace that suits the available sediment budget and not increase volumes in one bay at the expense of others. Gradual increases in volumes are to be preferred as the resulting beach is likely to be more stable. However, in some situations, it may be beneficial to ‘grow’ the beach rapidly to recover a sudden loss of material. In these cases, consideration should be given to importing material to make up the deficit, but cost may preclude this.

Sections 9.3.1 and 9.3.2 examine two typical scenarios and management approaches that can be adopted in each case.

Box 9.2 Adjustment of beach plan shape by adjusting groyne profiles at East Preston, West Sussex

For many years, there was a marked step-back in beach alignment at East Preston in West Sussex. This step-back (Figure 9.10) was presumed to be due to differing management practises by landowners befo the introduction of the Coast Protection Act 1949.

Figure 9.10 The step-back in seawall alignment behind Figure 9.11 Reconstructed groynes at East Preston, the groyne at East Preston, West Sussex, taken in the West Sussex, c1983 (courtesy Arun District Council) 1950s (courtesy Arun District Council)

When the existing timber groynes reached the end of their service life in the early 1980s, Arun District Council replaced them (Figures 9.11 and 9.12), but with a view to addressing the plan shape issue (note that in Figure 9.11 the groyne profile is much higher than the existing beach). Groynes were installed with over-length piles, driven to provide the ability to ‘plank up’ the groynes, as shingle gradually accreted. Planks were added in the ‘waist’ section, to gradually raise and push forward the crest without undue downdrift effects This was done at nominal cost as part of the council routine repair and maintenance works.

Groynes in coastal engineering 141 Box 9.2 Adjustment of beach plan shape by adjusting groyne profiles at East Preston, West Sussex (contd)

Figure 9.12 Aerial photograph of groynes at East Preston, Figure 9.13 Aerial view showing straighter beach West Sussex, 1985 (courtesy Arun District Council) alignment at the old step-back at East Preston, West Sussex, 2016 (courtesy CCO)

When the piles could take no further planking, they were fitted with lift piles (230 mm x 230 mm) bolted to the parent pile of the same dimension. This process was repeated, with some piles being lifted twice, over time, to achieve the desired result. The resulting groynes are structurally sound and have brought the beach seaward locally by about 20 m, providing a more linear beach plan shape (Figure 9.13) across the previous step-back and an easier beach to manage. Fortunately for Arun District Council, there were no critical assets at immediate risk which required an increase in beach width, and so the gradual improvement could be achieved without detrimental effects downdrift or the need to artificially nourish the beach.

9.3.1 Existing groynes significantly above existing beach profile Figure 9.14 shows an example where the groyne planks are well above the beach over most of the groyne length. There is little prospect of material bypassing the groynes across the top of the planks, except at the landward end of the structure, which will only occur under the more energetic conditions and at the upper end of the tidal range. The upper planks are not contributing to maintenance of the beach and are simply a burden on the structure Figure 9.14 Profile of a high timber groyne maintenance. This situation generally occurs some years following initial groyne construction, when the structure is likely to match with beach profiles more closely.

Beach nourishment is one solution to this as the system has the capacity to hold a significantly greater volume of sediment than is currently available. For example, in Figure 9.15, the piles extend some way above the top plank level and this could allow additional planks to be added over part of the beach profile to further improve capacity for beach nourishment to be added to Figure 9.15 High timber groynes at Bognor Regis, West Sussex (courtesy Arun District Council) the system.

142 CIRIA, C793 Lowering of the groyne profile is an alternative solution, as the groynes may be too efficient in their current condition. There is potential to remove some of the groyne planks and lower the groyne elevation, making potential long-term maintenance savings on planks and fixings. Care should be taken when selecting areas of the groyne from which to remove planks. For example in Figures 9.14 and 9.15 the following is evident: „ The crest of the groyne at the root adjacent to the seawall is already at an elevation where passage of material may occur across the groyne crest, so adjustment would not be sensible at this location. „ Over much of the remainder, there is potential to lower the groynes by as much as six planks high (about 1.5 m), while maintaining a similar level of efficiency.

9.3.2 Existing groynes at or below existing beach profile Where beach sediment is at or above the level of the groynes (Figures 9.16 and 9.17), the groynes may have performed satisfactorily. However, if required, additional capacity can be achieved by increasing the level of groyne planks. The implications of this on downdrift beaches should be considered as tuning of groyne crest profiles may have a negative effect on downdrift sites. These activities should be carried out carefully with beach monitoring and Figure 9.16 Low groyne elevation showing sediment transport across the modelling where appropriate. upper beach (courtesy Worthing Borough Council)

If it is considered that additional planks would be beneficial then this should be done incrementally. Fixing too many planks in one operation will have one of two outcomes: „ Leaving an excessive area of exposed planking – potentially providing a larger than desirable face of planking for waves to

reflect off (disturbing Figure 9.17 Profile of a low timber groyne accreted sediment) and/ or providing a surface for waves to run up and against, leading to localised overtopping and disturbance of the crest. „ Attracting too much material, to the detriment of nearby bays or unduly affecting the plan shape of the beach. Sometimes it can appear that extra planks would be possible for groynes located in a ‘headland’ area (ie at the apex of a beach with an overall convex plan shape). However, gaining more material in a bay that is already at the apex of a coastline will accentuate it, ultimately making it form a mini-headland, which may have a negative effect on other locations along the coast.

If it is decided that fixing extra planks is practicable and desirable, but the pile height does not allow more planks to be fixed, then additional pile height can be provided by bolting, or in some cases, scarfing an additional timber member or ‘lift pile’ onto the existing pile. Lift piles are typically fixed to raise the profile by not less than two planks in height (smaller lifts are rarely cost beneficial). Although this extends

Groynes in coastal engineering 143 the total length of the pile, revised design calculations may not be necessary to support this change because the existing pile is now receiving more support from the growing beach. Even if the pile visible above the beach is in poor condition, the portion buried in the beach may be in good condition and may be capable of supporting new elements fixed to the existing structure. Areas of rot in the pile can be spanned to maintain the structural integrity of the pile and any planks fixed to it as much as possible.

The ‘lift pile to host pile’ connection may need to be specifically designed. As a typical rule, a lift pile raising the height above the host by 0.5 m will need two bolts into the host pile and anything greater will require three bolts. It is also possible to fix a further lift pile to an existing lift pile; here it may be necessary to rely on the overturning resistance generated in the extra planking, rather than relying on the cantilever from the embedment, which could exceed original design parameters. Once a pile has been extended, planks are fixed in the usual manner (see Section 8.3.5).

9.4 GROYNE REMOVAL

The decision to remove (or replace) a groyne or groyne field should be taken with care but may be justified for the reasons set out in Section 2.3.3. As a guide to the approach, the following questions (with their suggested approach to finding the answers) may be useful: 1 Is the groyne field working as anticipated? Did it ever work properly? Reasons for lack of working may include a predominance of onshore-offshore transport rather than longshore transport. 2 Does it matter that they are not working? It may not matter if a policy of managed realignment or rollback is being adopted or alternative methods of protecting the coast from erosion are to be preferred. 3 Is it possible to remove them? Assessment of the likely erosional impacts of groyne removal on beach behaviour should be carried out, following the guidance on understanding beach behaviour set out in Chapters 3 and 4. It is also recommended to follow any available general guidance on decommissioning of coastal assets. 4 In what order should they be removed? If removal of groynes takes place as part of a groyne renewal scheme, the sequencing of the removal of the old groynes and the replacement with new groynes should be carefully managed to avoid unnecessary loss of beach material. Again, the guidance in Chapter 4 should be consulted. Generally, it is better to proceed from downdrift to updrift with the construction and demolition processes and to delay removing old groynes until the equivalent downdrift groyne has been installed. 5 What process should be followed for the physical removal of groyne elements? The process should follow the same general guidance for construction set out in Chapter 8. When removing an old groyne, where possible all elements of the groyne should be removed. This is a relatively easy operation for the planking using the equipment listed in Section 8.4. However, piles may have been driven well into the substrata and may resist extraction, even when specialist extraction machinery is used. In these cases, the pile may be cut off. Snapping of piles should be avoided as this will leave unsafe splinters and spikes in the beach. Piles are often cut off a minimum of 1 m below beach level, but beach variability should be considered before deciding the cut-off depth. Whatever decision is reached, about pile cut-off level, subsequent monitoring of the beach behaviour is recommended to check that the cut-off piles are not becoming exposed and creating a public safety risk.

144 CIRIA, C793 10 Inspecting and maintaining groynes This chapter provides information and guidance on the inspection and maintenance of groyne fields to deliver the design structural performance over the required service life (Figure 10.1). In particular, the chapter discusses the prediction of rates of groyne deterioration in order to plan proactive maintenance, the planning of inspection programmes and inspection processes, and specific proactive and reactive maintenance activities and techniques. The maintenance of non-timber structures is discussed briefly. The main sections of this chapter are:

„ Deterioration of timber groynes (Section 10.1) „ Groyne monitoring and inspection (Section 10.2) „ Maintenance practices for timber groynes (Section 10.3) „ Maintenance of non-timber groynes (Section 10.4)

Coastal management Functional requirements context and indicators (Section 2.1) (Section 2.1)

Physical coastal environment – hydraulic, geotechnical and morphological constraints (Chapter 3)

Start or continue to No Groyne removal use groynes? (Section 9.4)

Yes

Plan shape design of Evaluate and choose groyne groyne field (Chapter 4) type (Section 2.2) and material (Chapter 6)

Design groyne profile (Chapter 5)

Structural design or design review for groynes (outline and detailed) (Chapter 7)

Construction and/or major refurbishment (Chapter 8) Major

Minor Groyne repair and maintenance Groyne adaptation (Section 9.2) (Section 10.3) and adjustments (Section 9.3)

Small Beach changes Routine monitoring Large (Section 9.1) (Sections 9.1 and 10.2) Structural deterioration (Section 10.1) Post-incident monitoring (Sections 9.1 and 10.2)

Climate change Storm events (Section 3.1.4)

Figure 10.1 Parts of groyne management process covered in Chapter 10

Groynes in coastal engineering 145 Groynes will deteriorate over time because of their exposure to the marine environment and associated loading conditions. Ongoing maintenance will be required to protect their structural performance.

Required maintenance is likely to be undertaken on both a proactive (or anticipated) basis – using best case predictions of the likely deterioration, validated by on-site inspection programmes, and a reactive basis – driven by evidenced, unanticipated failures, eg post-storm events.

10.1 DETERIORATION OF TIMBER GROYNES

10.1.1 Introduction Understanding deterioration, where it occurs and what effect it is likely to have on a timber groyne is essential for planning required long-term maintenance strategies.

The rate of deterioration of timber groynes varies significantly according to sediment type, construction detail, wave climate and tidal range, and quality of the original design and construction. Damage to groynes may arise from the following: „ mechanical abrasion of groyne elements within the beach „ marine borer attack (gribble and/or shipworm) „ differential large-scale wave and sediment loading during storm wave conditions „ undermining following cross-shore beach loss during storms „ collisions from debris.

As groynes are constructed of many elements, effective design (see Chapter 7) and maintenance (this chapter) should take account of these damage mechanisms. Failure of the weakest elements may also mean damage to, or loss of, otherwise sound components. Similarly, failure of individual piles can result in major structural issues. So it is important to identify the site-specific hazards that may affect the specification of materials, maintenance intervals and service life.

Fixings for planking have historically been a weak point on timber groynes, with loss of planks from a structure invariably the result of failure of the fixings, rather than wear of the timber. On even a (short) groyne about 20 m long, constructed to a shallow depth, there are likely to be upwards of about 150 fixings per groyne. When considered as a maintenance issue and recognising both the cost of materials and labour, the number of fixings and the need for replacement timber, becomes an important cost issue. Both material and fixing style may have a significant effect on the durability of groynes. As recommended in Section 7.4.4, stainless steel will offer a much longer service life Figure 10.2 Corroded and abraded steel fixings than mild steel, which corrodes and then abrades rapidly (Figure 10.2). Use of bolts should ideally be restricted to double nutted connections between main structural members, with coachscrews being used for fixing planks to piles where possible. Discussion of maintenance practices for bolted connections can be found in Section 10.3.3.

10.1.2 Predicting deterioration rates for timber groynes Deterioration rates may be estimated by considering the performance of different materials under different exposure conditions. An understanding of deterioration and how maintenance activities might influence deterioration can inform ‘deterioration curves’ which can then be used to predict

146 CIRIA, C793 when interventions, such as proactive maintenance, may be optimal or necessary to maintain acceptable asset performance.

Deterioration of timber is a time-dependent process and is caused by variable agents of degradation such as attack by marine borers, abrasion and other sources of damage such as sudden large wave impact or unusual lateral loadings caused by a reversal in the prevailing longshore drift. Deterioration rates are influenced by these factors and by the type and quality of timber used and the relative quality of construction.

Timber does not deteriorate in a linear manner when exposed to biological agents of degradation and, once affected, the rate of deterioration tends to accelerate which makes prediction more challenging. However, abrasion, which is mechanical damage, can be predicted and expected to progress on a linear model although different rates of deterioration will occur at different positions on the groyne. Rates of deterioration also depend on several factors that are influenced by climate change (eg biological agent activity may accelerate in warmer temperatures) and guidance should be followed regarding the extent to which climate change should be considered when planning resilient maintenance programmes (Environment Agency, 2019).

Predicted deterioration rates are based on observations of degradation of similar systems and/ or degradation modelling. However, such predictions require validation for the local context using observations of deterioration of the groyne itself or nearby groyne fields – especially data on previous failure points, ie when and where this can occur. Deviations from predicted trends may occur in particular circumstances, eg the arrival of marine borers due to changing environmental conditions or opportunistic colonisation from driftwood. In addition, data from one site may not be directly applicable to another site even if service conditions, hazards and construction material are similar. This is due to the variable nature of timber and the fact that deterioration rates vary considerably along each groyne and also between groynes in a particular groyne field.

Given the variability of the different types of timber groyne constructions and exposure conditions, it is not possible to develop a single generic deterioration curve for timber. Deterioration curves need to be based on exposure conditions and the selected timber species. These curves may provide a crude estimate of deterioration over time. These may prove useful for predicting when it is likely a groyne(s) will progress from one condition grade (see Table 10.1) to the next, or, more significantly, when the groyne passes the threshold ‘point of no return’ in terms of effective performance. It is unlikely a groyne will move forward more than one condition grade unless subject to catastrophic failure, which may occur after a severe storm event or human activity affecting a groyne. Deterioration curves may be able to indicate when a structure will move from one condition grade to the next under the prevailing conditions. A deterioration curve for a whole groyne may be affected by strategies employed to counter the effects of deterioration of certain elements or components. Observed timber deterioration curves may be used to provide evidence to show that design modifications may be either successful or unsuccessful over time because of their implementation. Such an approach could optimise design, specification of materials and construction decisions on a nationwide basis depending on the geographic location and service conditions.

Box 10.1 presents examples of deterioration curves developed using the assessment methods described in Section 10.2. The curves compare Greenheart groyne performance under a range of different conditions.

Groynes in coastal engineering 147 Box 10.1 Comparison of groyne deterioration curves

The following examples show deterioration curves developed using the assessment methods described in Section 10.2 (Figures 10.3 to 10.6). The curves compare Greenheart groyne performance under a range of different conditions. The deterioration curves are based on general observations relating to overall groyne condition. The curves illustrate likely effective service life if no maintenance is undertaken (thick lines) contrasted with service life when groynes are subject to maintenance (thin lines). The condition curves assume that maintenance is undertaken at regular intervals although this may not be the case in practice. The threshold level on the curves is the point at which the groyne no longer performs effectively. The green section of the curve denotes that the groyne is in good condition. The yellow section denotes an acceptable level of deterioration where performance has not been adversely affected, for example, small gaps may develop between planks. The red shading denotes the level where the groyne no longer meets its performance requirements and maintenance is needed to restore functionality. Differences in the performance of timber depending on exposure conditions can be seen by comparing deterioration curves. Figure 10.3 shows the deterioration curves of Greenheart groynes at Milford-on-Sea and should be compared with the performance of Greenheart groynes at Bournemouth shown in Figure 10.4. Both sites are located on the south coast in Dorset and are separated by about 10 km. The beaches of both sites are characterised by a narrow tidal range. However, the sediment at Milford-on-Sea mostly comprises shingle (causing rapid deterioration) whereas that at Bournemouth is fine sand (slow deterioration). (In the case of Figure 10.4, the effectiveness of groynes positioned on Bournemouth beach is compromised even by small gaps developing between planks. So, there is no acceptable level of deterioration and no yellow zone on the figure.)

Figure 10.3 Deterioration curve for Milford-on-Sea, Hampshire

Figure 10.4 Deterioration curve for Bournemouth, Devon

While Figures 10.3 and 10.4 illustrate the difference in the performance of Greenheart under different conditions, a comparison of Figure 10.5 (Dawlish Warren) and Figure 10.6 (Withernsea) illustrates how similar performance may be determined at two different geographic locations on the UK coastline. Both beaches are characterised by a wide tidal range and sediment mostly comprising sand. Both deterioration curves indicate that the groynes may deliver a service life of about 40 years although periodic maintenance needs to be undertaken, typically at 15-year intervals.

148 CIRIA, C793 Box 10.1 Comparison of groyne deterioration curves (contd)

Figure 10.5 Deterioration curve for Dawlish Warren, Devon

Figure 10.6 Deterioration curve for Withernsea, Yorkshire

See: Williams (2018) and SCOPAC: https://scopac.org.uk/research/performance-of-timber-groynes-bournemouth

10.2 GROYNE MONITORING AND INSPECTION

10.2.1 Introduction The monitoring and inspection of groynes is important for: „ Supporting an understanding of the deterioration rates of the groynes and the impact of any site- specific characteristics and/or loadings on those rates. „ Identifying necessary repairs (required either as due to long-term deterioration or short-term storm damage). „ Evaluating the performance of any previous maintenance strategies in protecting the groyne. „ Supporting the development and planning of robust future maintenance and replacement works schedules and budgets.

Inspections should follow a consistent pattern, method and frequency (see Section 10.2.2), which will identify the agents of degradation and help inform rates of deterioration and understanding of modes of failure and where deterioration and damage is most likely to develop. Frequent and detailed inspection and monitoring leads to more effective asset management.

Coastal management teams in operating authorities will undertake some monitoring of structures as they carry out their land-based surveys, but individual structures should be monitored separately

Groynes in coastal engineering 149 regularly. Some teams undertake these more detailed surveys as a separate exercise reporting to the relevant local authority.

The principal service hazards for which the groynes should be inspected include loss of planking, undermining, excessive updrift structure loading, landward outflanking, wear of groyne elements and fixings, biological deterioration, and fixing loosening and loss.

Regular inspections can be quick visual assessments or detailed surveys. Invasive surveys do not need to be conducted regularly, but some authorities may occasionally take impact readings or timber cores to assess overall deterioration rates, for example, when a new type of timber has been used as a prototype experiment.

Experienced inspection staff, familiar with their area will recognise the onset of deterioration, whereas less experienced staff will rely on an accepted convention to report conditions. A good example of this would be by following guidance in Environment Agency (2012), but note that it only provides a common condition grading system. In addition, there will be a need to record what works (if any) are required or desirable.

10.2.2 Nature, timing and frequency of groyne inspections Patterns, methods and frequencies of inspection should be consistent, to enable life-cycle degradation to be monitored.

Ideally, structures should be inspected at least once or twice per year. They should be timed to allow planning of maintenance, which is often conducted outside of the summer season when recreational use of the beaches is high. Inspections may be planned to coincide with the end of the summer, enabling maintenance to be undertaken before the winter, or in the early spring following winter storms. Surveys carried out in winter months or in the early spring, when beach levels tend to be lower, are more likely to reveal exposed sections of structures and, specifically, worn piles. Some operating authorities with particularly vulnerable or sensitive groyne systems increase the inspection frequency over and above the suggested minimum two inspections, carrying out intermediate surveys using their maintenance teams or contractors who are empowered to undertake routine repairs when damage is found. These repairs can be measured and paid for afterwards with single walk-through verification by the coastal manager.

Additional or alternative ad hoc inspections may be appropriate to take account of: „ Storm events, which often lower beach levels adjacent to structures, as well as weakening or destabilising elements of the structure. Post-storm inspections are recommended to minimise further deterioration and to take advantage of the exposure of parts of the groyne that would otherwise remain hidden. „ Reports of damage submitted by members of the public, other body or other operating authority team.

Having established a regime for inspections, the inspector will decide whether the inspection will be one of the following: 1 Walkover surveys will be rapid exercises proceeding from one end of the beach/frontage to the other with a brief stop at each groyne to gain an impression of its overall condition grade. The journey from one groyne to the next can be used to gauge the effectiveness of the groyne by assessing the overall condition of the beach in that bay. Despite being relatively fast, it will allow the inspector to pick up where there are missing or poorly fitting planks and where there has been a marked change in the beach. Also, an inspector more familiar with the frontage will be able to detect more subtle changes and identify trends. For example, a walk-through inspection at the end of the winter season when beach levels are at the lowest may enable initial evidence of marine borer activity to be picked up. 2 Detailed inspections require more planning and time and will involve taking notes of each element of the structure and its full length inspected from both sides.

150 CIRIA, C793 To achieve consistency, detailed inspections should ideally be carried out at a MLWS tide and when there is a calm sea state. This will allow a thorough inspection in the intertidal zone, especially if the beach is characterised by a narrow tidal range. Winter and early spring inspections when beach levels are low may reveal sections of the groyne that are normally permanently buried. These are unlikely to be exposed to abrasion or biological attack. In any reconstruction, such undamaged members of a timber groyne may provide possible material for reuse. The breeding season for shipworm and gribble tends to be from late spring and through the summer. Evidence of marine borer activity may be picked up in the autumn months. Whatever the speed of inspection, fresh exposure of previously-buried planking (shown by a different colouration of the face of the planking on the updrift side) should be noted and its cause explored. A freshly exposed beach face, evidenced by a near vertical, well-graded and packed beach face, will indicate beach erosion and this should also be noted, and its cause explored. One explanation for the erosion may be a fault in the downdrift groyne structure allowing material to pass it more quickly than designed. 3 Post-storm inspections (conducted with due awareness of the inspector’s health and safety) will enable inspectors to assess the scope of any necessary repairs. An indication of gradual wear of planks and other elements may be noted on such an inspection, but this can be left for the next detailed inspection, as wear is a comparatively slow process.

10.2.3 Inspection logistics Direction of inspection Each inspector will have their own references, but detailed asset inspections often proceed from downdrift to updrift. Walk-through inspections can be undertaken in either direction, but working in the opposite direction (ie from updrift to downdrift) gives the inspector a spatial overview of the way in which the morphology of the beach is operating.

Working on foot and proceeding updrift, ie approaching the groyne from the leeside, the inspector will get a better view of the ‘gappiness’ of the groyne’s planking and wear to the piles supporting the planks. The inspector may then form an opinion of the general condition of the planking from the planked side, noting wear (abrasion), any rot developing, fixing soundness and any split, broken or missing planks.

An inspection of a relatively heavily groyned beach may be conducted on foot. However, all terrain vehicles (ATVs) and drones can be useful for very rapid inspections, such as when gaining a first impression after a storm. There may be challenges in moving the ATV from one groyne bay to another, but they are worthwhile considering for widely spaced groynes, where most of the inspection time would otherwise be taken up with the walking between groynes. However, there may be restrictions on their use associated with environmental issues such as nesting birds.

Accessibility of groynes for inspection One of the challenges of groyne inspection is that some parts of the structure are likely to be covered with beach sediment and cannot be inspected (unless costly excavations are carried out). However, on a positive note, it is mainly the exposed sections of the structure that are most susceptible to abrasion and are those most likely to require maintenance. Generally, it is reasonable to assume that groynes showing signs of wear on piles above the beach level will also be worn to some depth below beach level. Ideally inspections should be conducted when beach levels are at their lowest, although this may be difficult to achieve in practical terms.

Deeper and more permanently buried sections of structures are less likely to require maintenance. In some instances, timbers that have been buried and that have been in place for more than 25 years may be in ‘as new’ condition. Opportunities to view buried sections of the structure (eg during temporary excavations for maintenance) should be accompanied by a detailed inspection of all elements.

Groynes in coastal engineering 151 Beach profile data can be used effectively to plan and maximise inspection opportunities, especially if they are longer term records. The profile envelope will capture the upper and lower limits of the beach over time, the profile position (Figure 9.5) together with the mean profile. Levels adjacent to the nearby groynes may be inferred and provide a good indication of the potential zone of abrasion or marine borer damage to the structure. Inspection following a period of high beach levels is not ideal as most of the structure is likely to be buried. Surveys following periods of erosion created by sequences of storms present the opportunity to inspect the structure at maximum exposure.

Any inspections will need to be planned around beach accessibility (see also Section 8.1.5).

Health and safety Timing of any inspection should consider tides (times and heights). Maximising the available inspection period by starting as soon as possible after high water and then working over low water and continuing until the rising water level means it would be no longer safe to continue.

An inspection should include a health and safety risk assessment that considers the following: „ risks that recent storm(s) may have weakened structures, as traversing over weakened structures may lead to ‘slips, trips and falls’ „ actual soft sand or cliff falls (more likely if there has been recent heavy rain or frost) „ rapid changes in weather conditions (which can also affect ground conditions) „ early incoming tide (always keep one eye on the sea) „ emergency experienced en route (eg a member of the public in difficulty).

Other issues for the inspector to note in pre-planning include: „ provision of suitable PPE, including when working near to water, safety lines and/or life jackets „ any necessary tools (see Inspection toolkit) „ food and drink „ working alone or in pairs (For ease of getting to and from site, it may be useful to consider two inspectors working together but each dealing with a separate part of the beach. In this way vehicles can be left at the start and part-way along the route. Inspectors can stay in touch by short range radio if there is poor mobile phone coverage. If lone working is unavoidable then a buddy system should be set up.) „ public use of the beach (eg tourists or dog walkers (often a good source of local knowledge) and other leisure activities).

Inspection toolkit While the essential part of inspection is good observation and common sense, there are many other items that are useful to bring to the inspection (besides a notepad and pen/pencil or digital app device such as a tablet etc). Each organisation will have its own approach, but a check list to consider is as follows: „ schematic drawing of the groyne (using water resistant paper) „ electronic or paper forms (see Section 10.2.5) to record asset condition, including assessment of biological attack and/or mechanical abrasion „ spirit level (2 m) „ flexible tape measure (tailor’s tape) „ ruler (300 mm) „ adze „ hammer (geologist) „ multi-tool or knife, or chisel and bradawl for probing the timber surface „ mobile phone/two-way radio

152 CIRIA, C793 „ first aid kit „ PPE (additional to that used normally) „ safety glasses „ gloves „ waterproof clothing „ extra layer of warm clothing „ stick measure „ camera „ torch „ rope „ spray paint or another marker „ back-up notepad and pen/pencil.

Note that needles or other ‘sharps’, discarded by beach users or washed up, may be found during the inspections. It will be for the inspector to decide if they are willing or able to recover these sort of items (small sharps recovery kits are available). In any event, they should not be left on the beach for others to injure themselves and should at least be reported for recovery by trained personnel.

10.2.4 Specific inspection activities Inspections can take a variety of forms to meet specific asset management objectives. This section details specific inspection activities. Visual condition assessment is likely to be undertaken as part of a walk- through inspection, whereas marine borer or abrasion assessments will form part of more detailed or independent assessment activity. The key factors to consider in undertaking a detailed inspection of timber condition are: „ Inspect for evidence of marine borer attack. This should be carried out in the seaward third of the groyne and at the lowest part of the intertidal zone, directly above beach level. „ Inspect for abrasion. This may be more of a challenge to assess. The groyne should be examined along its length at low tide to establish where the maximum point of abrasion has occurred. This may typically be in the mid-third section of the groyne where exposure to volatile beach material is most severe. „ Inspect for decay. Decay is more likely to occur in the upper sections of the groyne towards the landward end and above the intertidal zone.

Overall visual condition assessment The Environment Agency (2012) has published guidance on assigning broad condition grades (see Table 10.1) to timber groynes. These overall condition grades will not reflect differences in deterioration rates across different sections of a groyne (eg the seaward end may deteriorate faster than the landward end). In addition, one critical section of a groyne in comparatively poor condition may adversely affect the condition grade for the whole structure.

Table 10.1 Groyne condition grades (from Environment Agency, 2012)

Numerical Condition Description grade summary General: cosmetic defects that will have no effect on performance. 1 Very good Key features: no significant defects. No significant gaps between planks or missing planks. No structural damage to walings, ties or fixings. No undermining.

General: minor defects that will not reduce overall performance of the asset. 2 Good Key features: minor defects. Minimal damage to or loss of planks. Minimal structural damage to ties, walings, or fixings. No undermining.

Groynes in coastal engineering 153 Numerical Condition Description grade summary General: defects that could reduce performance of the asset.

3 Fair Key features: some damage to, or absence of planks. Some movement, rotation or bulging possible but groyne still ensures arrest of majority of beach material drift. Minor damage to ties, walings and fixings. No undermining. General: defects that would significantly reduce the performance of the asset. Further investigation needed. 4 Poor Key features: structurally unsound now or in the near future. Substantial loss of or damage to planks significantly reducing arrest of beach material drift. Partial undermining. Significant movement or rotation of groyne, damage or loss of ties, walings or fixings. General: severe defects resulting in complete performance failure.

5 Very poor Key features: completely failed or derelict. Loss of a significant number of planks and groyne no longer arresting drift of beach material. Severe movement, rotation or undermining. Ties, walings and fixings damaged or missing.

Detailed inspection for marine borers When looking for marine borers, the inspector has to know where to inspect the structure and what to look for. The inspection may be broken down into two sections: 1 Gribble tend to inhabit sheltered parts of a groyne and may be found at the pile/plank interface on the landward face of piles on the downdrift elevation of a groyne in the intertidal zone. Their presence is often indicated by the characteristic galleries present immediately below the surface of the timber. These maybe exposed by scraping away the surface of the timber with a sharp adze. In many cases, abrasive tidal action will reveal the galleries (Figure 6.11). 2 Shipworm are harder to detect. The only visible parts of the animal are the siphons, which are often hidden within marine fouling. Unless the combination of shipworm attack and abrasion has exposed the tunnels, the only easy and practicable method to detect shipworm is to strike off with an adze an edge of timber from a number of selected piles and planks on the more sheltered landward face of the groyne in the intertidal zone. Where practicable, an assessment should be made to target sections where there may be sapwood, particularly in Greenheart piles. The outer 5 mm to 10 mm of timber should be removed from the edge of the pile over about 500 mm at the lowest part of the pile. Removal of the outer layer of timber will reveal underlying tunnels if there has been any infestation (shipworm tunnels have been revealed in this way in Figure 6.13).

Marine borer damage may be evaluated using the visual assessment categories in Table 10.2.

Table 10.2 Visual assessment categories used to estimate marine borer attack

Numerical Amount of surface attack caused by gribble Amount of attack caused by shipworm (teredinids) assessment (limnoriids) as % surface area as % volume category

0 No attack. No evidence of shipworm attack.

Minor attack. Minor attack. Single or a few galleries covering not 1 more than 10% of surface area of the timber. Isolated tunnels exposed by erosion of the timber/ use of an adze.

Moderate attack. More than 10% of the total surface Intermediate attack. 2 area of the timber was covered with galleries. Cross- Tunnels penetrate to a depth of 10 mm into the section dimensions practically unchanged. timber. The damage tends to be localised.

Extensive attack fully covering the surface of the Heavy attack. timber. Some reduction in cross-section observed 3 and gaps between groyne planks and between Tunnels penetrate to a depth greater than 10 mm, planks/piles are observed. Loss of cross-section/or but less than 30 mm and the damage is present in a single plane does not exceed 25%. throughout the component.

Severe attack. Significant loss of cross-section or in Severe attack. a single plane is clearly visible but does not exceed 4 50%. Large gaps between timber components Tunnels penetrate to a depth of greater than 30 mm visible. and are present throughout the component

154 CIRIA, C793 Numerical Amount of surface attack caused by gribble Amount of attack caused by shipworm (teredinids) assessment (limnoriids) as % surface area as % volume category Failure. Timber is easily broken away from the Failure. More than 50% loss in cross-section or structure and the damage penetrates more than 30 5 in a single plane observed. Large gaps between mm. components evident. Damage is widespread throughout the component.

Detailed abrasion inspection Abrasion tends to be worse at the groyne/beach interface and about one-third along the groyne from the seaward end. An assessment of section loss of piles may be made by wrapping a tailor’s tape measure along the accessible faces, which allows about three-quarters of the pile perimeter to be measured (Figure 10.7).

An estimate of cross-sectional area can be made by applying the formula:

2 2 Figure 10.7 Measuring three-quarters of the pile Cross-section (mm ) = (n x 1.3/4) perimeter (courtesy John Williams)

Where n = cumulative length (mm) of three accessible faces.

This value may be compared against the specified cross-section and an estimate of section loss over time may then be made.

In some cases, where piles extend above the height of the groyne planks, as assessment of cross-section may be made using the following formula and this will allow the estimation of section loss due to ‘necking’ abrasion. Cross-section (mm2) = (n/4)2 where n = cumulative length of all four accessible faces.

Groyne planks, especially planks lower in the groyne that show signs of ‘scalloping’ abrasion coincident with swash/backwash action, may be assessed by using a spirit level and a ruler. These measure the localised loss in section that can be compared against the specified cross-section (Figure 10.8).

Abrasion measurements provide a ‘snapshot’ of abrasion at a single point on a single component. When assessing a groyne and its constituent components a general assessment may be made by visually estimating the general pattern of abrasion across the entire structure.

Abrasion may be quantified using the guidance in Table 10.3.

Figure 10.8 Assessing scalloping abrasion of groyne planks (courtesy John Williams)

Groynes in coastal engineering 155 Table 10.3 Visual assessment categories used to estimate abrasion

Numerical Estimated loss of section caused by abrasion and subsequent erosion of assessment category the timber component (%) 1 No observable erosion.

2 Light erosion estimated to at less than 5% of the cross-section.

3 Minor erosion estimated to at between 6% and 25% of the cross-section.

4 Heavy erosion estimated to be between 26% and 49% of the cross-section.

5 Severe erosion estimated to exceed 50% of the cross-section.

Abrasion can also occur at interfaces between elements. These should be inspected in detail to identify evidence of deterioration and possible mitigation measures (see Box 10.2).

Box 10.2 Inspecting a timber waling/sheet pile interface

Walings are longitudinal strengthening members and they may attract marine growth, eg molluscs and/or weed, which will retain moisture on their underside, leading to early deterioration. The waling may appear sound, but the underside could be hollow, leaving the component, and the groyne, weakened. The inspector should check the underside of walings as standard procedure. Poorly-mated faces of scarf joints are another source of early onset of rot. In both instances and where rot is evident, there is not much that can be done other than to replace the whole timber. However, it should be recognised that a waling may only be necessary in overall groyne strength terms for the early part of its service life. Once the groyne has become ‘established’ and the beach stabilised, the waling may not be required and so it should not be automatically replaced if found to be defective – its replacement should be Figure 10.9 Differential wear of end grain of sheet piles considered on a case-by-case basis. and adjoining waling Where vertical timber sheet piles are driven into the beach and fixed to a waling, there is the potential for both the top of the sheet piles and the walings to become exposed to the zone of active sediment movement. However, the end grain at the top of the sheet piles will resist abrasion far better than will the waling (Figure 10.9 shows where the grain runs with the length of the timber element). Over time, the less abraded sheet piles will retain the sediment on the waling forming a gulley and exacerbating the abrasion, eventually leading to a situation where the fixings connecting the sheet piles to the waling are compromised. This situation can be identified during inspections. A practical mitigation measure is to fix a cover strip over both the waling and end grain, removing the opportunity for differential wear. Ideally, direct exposure of the waling and tops of the sheet piles should be avoided at first.

Detailed inspection for soft rot When inspecting existing soft wood timber groynes, buried planks should first be exposed by excavation. The extent of decay can be assessed by striking with a hammer (hammer soundings) or by assessing the ease of penetration with a power drill.

10.2.5 Recording inspections Repeatability, comparability and consistency in recording surveys are essential to enable temporal or spatial trends in asset degradation to be identified. Because of the degree of variability and change associated with the whole-life management of groynes or groyne systems, data management is an important issue, whether or not modern Building Information Modelling (BIM) systems are employed.

If a formal inspection proforma is adopted, a robust recording system will include a range of approaches to data capture. Commonly-used approaches include a combination of photographs, written inspection notes and annotation of schematic sketches or drawings of the structure that is inspected. Emerging methods such as laser scanning may also be considered.

Photographs taken from common locations at each inspection can help to build up a picture of the evolution of the structure over time. Many cameras and mobile phones are now equipped with wide

156 CIRIA, C793 angle or ‘panoramic’ features that enable a whole groyne to be captured in a single photograph. However, close-up photos are important for recording details.

Annotated sketches or drawings provide a particularly useful method for more detailed recording (Figure 9.4). Digital versions of such sketches are now preferred for long-term data management.

A simple field form is shown in Figure 10.10, which provides a schematic layout showing all timber elements of each groyne. The inspection enables the schematic to be annotated with observations of performance and proposed remedial measures.

Figure 10.10 Generic inspection form for timber groyne maintenance (courtesy New Forest District Council)

As an alternative, various apps and electronic methods of recording observations have been developed. Box 10.3 describes the approach used by New Forest District Council.

Box 10.3 Electronic groyne condition recording system (courtesy New Forest District Council)

The New Forest District Council timber groyne inspection form comprises three main parts that relate to timber groyne surveys: dimensions (Figure 10.11), condition (Figure 10.12) and works completed (Figure 10.13). Photographs of the structure (stored with the database) can be used to supplement the information entered on the form. Fields are also available for storing information such as groyne number, CPS reference (coast protection survey), location co- ordinates, inspector name, date and summary text. The dimensions area of the form (Figure 10.11) provides the basis for recording baseline information about the structure, which are unlikely to change during its lifetime. However, the number of planks and crest elevation may vary during the life cycle of the structure, as it is raised or lowered to better manage sediment volumes on the beach. Basic groyne structure details can be recorded (length, crest level of groyne at root and outer end, and numbers of piles, planks, bolts and plates etc). The form also allows Figure 10.11 Timber groyne inspection form – dimension for recording details of the actual timber sizes, namely pile form width, depth and length, and plank width, depth and length. Beach-structure interaction can be recorded from simple dip measurements made with a tape measure. This provides an indication of the efficiency performance of the groyne, ie whether the groyne is retaining material. The condition area of the form (Figure 10.12) allows the user to enter the construction date and a consistently assessed overall condition grade to provide an overview of structure condition. Descriptions of any works or materials required to return the groyne to sound condition can also be recorded, together with a costing based on an agreed schedule of rates.

Groynes in coastal engineering 157 Box 10.3 Electronic groyne condition recording system (courtesy New Forest District Council) (contd)

The Council’s condition grade differs slightly from that of the Environment Agency and has a focus on the necessary interventions:

„ No works required. „ Some signs of wear. All planks and piles ok. Returnable to as-built condition with replacement of some bolts. „ Planks/piles missing. Groyne being undercut. New planks/piles required to return to satisfactory condition. „ Groyne is ineffective. Total replacement required. When entering works or materials required to repair the groyne, details such as number required, and cost rates allow an overall cost to be calculated. A description field at the bottom of this part of the form allows any text to be entered.

Figure 10.12 Timber groyne inspection form – condition form

The works completed section of the form (Figure 10.13) is similar to the condition area except here the user can list and cost up works completed, and materials used. At the bottom of this part of the form is a description field where any comments regarding works may be entered. The analysis tool makes provision for a life-cycle analysis of structure inspections, maintenance activities and costs, and the relative efficiency of any repairs. All locations where timber groyne surveys exist can be accessed, and data previously given on the timber groyne inspection form can be tabulated and graphed over time. Expenditure and work required are graphed as cost over time. Groyne condition is graphed as condition (grade) over time. Other data that will have been entered such as groyne dimensions, works Figure 10.13 Example of works completed section for the completed expenditure and work required to return groyne same groyne as shown in Figure 10.12 to sound condition (grade 1), displayed in a tabular format.

10.3 MAINTENANCE PRACTICES FOR TIMBER GROYNES

As with many maintenance activities, procedures and processes have been developed iteratively at various locations over many years. Some of these processes have resulted in increasingly efficient management. This section does not to provide a prescriptive solution to maintenance, but documents a range of alternative approaches, together with their associated risks and successes. Some of the techniques have generic applicability and others may be site specific.

10.3.1 Introduction: proactive and reactive maintenance Maintenance procedures may be divided into reactive and proactive measures.

Reactive measures include the following (see also Table 10.4): „ Replacement of missing, broken, worn or rotten planking. This is a relatively easy task that involves removing the old plank and any fixings, and fitting a new plank. Every effort should be made to remove old fixings, as cutting or grinding off and leaving the thread embedded will lead to ‘nail sickness’. If repairs are required repeatedly, over time there will be nowhere for the new fixing and rust will spread into the timber, quickly blunting drills used for future repairs. It is often the easy solution to use a disc cutter or angle grinder to take off the protruding heads of old fixings, but it may well be possible to remove them completely with a little extra effort and a good quality pipe wrench.

158 CIRIA, C793 Any plank replacement activities should follow the construction guidance given in Section 8.3.5. Ensuring a tight butt joint between planks will slow down the rate of abrasion at plank edges. This reduces the rate of wear associated with the transport of larger sized particles of sediment and the effect of small pebbles wedging open small gaps between the planks. The avoidance of lap joints also removes sheltered refuge for marine borers. „ Mitigating wear or rot in piles. Fixing extra components (see Section 10.3.4) that transfer structural loading capacity to the upper part of the pile. Typically, the components comprise of timber of a section similar to the host pile, but they should not be done with sections of width smaller than 150 mm. Introducing new material via a scarf joint. However, cutting timbers on site (especially the host pile), to the necessary tolerances, may not always be practicable.

„ Repair of storm damage. Loss of structural connection to seawalls or disconnected members (eg walings) and anything fixed to them (eg timber sheet piles) can occur because of storms. Repair of this damage can be more involved and may be outside of the scope of an in-house team. If the damage is substantial, it is likely to have occurred to a critical asset, in a vulnerable location, with high wave loading, in which case specialist contractors should be involved as soon as practical. „ Deterioration and damage to beach accesses. Access to the beach from say the promenade above, or over the groyne from one bay to the next may well be facilitated by timber or concrete steps. Depending on the exposure and the character of the beach (including sediment size), these accesses can deteriorate quickly. For reasons of public safety, they should be inspected as part of the groyne inspection regime and repairs considered. The nature of the inspection and repair is outside the scope of this guide.

Table 10.4 Reactive maintenance measures for timber groynes

Element Type* Cause Method Comment Fix strengthener (to span Consider ways to protect/reduce Necking Abrasion across necking). Re-fix effect planking to new piece Marine borer Replace elements affected May not be in crucial section of Wetting/drying structure Rot Poor quality timber May be due to mis-aligned/ used in construction/ Replace elements affected twisted piles. poor workmanship Leaving permanently damp areas

If minor then cutting beyond Natural grain split can relieve stress – movement if larger then mild steel (galvanised) clamp(s) Relevant only where pile is driven – consider leaving pile Splitting rings on after driving (rather than Driving stresses using temporary driving ring). Avoid leaving ‘shoulder’ below ring – better to transition round to square. Over-stressing –wave Replace or fix strengthening Strengthen or reinforce groyne in loading piece as appropriate that area to reduce stresses May need to be re-piled Life may be extended with ties or Over-stressing – loss Leaning with longer piles to achieve struts to provide extra support – of beach volume

Pile better penetration into bed often requires piling work

Groynes in coastal engineering 159 Element Type* Cause Method Comment Structural damage due Missing section of to wave action or other Replace section of waling waling reason Water ponding or use Wet rot of softwood timber Replace section of waling

Waling (upper) susceptible to wet rot Fix new waling directly Beach level dropped below damaged. Re-fix exposing normally Often caused when shingle runs Abrasion/loss of sheet piles to new waling. buried element – on grain of waling and is retained fixing Fix longitudinal cover board timber and fixing by harder end-grain of sheet pile and replace old waling with abraded new plank to fill gap Loss of beach Long term trend of Major intervention likely to leading to beach beach lowering be needed underrun (see Section 9.2.1) or Fix second, lower level, Identifiable one-off or poorly secured lower waling to provide two point ‘recoverable’ situation fixing Waling (lower – giving support to vertical sheet piles levels of groyne

May need some excavation Missing Storm damage Replace Urgency will be dependent upon a number of factors (see text)

Missing Age/wear

Missing Poor fixing

Missing Rot Under-run/loss of fines Endeavour to seal bottom Gaps between through gaps between of planking to bed strata or planks

Planking planks with sufficient penetration

Proactive measures include those listed in Table 10.5, and their focus is typically on one or more of the following: „ early replacement of deteriorated components „ mitigating wear to some components by slightly raising or lowering planking levels.

Table 10.5 Proactive structural maintenance or repair measures for timber groynes

Type of proactive Element Reason Method Comment measure Can be the first maintenance Can be good use of off cuts of operation (ie 12 months after planking. construction) when the beach Only required in high abrasion PIles Sacrificial pads To combat abrasion. has settled, and the abrasion zone – beaches with steep slope zone has become evident. may only require seven to eight May require one, two or three piles to be treated in this manner, sides to be protected. shallow slopes may require more.

Waling No proactive operations required unless major re-profiling proposed (see Chapter 9)

To reduce abrasion, Fix new plank on top of existing Adding additional where shingle is May be beneficial to existing plank Planking – may need the existing to be planks running over the or pile. replaced if full section lost. structure.

10.3.2 Replacing worn/damaged timber piles Piles that are beyond service life and not repairable will need to be replaced. This may be beyond a maintenance operation, so it will need to be included in a capital scheme. The typical total cost of replacement is about five times the cost of the timber. This high cost reflects the plant required (eg piling rigs to deliver typical penetration depths of the order of 5 m) and the complex process of disconnecting all planks, pulling the old pile, driving a new pile, and finally reconnecting all of the planks to the new pile. Mobilisation of plant to replace only one pile is not an efficient process.

160 CIRIA, C793 If the pile is only damaged over the top 0.7 m, an extension should be spliced onto the existing pile and the damaged portion removed. To do this the lower section (2 m) of the pile would need to be in good condition to fix the extension pile onto it.

If it is necessary to install a new pile, there are two main options available: 1 Drive a new pile alongside an existing pile (see also Sections 8.3.1 to 8.3.3). The new pile can be pitched between the planks and the waling and clamped tight to the side of the existing pile using steel clamps. The pile can then be driven as tight as possible to the existing pile using the temporary walings and crossheads. Common practice is to drive the pile with a chisel point (rather than the common pyramidal point) to encourage the pile to be pushed onto the planking line. However, there will be a tendency for beach material to come between the replacement pile and the existing (pile or plank) kicking the pile off vertical. Direct driving of piles may also result in twisting, and difficulties in keeping piles vertical or on line. There may also be a requirement to remove planks and replacement planking to maintain the required staggered joints and fixings to the pile. 2 Removing and replacing a damaged pile. This should only be contemplated when the pile is badly damaged at depth. However, if the pile has reduced in section or is rotten then it will be very difficult to extract and other options like extensions or pile replacement alongside should be used. Ideally, the approach should be to release and remove some of the planks, waling etc by removing the fixings, so that the pile is free standing but supported. One option is then to remove the pile using a clamp on a vibrating hammer. An alternative option is to ‘pull’ the pile out using a large excavator – taking care not to damage any of the adjoining timbers. This can be done by fixing a clamp to the pile together with a large (50 mm diameter) dowel through the pile where chains could be attached. The position of any new pile is set and maintained by temporary walings fixed at both high level and low-level spanning between the piles either side of the replacement, crossheads are then fixed to the walings to position the pile laterally. The pile is then pitched and driven (although it may be possible to dig the pile in rather than driving it). To complete the driving a ‘dolly’ (spacer) may need to be fixed to the top of the pile to avoid the piling hammer (or surrounding parts of the piling rig) hitting the planking/waling. The various groyne components can then be re-fixed onto the pile. Further information on piling techniques is given in Chapter 8. 10.3.3 Maintaining bolted fixings Analysis of damage to groyne structures suggests that failure of bolted fixings is important and should be avoided by simple regular checking and replacement of worn or damaged bolts. Failure to maintain small components of structures, including replacing corroded fixings, may result in rapid degradation, loosening or loss of the fixing and the resulting loss of expensive timber. The cost of maintenance of the fixings is minimal compared with the loss of timbers.

Stainless steel bolts can be replaced more easily than mild steel because they do not seize up with corrosion. However, the threads may be stripped, or the nuts and bolts may have jammed in the presence of fine sand, which may mean that the fixings are not reusable. This is a frequent issue on fine sand beaches, but less likely on shingle beaches.

Where bolts have been used to attach planks to piles, consideration should be given to replacing these bolts with coachscrews (see Box 7.3).

10.3.4 Plated connections between planks Planks may be joined with plates (Figure 10.14) to optimise the use of the available lengths of planks and avoid waste. However, the use of plates in this way may introduce a weakness into the structure, and corrosion of the plates or of the fixings through the plates has been observed to lead to failure of the joints and subsequent loss of timbers from structures. Where possible it is recommended that such points

Groynes in coastal engineering 161 of weakness are eliminated, but typically this will require all planks to be removed and the lengths of planks adjusted so that joints always occur on the pile centres. Despite the additional short-term cost for the extra effort in rearranging the planks, the risk of loss of planks from the structure is significantly reduced and such maintenance is likely to be cost effective in the longer term.

Figure 10.14 Planks joined with steel plates (courtesy New Forest District Council)

10.3.5 Pile abrasion protection systems The life of piles may be very short at locations where abrasion is a major issue. Smaller tidal ranges together with wave action and variable beach levels may result in aggressive abrasion that causes wear to a concentrated zone of the piles (see Box 10.4).

To reduce life cycle costs, pile protection systems (also discussed in Section 7.6.4) may be used to prolong the life of the piles by up to 20 years. They can be added much more quickly than replacement piles, with a maximum of 10 units able to be replaced over a low water period as opposed to three piles over the same period. A pile protection system is simply comprised of sacrificial timbers fixed to the existing pile over the length of the pile which is vulnerable to erosion. Such timbers may be cost-efficiently sourced from planks removed from the groynes during maintenance operations, providing they are not too worn. Pile protection planks should be bolted or screwed to the vertical faces of the piles to provide direct protection against abrasion (Figure 10.15). Pile protection should be fitted over the zone of maximum wear – often centred on the mid-tidal elevation. The required length of the pile protection units will vary according to the characteristics of each site, but typically may be 1 m to 2 m in length.

Figure 10.15 Recently attached pile protection units showing minimal wear (courtesy New Forest District Council)

If adopted, pile protection should be fitted to two or three faces of the piles, Although most wear may well be identified on the seaward and downdrift faces of the piles, some may also be evident on the landward face which arises from backwash of sediment (see Box 10.4). Some beach excavation may be required during installation to ensure that the active abrasion zone is fully covered.

162 CIRIA, C793 Fixings for pile protection can be installed only using hand tools. A single pile protection board can be fixed with two coach screws (preferable stainless steel) directly to the piles, avoiding the need to undo existing planks from the piles. Typically, 50 to 60 per cent of coach screws can be reused. As discussed earlier, heads of the coach screws should be recessed into the timber for better protection and future reuse.

Pile protection systems can be fitted at various stages in the structures’ life but are more easily and orderly fixed to new piles, when the edges of the pile are square and have not been subject to abrasion. Joints between the pile face and the pile protection units can be made more tightly, which will also reduce the possibility of attack by gribble.

Pile protection units should be regularly monitored and replaced when they wear through, ideally before wear starts to occur on the king pile. This will make it possible for neater repairs and minimise the risk of infestation with gribble worm in the gaps between the pile protection and the pile.

Box 10.4 Pile protection at Milford-on-Sea, Hampshire

On the high energy gravel beach at Milford-on-Sea the life of some of the piles may be as short as three to five years. The remainder of the piles generally remain undamaged and may be in nearly perfect condition if removed. The most badly affected piles are those located in the mid-tide section of the groynes. Abrasion may typically affect 1 m to 2 m of the pile length. The principal hazard affecting groynes at Milford-on-Sea is abrasion. The beach is narrow and steeply inclined and comprises a mixture of sand and shingle, but predominantly shingle, and the location characterised by a narrow tidal range. Historically, Greenheart piles have provided a service life of five to seven years. Not only have the piles been the life-determining factor, but they are also the most expensive component to procure and install. Recent modification to the groyne design has incorporated Greenheart sacrificial pile protection. Most wear is likely to be seen on the seaward and downdrift faces of the timber piles and these faces are frequently fitted with pile protection units (Figures 10.16a and 10.17). While it is more expensive, pile protection is now fixed to all sides of partially worn piles (Figure 10.16b). The planks are fixed to the sides of the piles after installation and are comparatively easy to install and maintain. The Greenheart plates are 75 mm thick and, at the beach/groyne interface where beach material is most volatile and abrasion greatest, deliver an average of five to six years of protection to the seaward elevation of the pile a similar lifespan to the equivalent unprotected pile (Figure 10.18). So, one cycle of installation of sacrificial pile protection doubles the service life of the pile, based on previous estimates of life expectancy. A strategy of maintaining pile protection, replacing when it has almost worn through, may deliver much longer groyne service life. Pile protection can also be retrofitted to piles that are showing signs of abrasion.

a b

Figure 10.16 Pile protection fitted to two sides (a) and three sides (b) of the pile

Groynes in coastal engineering 163 Box 10.4 Pile protection at Milford-on-Sea, Hampshire (contd)

Note that the pile protection on the seaward side has abraded more, exposing the underlying pile, compared to the sacrificial plank on the landward side.

Figure 10.17 Cross-section of timber protection to pile face

Figure 10.18 Examples of worn pile protection system

10.3.6 Maintenance tools and equipment Tools A well-organised collection of tools will be required to carry out maintenance of timber groynes (Figure 10.19, see also Section 8.4). These will include common small tools such as hand saws, adze (for trimming and shaping), spanners, hammers (sledge and club) and pry bars. In addition, there may be a requirement for specific tools specially tailored to working on the groynes in the most efficient way possible. All these specific tools are focused on a particular task. Examples include: „ a specially-manufactured bolt puller machine for pulling out awkward bolts „ chain saws of a suitable blade size and power rating

164 CIRIA, C793 „ augured drills, each set up with specific diameter drill bits and a spare drill „ different petrol mixes in the different petrol cans for the three-stroke machines „ screwdriver to clear away grit etc from a countersunk bolt head „ set square to clear away drilling scurf from between two pieces of wood to ensure there are no gaps.

Contractors will have their own preferences for tool types, but whichever system is favoured, it is efficient for all tools (eg drills and wrenches) to be powered in the same way, eg if tools are air powered, a compressor and air hoses will also be required. Other alternatives are tools driven by a hydraulic power pack or petrol/diesel motor.

Figure 10.19 Full collection of hand tools (courtesy New Forest District Council)

Equipment Equipment for maintenance work will be much the same as for original construction although some may be scaled down in size, eg JCB or 15-tonne 360 degrees excavator in place of a larger machine. This is because it will not be necessary to have the higher output machines that are used to ensure productivity for construction. Manoeuvrability is key for maintenance works.

Health and safety Operatives should be fully trained in the use of each tool and piece of equipment. This training should explain not only how to use the equipment but also provide information on the various associated risks and any guidelines available. The risks and guidelines (eg for vibration causing hand/arm muscle, tendon and nerve damage) should be understood and accounted for in the planning and supervision of all works.

10.3.7 Materials supply for maintenance The main question for the operating authority is to decide whether to have a local stockholding (by each local authority or ‘nominated’ storage by retained contractor) or whether to organise a supply to site direct from the supplier for each operation. In general, a rolling programme of small-scale repairs are more suited to stockholding by the operating authority whereas specific, well-planned refurbishment jobs can benefit from contract supply.

If it is decided that it is important to have ready access to dedicated stocks to deal with emergency situations arising, care should be taken in storage if the timber is not to be rendered unfit before it gets into service. Timber stored indoors will dry out but conversely timbers stored in damp, exposed conditions, without effective air circulation around the stacks or between pieces, may experience premature rot. Stocks should be ‘turned over’ so that it is not a case of last in – first out. Suitable stacking will help reduce workforce strain and improve efficiency, whether by reducing manual handling and reduce the safety risks of plant manoeuvring for lifting/loading in confined spaces.

Groynes in coastal engineering 165 It will be necessary to stock a range of timber sections for the various component requirements (piles, waling and planks) and these in a range of lengths if waste is to be avoided. Of these, stocking a typical length of, for example, planking is probably the highest priority for rapid maintenance. In addition, if saved, offcuts can be used for pile abrasion protection (see Section 10.3.4).

If timber is specifically procured for a project, it is important to determine quantities (section and length) in advance to avoid increased costs, making a suitable contingency for unforeseen repairs (see also Sections 8.1.2 and 8.1.3).

In-house teams will be more accustomed to working on their ‘patch’ but may not have the expertise, plant or ability to undertake larger or more difficult tasks and may be less able to plan for and mitigate risks that can occur. Contractors used on a long-term basis (due to location or availability) will develop knowledge of sections of coastline. It is likely that they will have a higher hourly rate and require greater contract administration and supervision costs than in-house teams, leading to higher overall costs, but this may be offset by their ability to tackle a wider range of jobs and ability to work tidal shifts.

If timber is to be reused (see Box 10.5), then a careful assessment of its performance and the parts of the timber that may be reused is necessary. Questions that will need to be addressed include: „ Can the timber be re-machined to deliver smaller section sizes, eg creating planks out of worn piles? „ If the timber is to be re-machined, can the metal work be removed? „ Is there a need to re-grade the timber? „ If reuse is not possible, the timber is likely to have an alternative down-stream value in terms of other forms of recycling and these should be explored.

Box 10.5 Reuse of groyne timber at Bournemouth, Dorset

During 2015 to 2018, time-expired groynes at Bournemouth were deconstructed with the objective of evaluating the performance of different species of timber and evaluating timber recovery rates. The research indicated that, in the case of Ekki planks, about 90 per cent of the planks were recoverable. This was due to Ekki’s excellent resistance to abrasion and marine borer attack. Lower recovery rates were recorded for Greenheart planks. Greenheart piles were not suitable for reuse on account of a combination of insufficient pile length and loss of section caused by gribble. However, piles could be re-sawn to yield planks suitable for reuse in the Bournemouth groyne field.

10.3.8 Temporary works – excavations and bunding Excavations for maintenance works (see also Section 8.5.6) should be avoided as much as possible given the limited tidal construction window. In practice, modest excavation may only be necessary in the following situations: „ Where the ‘faulty’ plank is partly buried or where the beach has dropped to near or below the bottom plank and planks are required at a lower level to prevent underrun of material. „ To fix sacrificial pads, pile strengtheners or lift piles, where there is not enough exposure of the host pile above beach level.

In these cases, excavations greater than 0.5 m deep are rare and so there is normally no need for temporary support. Water may still collect in these shallow excavations but extended trenching (away from the main excavation and to a lower level) is often all that it required to keep the excavation dry and suitable to work in. Care should be taken on backfilling as the reinstated material will remain soft and not fully compacted naturally for several tides.

More significant excavations may be needed for major repairs to structures. These may require some form of temporary works to provide protection to operatives and to safely extend the working window. Bunds of sand or gravel around the structure may facilitate this.

Criteria controlling the size of the protective bunds includes the depth of the structure within the beach and tidal range. The examples shown in Figures 10.20 and 10.21 indicate two sites with similar

166 CIRIA, C793 tidal range characteristics (about 2 m), but quite different physical characteristics. For example, in Figure 10.20 excavation is carried out to a shallow depth at which the groyne planks close onto a clay surface. By comparison in the Bournemouth example (Figure 10.21), the mobile material extends significantly deeper and the required excavation depth is greater, necessitating the larger scale and more complex temporary sand bunds to provide a safe working environment. (Figure 10.21 shows new construction, but a similar approach would be required for maintenance.) Figure 10.20 Creation of sand bunds for temporary works (courtesy New Forest District Council)

Figure 10.21 Extensive temporary bund surrounding groyne under construction

10.3.9 Navigation markers The design of navigation markers (which mark the seaward ends of groynes) is discussed in Section 6.10. As they often extend 2 m above MHWS, it is difficult to keep them clean (ie free from bird deposits). They require painting periodically and, as they are exposed to wave action, fixings are prone to work loose under vibration.

Where the supporting structure is substantial, maintenance requirements will be low but accessing the marker may be more difficult. A lighter steel structure may facilitate easier access to the marker itself, but the bolts fixing the steel sections to the seaward end of the groyne can work loose with the wave action and need to be tightened regularly or replaced if unduly worn. These items should be on the detailed inspection schedule, although a useful early sign of a loose connection will be a leaning marker structure.

When they can be accessed from land at low water, ladders or a temporary staging can be used to provide access. Alternatively, a suitably-equipped hydraulic machine with a ‘man-basket’ can be used. Boat access will be required for those below low water.

10.4 MAINTENANCE OF NON-TIMBER STRUCTURES

„ Steel sheet piling. Steel sheet piles may be used in place of timbers to prevent undermining of pile and plank groynes. If exposed to shingle abrasion (which, with good design, they should not be), the steel will wear very quickly and lose integrity. The steel will both present a hazard to beach

Groynes in coastal engineering 167 users, and not fulfil its intended purpose of retaining beach material. In this situation the steel can either be replaced with timber (an expensive operation) or cut down and re-fixed to at lower level and a cover strip attached to reduce the probability of a failure recurrence (see Section 6.5). „ Polymers. Maintenance will depend upon the type of polymer used. Abrasion will release particles into the environment

(Figure 10.22) and this effect Figure 10.22 Erosion gullies on surface of polymer groyne plank at requires careful consideration Withernsea, Yorkshire (see Section 6.3.6). „ Concrete. Dupray et al (2010) provides extensive guidance on concrete repair and rehabilitation. This includes aspects appropriate to both original concrete construction (eg concrete mix design, aggregate size and type including abrasion resistance, construction aspects such as shuttering, working time and curing time) and to concrete repair techniques using both concrete and various epoxy or polymer systems. „ Rock/armourstone. CIRIA, CUR, CETMEF (2007) provides extensive guidance on the monitoring and maintenance, and construction of armourstone structures. Maintenance may be required due to poor material, incorrect sizing of original rock or ground/beach erosion/settlement. Replacement of isolated dislocated rock pieces is possible, but more extensive repairs may involve dismantling of the parts of the structure and reconstruction to achieve the necessary interlock/stability of the armourstone.

168 CIRIA, C793 References

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Groynes in coastal engineering 173 USACE (1984b) Shore protection manual, volume 2, Coastal Engineering Research Center, Department of the Army Waterways Experiment Station, US Army Corps of Engineers, Vicksburg, Mississippi, USA http://ft-sipil.unila.ac.id/dbooks/S%20P%20M%201984%20volume%202-1.pdf USACE (2002) Coastal engineering manual, EM_110-2-1100, Coastal Engineering Research Center, Department of the Army Waterways Experiment Station, US Corps of Engineers, Vicksburg, Mississippi, USA https://www.publications.usace.army.mil/USACE-Publications/Engineer-Manuals/u43544q/636F617374616C20656E67696E6565726 96E67206D616E75616C/ VAN RIJN, L C (2004) Principles of sedimentation and erosion engineering in rivers, estuaries and coastal seas, Aqua Publications, Amsterdam, The Netherlands (ISBN: 9-08003-566-1) VAN RIJN, L C (2014) “A simple general expression for longshore transport of sand, gravel and shingle”, Coastal Engineering, vol 90, August 2014, Elsevier BV, London, UK, pp 23–39 WILLIAMS, J (2018) Field based engineering assessment of timber groynes, unpublished report for Environment Agency, Bristol, UK WIGGINS, M, SCOTT, T, MASSELINK, G, RUSSELL, P and MCCARROLL, R J (2019) “Coastal embayment rotation: Response to extreme events and climate control, using full embayment surveys”, Geomorphology, vol 327, 15 February 2019, Elsevier BV, UK, pp 385–403 WORLD STEEL ASSOCIATION (2012) Sustainable steel: At the core of a green economy, World Steel Association, Brussels, Belgium https://www.worldsteel.org/en/dam/jcr:5b246502-df29-4d8b-92bb-afb2dc27ed4f/Sustainable-steel-at-the-core-of-a-green-economy.pdf YOUNGQUIST, J A, MYERS, G E, MUEHL, J H, KRZYSIK, A M and CLEMONS, C M (1994) Composites from recycled wood and plastics, USDA Forest Service, Forest Product Laboratory, Madison, USA

Statutes Acts Arterial Drainage (Amendment) Act, 1995 Coast Protection Act 1949 (c.74) Flood and Environment Protection Act 1985 (c.48) Flood Risk Management (Scotland) Act 2009 (asp 6) Marine and Coastal Access Act 2009 (c.23) Merchant Shipping Act 1988 (c.12) Town and Country Planning Act 1990 (c.8) Water Resources Act 1991 (c.57) Wildlife and Countryside Act 1981 (c.69)

Directives Council Directive 92/43/EEC of 21 May 1992 on the conservation of natural habitats and of wild fauna and flora (Habitats Directive) Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2000 establishing a framework for Community action in the field of water policy (Water Framework Directive, WFD) Directive 2009/147/EC of the European Parliament and of the Council of 30 November 2009 on the conservation of wild birds (Birds Directive) Directive 2014/52/EU of the European Parliament and of the Council of 16 April 2014 amending Directive 2011/92/ EU on the assessment of the effects of certain public and private projects on the environment (EIA Directive)

Regulations Regulation (EU) No 995/2010 of the European Parliament and of the Council of 20 October 2010 laying down the obligations of operators who place timber and timber products on the market The Construction (Design and Management) Regulations 2015 (CDM 2015) (No.51) The Conservation of Habitats and Species Regulations 2017 (No.1012)

Standards

British BS 5930:2015 Code of practice for ground investigations BS 5930:1999+A2:2010 Code of practice for site investigations (withdrawn) BS 8500-1:2015+A2:2019 Concrete. Complementary British Standard to BS EN 206. Method of specifying and guidance for the specifier BS 8500-2:2015+A2:2019 Concrete. Complementary British Standard to BS EN 206. Specification for constituent materials and concrete BS 6349-1-1:2013 Maritime works – General. Code of practice for planning and designs for operations BS 6349-1-4:2013 Maritime works – General – Code of practice for materials BS 6349-2:2019 Maritime works. Code of practice for the design of quay walls, jetties and dolphins

174 CIRIA, C793 European BS EN 13383-1:2013 Armourstone. Specification (withdrawn) BS EN 1992-1-1:2004+A1:2014 Eurocode 2: Design of concrete structures. General rules and rules for buildings BS EN 1997-1:2004+A1:2013 Eurocode 7. Geotechnical design. General rules BS EN 1912:2012 Structural timber. Strength classes. Assignment of visual strength grades and species BS EN 206:2013+A1:2016 Concrete. Specification, performance, production and conformity BS EN 275:1992 Wood preservatives. Determination of the protective effectiveness against marine borers BS EN 335:2013 Durability of wood and wood-based products. Use classes: definitions, application to solid wood and wood- based products BS EN 350:2016 Durability of wood and wood-based products. Testing and classification of the durability to biological agents of wood and wood-based materials International ISO 21650:2007 Actions from waves and currents on coastal structures

Groynes in coastal engineering 175 Further reading

DEFRA (2013) Flood and coastal resilience partnership funding, Department for the Environment, Food and Rural Affairs, London, UK https://www.gov.uk/government/publications/flood-and-coastal-resilience-partnership-funding ENVIRONMENT AGENCY (2018) Flood and coastal defence: develop a project business case, Environment Agency, Bristol, UK https://www.gov.uk/guidance/flood-and-coastal-defence-appraisal-of-projects ENVIRONMENT AGENCY (2019) Grant in aid forms: flood risk management authorities, Environment Agency, Bristol, UK https://www.gov.uk/government/publications/capital-grants-for-local-authorities-and-internal-drainage-boards MHCLG (2019) National planning policy framework, Ministry of Housing Communities and Local Government, London, UK (ISBN: 978-1-5286-1033-9) https://www.gov.uk/government/ publications/national-planning-policy-framework-2

176 CIRIA, C793 CIRIA members ABG Geosynthetics Ltd Kier Group plc Active Building Centre Laing O’Rourke Civil Engineering Ltd AECOM Ltd London Underground Ltd AMC Environmental Ltd Loughborough University Arcadis Consulting (UK) Ltd Maccaferri Ltd ARL Training Services Ltd Marshalls Plc Arup Group Ltd Ministry of Justice Atkins Consultants Ltd Mistras Group Ltd Autodesk Ltd Morgan Sindall Construction and Balfour Beatty Civil Group Infrastructure Ltd BAM Nuttall Ltd Mott MacDonald Group Ltd Barratt Developments Plc Network Rail Black & Veatch Ltd Newcastle University BSG Ecology Northumbrian Water Ltd BWB Consulting Ltd OES Consulting Ltd City of Glasgow College Pinssar (Australia) Pty Ltd City University of London Polypipe Costain Ltd Rail Safety and Standards Board COWI UK Ltd Royal HaskoningDHV Ltd Curtins Consulting SafeLane Global Ltd Darcy Products Ltd Sir Robert McAlpine Ltd Durham University SLR Consulting Ltd E3P Smith and Williamson LLP Environment Agency Southern Water Services Ltd Esri UK & Ireland Stantec Galliford Try Plc Stuart Michael Associates Gatwick Airport Ltd T&S Environmental Ltd Gavin & Doherty Geosolutions Ltd Temple Group Ltd Geobrugg AG (UK office) Thames Water Utilities Ltd Geofem Ltd TOPCON (Great Britain) Ltd Geotechnical Consulting Group Transport Scotland Golder Associates (UK) Ltd UK Green Building Council Grosvenor Britain and Ireland United Utilities Plc Heathrow Airport Ltd University College London Henderson Thomas Associates Ltd University of Birmingham Highways England University of Bristol High Speed Two (HS2) Ltd University of Cardiff HR Wallingford Ltd University of Edinburgh Hydro Water Management Solutions Ltd University of Liverpool Imperial College London University of Reading Institution of Civil Engineers University of Southampton Ischebeck Titan Ltd (Ground Engineering WSP Department) Zero Waste Scotland J Murphy & Sons Ltd James Fisher Testing Ltd April 2020 C793

Groynes are long, narrow structures built approximately perpendicular to the shoreline. They are designed to control longshore transport of sediment on beaches and to deflect nearshore tidal currents. Their principal purpose is to slow the longshore movement of beach material by interrupting the movement of material and trapping it on the updrift side of the groyne. They also help to build beaches that protect land behind the beach from flooding and erosion. Groynes may extend across part or all of the intertidal zone and are normally grouped together to form groyne engineering coastal in Groynes systems or fields. This guide: ? explains the role of groynes in the context of wider shoreline management and sediment transport processes ? provides guidance on setting the layout and profile of groynes to optimally influence transport of beach material and to retain the desired beach shape and profile ? provides a framework for the design and maintenance of all types of groynes with particular emphasis on timber and other 'narrow footprint' groynes. (Narrow footprint groynes are those whose width is very narrow in comparison with their length and are commonly supported on vertical piles.) Groynes in ? provides advice on good practice approaches to construction, routine and periodic maintenance and to the use of sustainable material during repair and replacement ? focuses predominantly on UK experience and situations (eg macro-tidal) but with appropriate coastal engineering reference to international experience. CIRIA

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