Toe Structures Management Manual

Toe structures management manual Project: SC070056/R The Environment Agency is the leading public body protecting and improving the environment in England and Wales. It’s our job to make sure that air, land and water are looked after by everyone in today’s society, so that tomorrow’s generations inherit a cleaner, healthier world. Our work includes tackling flooding and pollution incidents, reducing industry’s impacts on the environment, cleaning up rivers, coastal waters and contaminated land, and improving wildlife habitats. This report is the result of research commissioned and funded by the Environment Agency’s Evidence Directorate. Published by: Author(s): Environment Agency, Horison House, Deanery Road, Andy Bradbury Bristol, BS1 9AH Jonathan Rogers www.environment-agency.gov.uk Dick Thomas ISBN: 978-1-84911-290-1 Dissemination Status: Publicly available © Environment Agency – December 2012 Keywords: All rights reserved. This document may be reproduced Coastal structures, structure toe, asset management, with prior permission of the Environment Agency. condition assessment, beach, scour The views and statements expressed in this report are Research Contractor: those of the author alone. The views or statements Halcrow Group Ltd expressed in this publication do not necessarily Ash House, Falcon Road, Sowton, Exeter, Devon represent the views of the Environment Agency and the EX2 7LB Environment Agency cannot accept any responsibility for Tel: 01392 444252 such views or statements. Environment Agency’s Project Manager: Further copies of this report are available from our Dave Hart, Manley House, Exeter publications catalogue: http://publications.environment- agency.gov.uk or our National Customer Contact Collaborator(s): Centre: T: 08708 506506 None E: [email protected]. Project Number: SC070056 Product Code: LIT 7651 ii Toe structures management manual Evidence at the Environment Agency Evidence underpins the work of the Environment Agency. It provides an up-to-date understanding of the world about us and helps us to develop monitoring tools and techniques to manage our environment as efficiently and effectively as possible. The work of the Environment Agency’s Evidence Directorate is a key ingredient in the partnership between research, guidance and operations that enables the Environment Agency to protect and restore our environment. This report was produced by the Research, Monitoring and Innovation team within Evidence. The team focuses on four main areas of activity: • Setting the agenda, by providing the evidence for decisions; • Maintaining scientific credibility, by ensuring that our programmes and projects are fit for purpose and executed according to international standards; • Carrying out research, either by contracting it out to research organisations and consultancies or by doing it ourselves; • Delivering information, advice, tools and techniques, by making appropriate products available. Miranda Kavanagh Director of Evidence Toe structures management manual iii Executive summary This manual is intended to provide those responsible for flood defences, coastal erosion protection structures and other coastal structures with practical guidance on how to determine, monitor, assess and mitigate for the effects of toe scour. It describes the different types of toe protection structures and provides illustrations of typical designs that are often used as solutions. The case studies on beach lowering and scour management provide real examples of both good and bad practice, and discuss lessons learnt from past schemes. Scour, in specific relation to coastal engineering projects, can be defined as ‘the removal, by hydrodynamic forces, of erodible bed material in the vicinity of coastal structures’. This definition distinguishes scour from the more general erosion and notes that the presence of a reflecting structure contributes to the cause of scour. Scour that affects coastal structures can lead to partial damage, or in extreme cases, complete failure of the structure. A comprehensive survey published by CIRIA in 1986 concluded that scour at the toe of structures represented the most prevalent and serious form of damage to seawalls in the UK. Toe scour is a serious and costly problem – moreover, it is one that is not limited to any particular environment or generally to any particular type of seawall. Toe protection provides insurance against scouring and the undermining of a structure. It provides additional armouring of the beach or base of a defence in front of the structure which prevents waves and currents from scouring and undercutting it. This manual presents, in a logical way, how relevant issues should be covered in the assessment, management and design of the toe of coastal defence structures. These issues include: • monitoring of beach levels and structure condition (including links to coastal monitoring programmes) – Chapter 3; • trigger points for action during the life of the asset (including links to ‘performance features’) – Section 3.4; • maintenance and replacement of elements of the asset or of the whole structure – Chapter 4; • environmental and sustainability issues surrounding scheme/structure planning, design and operation – Chapters 2, 3, 4 and 5; • inputs to the assessment and design processes (including relationship to defence fragility, both for asset management and new design) – Chapter 5; • selection of mitigation options to enhance or prolong performance of the asset – Chapter 2 and Section 5.3. The manual is supported by appendices containing information on scour processes, methods of predicting scour and a number of case studies from around the UK to illustrate particular management approaches and draw on the experience of the techniques that have been employed. iv Toe structures management manual Acknowledgements This manual has been developed from the outputs of the Environment Agency’s Toe Structures for Coastal Defences project (SC070056) undertaken by HR Wallingford, which provided much of the research base for the content of this manual. The development of the guidance was directed by a steering group consisting of: Nick Lyness (chairman) Environment Agency Geoff Baxter Environment Agency David Hart Environment Agency Assistance and review was also received from: Brian Farrow North Norfolk District Council Chris Hayes Environment Agency Steve McFarland SEPA Adrian Phillpot Environment Agency Alan Williams Coastal Engineering UK Ltd The project team is grateful to the members of the steering group and those who contributed time, information and images to the development of the manual. Toe structures management manual v Contents 1 Introduction 1 1.1 Background and context 1 1.2 Concept of toe protection 3 1.3 Structure of the manual 5 1.4 The asset management cycle 8 1.5 Target audience 8 1.6 Acknowledgements 9 2 Toe structure types and materials 10 2.1 Introduction 10 2.2 Types of toe structure 11 2.3 Materials 21 2.4 Choice of structure type and materials 33 3 Asset management 36 3.1 Introduction 36 3.2 Management overview 37 3.3 Life-cycle failure modes 39 3.4 Planning a management programme for structure toes 44 3.5 Defining ‘critical’ and ‘trigger/alert beach levels’ 46 3.6 Beach monitoring 48 3.7 Beach depletion and foreshore down-cutting 68 3.8 Structure condition monitoring 69 4 Maintenance 81 4.1 Introduction 81 4.2 Issues associated with maintenance of existing toes 83 4.3 Maintenance of toe structures 92 5 Toe structure design 109 5.1 Introduction 109 5.2 Identification of the problem 110 5.3 Project appraisal 111 5.4 Design principles 114 5.5 Undermining 118 5.6 Liquefaction at the toe 123 5.7 Geotechnical stability 124 5.8 Resistance to wave and current loading 134 5.9 Hydraulic performance 143 5.10 Effects on coastal processes 143 vi Toe structures management manual 5.11 Public safety 144 5.12 Natural environment 145 5.13 Heritage and visual impact 146 5.14 Amenity 147 5.15 Construction issues 148 List of abbreviations 152 References 154 Appendix A: Scour processes 164 Appendix B: Predictive methods 195 Appendix C: Case studies 225 Table 2.2 Loss of thickness (mm) due to corrosion for piles and sheet piles in sea water 1 23 Table 3.1 Examples of detailed intrusive and non-intrusive assessment techniques for defence structures and/or beaches 77 Table 4.1 Structure repair/restoration options and their applicability to the maritime environment (based on CIRIA 2010b) 94 Table 4.2 Options for repair works related to defects in concrete and concrete reinforcement and their applicability to the maritime environment (adapted from BSI 2006) 94 Table 5.1 Toe related failure modes of main structure types 115 Table 5.2 Relevance of criteria to toe structure yypes 118 Figure 1.1 Definition of the structural toe of a defence as opposed to the ‘visible’ toe 3 Figure 1.2 Layout of the guide 7 Figure 1.3 Document information in relation to the asset management cycle 8 Figure 2.1 Underpinning of seawall toe with steel sheet piles (courtesy HR Wallingford) 15 Figure 2.2 Sheet pile underpinning 16 Figure 2.3 Cribwork and concrete block fill, Norfolk (courtesy of North Norfolk District Council) 16 Figure 2.4 Extensive crack in masonry toe (courtesy HR Wallingford) 17 Figure 2.5 Gabion baskets of rock (courtesy HR Wallingford) 18 Figure 2.6 Rock infill of scour trough, Le Dicq, Jersey (courtesy HR Wallingford) 19 Figure 2.7 Timber bulkhead with rock toe protection at Lepe, Hampshire 19 Figure 2.8 Extended scour apron, masonry steps and armour, St Ouens Bay, Jersey

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