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Understanding and Predicting Beach Morphological Change Processes Associated with the Erosion of Cohesive Foreshores - Scoping Report

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Understanding and Predicting Beach Morphological Change Processes Associated with the Erosion of Cohesive Foreshores - Scoping Report PB10800-CVR.qxd 28/4/05 12:25 PM Page 1 Joint Defra/EA Flood and Coastal Erosion Risk Management R&D Programme Understanding and predicting beach morphological change processes associated with the erosion of cohesive foreshores - scoping report R&D Technical Report FD1915/TR Joint Defra/EA Flood and Coastal Erosion Risk Management R&D Programme Understanding and predicting beach morphological change processes associated with the erosion of cohesive foreshores – scoping report R&D Technical Report FD1915/TR Author: David S. Brew Produced: March 2004 Statement of use This document is a scoping study, to current best practice, of the processes of weathering and erosion of cohesive shore platforms and their interactions with adjoining beaches (and cliffs). The study identifies R&D needs and recommendations to address gaps in understanding of these types of shoreline. Dissemination Status: External: Released to Public Domain Internal: Released to Regions Keywords: erosion, weathering, shore platforms, beaches, cohesion, consolidation, coastal processes Research contractor: This document was produced by David S Brew at: Posford Haskoning, Rightwell House, Bretton, Peterborough, PE3 8DW Tel: 01733 334 455 Fax: 01733 333 538. With contributions from the British Geological Survey (Peter Balson, Stephen Pearson and Peter Hobbs), University of Sussex (Rendel Williams, David Robinson and Cherith Moses) and University of Bristol (Mike Walkden) Defra project officer: Jonathan Rogers (Mouchel Parkman) Tel: 01932 337 373 Fax: 01932 354 773 Email: [email protected] Publishing organisation Defra Flood Management Division Ergon House Horseferry Road London SW1P 2AL Tel: 020 7238 3000 Fax: 020 7238 6187 www.defra.gov.uk/environ/fcd Crown copyright (Defra); 2005 Copyright in the typographical arrangement and design rests with the Crown. This publication (excluding the logo) may be reproduced free of charge in any format or medium provided that it is reproduced accurately and not used in a misleading context. The material must be acknowledged as Crown copyright with the title and source of the publication specified. The views expressed in this document are not necessarily those of Defra or the Environment Agency. Its officers, servants or agents accept no liability whatsoever for any loss or damage arising from the interpretation or use of the information, or reliance on views contained herein. Published by the Department for Environment, Food and Rural Affairs. Printed in the UK, April 2005 on recycled material containing 80% post-consumer waste and 20% totally chlorine free virgin pulp. PB No. 10800 ISBN 0-85521-147-4 ii Contents Executive summary………………………………………………………………. vii Glossary……………………………………………………………………………. x 1. Introduction………………………………………………………………. 1 1.1 Aims of this scoping report……………………………………………… 1 1.2 Definition of a consolidated cohesive shore platform…………………. 1 2. Processes of weathering and erosion……………………………….. 5 2.1 Erodibility of cohesive sediments……………………………………….. 5 2.2 Power of energy inputs to erode……………………………………….… 9 2.3 Processes of platform weathering and erosion………………………… 9 3. The platform-beach-cliff system……………………………………… 21 3.1 Sandy shorelines v cohesive shorelines……………………………….. 21 3.2 Cliff recession-platform downcutting relationship……………………… 21 3.3 Beach-platform interaction……………………………………………….. 23 3.4 Sea-level rise and storminess…………………………………………… 25 4. Measurement techniques………………………………………………. 27 4.1 Techniques to measure platform downcutting…………………………. 27 4.2 Techniques to analyse beaches and broad-scale platform morphology………………………………………………………………… 28 4.3 Remote sensing…………………………………………………………… 31 4.4 Techniques to measure material properties in the laboratory……….. 32 4.5 Techniques to measure material properties in situ…………………… 34 4.6 Numerical modelling………………………………………………………. 37 5. Case examples…………………………………………………………… 39 5.1 United Kingdom……………………………………………………………. 39 5.2 Great Lakes………………………………………………………………… 49 5.3 Other areas…………………………………………………………………. 50 6. Preliminary management advice……………………………………… 52 6.1 Management issues………………………………………………………. 52 6.2 Management options……………………………………………………… 53 7. Research needs and recommendations…………………………….. 57 7.1 Weathering and erosion processes……………………………………… 57 iii 7.2 Relationship between platform morphology and physical environment……………………………………………………………….. 58 7.3 Relationship between geotechnical parameters and sediment erodibility……………………………………………………………………. 59 7.4 Numerical models………………………………………………………….. 59 7.5 CSG7 research proposal…………………………………………………. 60 8. References………………………………………………………………… 61 Appendix A – consultation report 89 List of figures Figure 1.1 Cohesive shore platform at The Naze, Essex, demonstrating the discontinuous nature of overlying non-cohesive sediment. Figure 1.2 Cohesive shore platform at Sidestrand, north-east Norfolk, demonstrating the discontinuous nature of overlying non-cohesive sediment. Figure 1.3 Example of a cohesive shore platform backed by a cliff – glacial till at Happisburgh, north-east Norfolk. Figure 1.4 Example of a cohesive shore platform backed by a sea wall with no cliff – Holocene mud at Sea Palling, north-east Norfolk. Figure 1.5 Cross-section through a cohesive profile comprising a shore platform and cliff (from Kamphuis, 1987). Figure 2.1 The cohesive shore platform on the Isle of Sheppey, north Kent, showing development of a fissure in the platform surface. Figure 2.2 Part of the cohesive shore platform at Holderness showing a rock capped pedestal of cohesive sediment, about 10 cm high, developed through erosion of the adjacent platform (from US Army Corps of Engineers, 2002). Figure 2.3 Distinction between concave and convex shore profiles as described for the Great Lakes (from US Army Corps of Engineers, 2002). Figure 2.4 Example of beach and beach ridge movement along the Holderness coast, illustrating how the exposure of the shore platform can vary (from Pringle, 1981). A – morphology after absence of northerly storms. B – morphology during northerly storm. C – morphology immediately after northerly storm. D – morphology a few weeks later. iv Figure 2.5 Example of bioerosion on the cohesive platform on the Isle of Sheppey, north Kent. Figure 2.6 Example of bioerosion on the cohesive platform on the Isle of Sheppey, north Kent. Figure 2.7 Polygons on the surface of the cohesive shore platform on the Isle of Sheppey, north Kent. Figure 2.8 Polygons on the surface of the cohesive shore platform at The Naze, Essex. Figure 3.1 Profile of an eroding cohesive shore with backing cliff. Vertical lowering of the nearshore must accompany horizontal recession of the cliffs in order to maintain an equilibrium (from Davidson-Arnott and Ollerhead, 1995). Figure 4.1 A micro-erosion meter and ancillary equipment. a) micro-erosion meter; b) template used as a guide during installation of pins; c) levelling bar; d) steel pins (from Askin and Davidson-Arnott, 1981). Figure 4.2 Aerial photograph of the cliffs, beach and shore platform at The Naze, Essex. Figure 4.3 Schematic layout of the in situ erosion flume (ISEF), 1) propeller; 2) perspex cover plate; 3) elecromagnetic flow meter; 4) optical turbidity sensor (from Houwing, 1999). Figure 4.4 Schematic diagram of ISIS (instrument to measure erosion shear stress in situ) showing the major components of the system for a field deployment (from Williamson and Ockenden, 1996). Figure 4.5 Schematic diagram showing in situ annular flume (from Widdows et al., 1998b). Figure 5.1 View alongshore of the shore platform at The Naze, Essex. Figure 5.2 Close-up of the shore platform at The Naze, Essex. Figure 5.3 Shoreward view the London Clay cliffs, beach and shore platform on the Isle of Sheppey, north Kent. Figure 5.4 Cohesive shore platform comprised of Barton Clay Formation at Lee-on-the-Solent, Hampshire (from Kemp, 1999). Figure 5.5 Beach slope and toe erosion at Lee-on-the-Solent showing narrow exposure of the shore platform and undercutting of wooden groynes (from Kemp, 1999). v Figure 5.6 Sea wall failure at Lee-on-the-Solent (from Kemp, 1999). Figure 5.7. Beach recharge at Lee-on-the-Solent 1997 (from Kemp, 1999). Figure 5.8 Shore platform along Holderness. Figure 5.9 Shore platform and cliff along Holderness, illustrating presence of armoured mudballs. Figure 5.10 Cohesive shore platform at Sidestrand, north-east Norfolk. Figure 5.11 Cohesive shore platform at Sidestrand, north-east Norfolk. Figure 5.12 Timber revetments at Happisburgh. Figure 5.13 Remains of groynes at Happisburgh. vi Executive summary There are significant stretches of cohesive shore platform in the United Kingdom where variable amounts of sand and gravel overlie cohesive clay materials (such as Holocene mud, glacial till and London Clay). Many stretches lie along the most rapidly eroding shorelines in the country (Holderness, Essex and north Kent) and pose significant problems for management. The process of downcutting of the shore platform and the interaction between the cohesive and non-cohesive components is not well understood. The coastal community needs to be better able to manage cohesive platforms because of their value as habitats and their importance in controlling the functioning of the wider coastal system, including beach form and sediment budgets. This importance is not limited to areas in which platforms are normally visible; in other locations they may rarely be revealed but still have a significant geomorphological role, particularly in regulating recession rates. In 2001, the
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