PB11207-CVR.qxd 1/9/05 11:42 AM Page 1 Joint Defra/EA Flood and Coastal Erosion Risk Management R&D Programme Understanding barrier beaches R&D Technical Report FD1924/TR Joint Defra/EA Flood and Coastal Erosion Risk Management R&D Programme Understanding Barrier Beaches R&D Technical Report FD1924/TR Produced: February 2008 Author(s): Stripling, S. Bradbury, A.P. Cope, S.N., Brampton, A.H. Statement of use This report scopes the state-of-the-art understanding of barrier beaches. It sets the foundations for improving the science and application of management tools, and for developing sound management guidance. Such guidance will help flood and coastal erosion risk management policy makers and practitioners make better decisions on barrier beach management practice. It should be noted that it does not constitute official government policy or guidance but provides information for understanding the geomorphological, physical and morphodynamic processes influencing barrier beach evolution and management. It will be of particular relevance for coastal managers and researchers engaged in enhancing understanding and management of these dynamic features. Dissemination status Publicly available Keywords: Barrier-beaches, spits, management, morphology, classification, breaching, overtopping, overwashing, modelling, monitoring Research contractor: HR Wallingford Ltd, Howbery Park, Wallingford, OXON, OX10 8BA Tel: 01491 835381 (This report comprises HR Report SR 683.) Defra project officer: Stephen Jenkinson, Ergon House, London Publishing organisation Department for Environment, Food and Rural Affairs 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);(2008) 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 (August 2008). Printed on material that contains a minimum of 100% recycled fibre for uncoated paper and 75% recycled fibre for coated paper. PB No. 12984 ii R&D Technical Report FD1924/TR - Understanding Barrier Beaches Executive summary Barrier beaches around the UK are important, not only in terms of defences against flooding but also in their own right as important coastal geomorphological features. A lack of detailed understanding of how these beaches evolve and of models to predict their performance as flood defences, together with constraints on acceptable methods of intervention, make the successful and cost-efficient management of barrier beaches a challenging task for coastal managers. The need for better management of such beaches as both flood defences and natural heritage areas will inevitably increase in the face of rising sea-levels. The essential feature of a “barrier beach” is that it has a distinct crest separating the seaward beach face and a well-developed back-slope. In many cases, such beaches have (or once had) an area of water on their landward side, whether an estuary, lagoon or brackish-water. Beaches with these characteristics may be further sub-divided into “barrier islands”, “spits” and “barrier beaches”, depending on whether they have none, one, or both of their ends attached to a land mass. Distinctions such as this, however, can become blurred although this may not be particularly important from the viewpoint of managing such features, as opposed to the geomorphological approach to describing and classifying them. Barrier beaches often form a natural flood defence to low-lying land behind them. However, man’s chosen land-use, e.g. for residential and business properties, can often mean that the natural standard of defence afforded by the barrier is inadequate. Barrier beaches are often overtopped by large waves, they leak, can roll-back landward, and ultimately may breach. All of these events can give rise to unacceptably high flood-risks, and are likely to become more frequent as sea levels rise further. These flood-risks can justify intervention to improve the standard of protection that barrier beaches provide, but the natural heritage interests of such barrier beaches can constrain what type of intervention is acceptable. At present there is scant guidance available which enables a balance between intervention and natural heritage interests to be achieved, and coastal managers are sometimes left to struggle through on a trial and error basis when seeking solutions. This study has provided justification and scope for further research into the performance of barrier beaches as flood defences. The Performance-based Asset Management Systems (PAMS) research programme, funded by Defra and the Environment Agency, has furthered performance-based flood-risk assessment through, for example, the concepts of fragility, resilience and deterioration. PAMS is specifically designed for the identification and prioritising of works needed to manage existing flood defences. Part of the reason for the lack of management guidelines for barrier beaches is our relatively poor understanding of the processes driving their short-term morphology and long- term evolution. The recommendations for further research, and the further research itself, will be embedded in the Environment Agency’s Sustainable R&D Technical Report FD1924/TR - Understanding Barrier Beaches iii Asset Management theme. As such, the proposed research, whilst still firmly and unavoidably centred upon improved process understanding, is placed into the context of performance-based flood-defence management. Ultimately, the research is expected to provide the framework for the evolution of a “Best Practice” guide. The main outputs of this study include: • A review of scientific literature and of existing predictive process methods for overtopping, through-flow and morphological change of barrier beaches has been carried out. • A review of barrier beach management methods has been carried out in consultation with individuals responsible for management of the beaches. This review includes discussion on monitoring and appropriate analysis. Case histories detailing state-of-the-art at 12 selected sites have been presented. • A catalogue providing site-specific information on barrier beaches around England and Wales, including an on-line GIS database that provides information relating to these beaches. Details such as description, location, dimensions, current management practice, photographic record and links to further information, are consequently held on one publicly available website www.barrierbeaches.org.uk. This web-site is also used to disseminate the findings of this scoping study. • Recommendations for a future phased-research programme within the Environment Agency’s Sustainable Asset Management theme have been made. This programme is designed to enhance our knowledge and understanding together with our predictive capabilities. It will include monitoring their condition, performance and the flood-defence standard that they offer, as well as develop predictive tools. Ultimately a “Best-Practice” guidance for cost-effective and environmentally acceptable management of barrier beaches as natural flood defences will be produced. iv R&D Technical Report FD1924/TR - Understanding Barrier Beaches Contents Executive summary iii Contents v 1. Background and objectives of study 1 1.1 Statement of problem 1 1.2 Objectives of this research project 4 1.3 Layout of report 5 2. Review of process understanding 7 2.1 Barrier beaches – definitions 7 2.2 Review of geomorphological classifications 10 2.3 Barrier distribution around the globe 12 2.4 Characteristics of barriers 14 2.5 Barrier beach profile features 22 2.6 Longshore processes specific to barriers 43 2.7 Evolutionary timescales 44 2.8 Recommendations for further work 47 3. Methods of study 49 3.1 Desk based investigations 49 3.2 Field based investigations 50 3.3 Modelling approaches 52 3.4 Recommendations for further work 74 4. Application of state of the art to coarse-clastic barriers 77 4.1 Conceptual models of morphodynamic forcing 77 4.2 Application of the Shingle model to barrier beaches 79 4.3 Application of Dimensionless Barrier Inertia model 84 4.4 Recommendations for further work 94 5. Case histories 97 5.1 Introduction 97 5.2 Porlock 99 5.3 Medmerry/ Selsey 114 5.4 Chesil 128 R&D Technical Report FD1924/TR - Understanding Barrier Beaches v 5.5 Slapton Sands 144 5.6 Hurst Spit 152 5.7 East Head 172 5.8 Hayling Island 180 5.9 Easton Broad 188 5.10 Sand Bay 194 5.11 Pett Levels 199 5.12 Westward Ho! 205 5.13 Dawlish Warren 209 5.14 Recommendations for further research 214 6. Review of current management methods 215 6.1 Introduction 215 6.2 Monitoring methods 216 6.3 Assessment of barrier beach performance 225 6.4 Review of mitigation measures 226 6.5 Recommendations for further research 236 7. Conclusions and research needs 239 7.1 Conclusions 239 7.2 Perceived weaknesses 241 7.3 Recommendations for future research 243 7.4 Delivery tools