Planning for Flood Risk Management in the Thames Estuary Technical Scoping Report March 2003 Planning for Flood Risk Management in the Thames Estuary Technical Scoping Report Environment Agency Rio House Waterside Drive Aztec West Almondsbury Bristol BS32 4UD March 2003 Publishing organisation Environment Agency Rio House Waterside Drive Aztec West Almondsbury Bristol BS32 4UD Tel: 01454 624400 Fax: 01454 624409 © Environment Agency March 2003 All rights reserved. No part of this document may be produced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise without the prior permission of the Environment Agency. Statement of use This document provides information for use by the Environment Agency in scoping the Planning for Flood Risk Management in the Thames Estuary project. Contract Statement This report presents the scoping exercise conducted by HR Wallingford for Planning for Flood Risk Management in the Thames Estuary. The Environment Agency, whose representative was Sarah Lavery, commissioned the study and the HR Wallingford Job Number was DAS 0806. Mervyn Littlewood and Matt Crossman prepared the report with key contributions from Paul Sayers, David Ramsbottom, Dr Richard Whitehouse, Dr Peter Hawkes, Dr Mike Dearnaley, Roy Atkins and Silvia Segura. The HR Wallingford responsible Director was Dr Jane Smallman. Key members of client and project team Prepared by ................................................................................... Mervyn Littlewood and Matt Crossman HR Wallingford, Project Managers Approved by ................................................................................... Dr Jane Smallman HR Wallingford, Director ................................................................................... Sarah Lavery Environment Agency, Project Manager HR Wallingford accepts no liability for the use by third parties of results or methods presented in this report. The Company also stresses that various sections of this report rely on data supplied by or drawn from third party sources. HR Wallingford accepts no liability for loss or damage suffered by the client or third parties as a result of errors or inaccuracies in such third party data. PLANNING FOR FLOOD RISK MANAGEMENT IN THE THAMES ESTUARY TECHNICAL SCOPING REPORT - ii - EXECUTIVE SUMMARY Large areas of the Thames Estuary are potentially at risk from tidal flooding and are currently protected to a high standard by the defences constructed in the late 1970s and early 1980s, which included the Thames Barrier at Woolwich and raised defences eastwards to Southend-on-Sea and the Isle of Grain. Most of these flood defences were designed to last until approximately 2030. The Environment Agency has recently started the process of planning their future strategy for managing flood risk in the Thames Estuary in order to ensure that it is in place before large-scale works are required. This report provides a context for the defences, it reviews available information and identifies key opportunities and constraints relating to the physical environment, existing management framework for the defences, assets at risk from flooding and appropriate boundaries for future studies. Flood risk within the estuary is not simply dependent on the water level at sea (which itself results from a combination of factors), but influenced by a complex combination of meteorological factors, fluvial flows, estuary morphology and the operation of the flood defences. The area of influence extends to the whole of the Thames catchments and some distance into the North Sea Due to the size and importance of the estuary the development of a flood risk strategy will clearly be a difficult undertaking, demanding a good understanding of a wide range of processes and issues including hydrodynamics, environmental, economic and social factors associated with flooding and potential schemes. Reliable decision-making within such a complex environment will require the development and application of a well-structured approach drawing on existing best practice and knowledge as well as the results of the latest research.. It will undoubtedly be necessary to adopt a tiered approach to decision-making, solving issues at a manageable local scale, but within a regional framework that recognises opportunities and constraints on an estuary wide scale. The summary of hydraulic processes within the estuary identifies considerable gaps in information and understanding where further data will probably be required. One significant concern relates to the reliability of water level records and predictions. There is also only limited information available on the performance and condition of the existing defences and some uncertainty as to the accuracy of the indicative floodplain mapping published by the Environment Agency. This situation will be improved with the development of the National Flood and Coast Defence Database and completion of a number of studies in 2003. The initial assessment of assets at risk of flooding divided the indicative tidal floodplain between Teddington and Foulness Point / Whitstable into 43 flood risk compartments covering a total 468 km2 with capital values totalling more than £99,000 million. Recommendations for the scope and timing of further studies are presented within ‘Advisory Report No. 1 in support of project development’ which has been prepared concurrently. For further information regarding the information contained in this report please contact Paul Sayers or Jane Smallman at HR Wallingford. PLANNING FOR FLOOD RISK MANAGEMENT IN THE THAMES ESTUARY TECHNICAL SCOPING REPORT - iii - PLANNING FOR FLOOD RISK MANAGEMENT IN THE THAMES ESTUARY TECHNICAL SCOPING REPORT - iv - CONTENTS EXECUTIVE SUMMARY iii 1. Introduction 1 2. The Thames Estuary – A Context 2 2.1 Historical development of the Thames 3 2.2 Hydraulic drivers of flood risk 9 2.3 Hydraulic drivers and ecology 17 2.4 Overview of the defence system 18 2.5 Sediment processes and morphological response 20 2.6 Socio – economic constraints and opportunities 22 2.7 Recommendations 24 3. The Physical Environment 26 3.1 Hydraulic drivers 26 3.2 Sediment processes and morphology 36 3.3 The existing flood defence system 40 3.4 Environmental responses 43 3.5 Recommendations 43 4. Existing Management Framework 45 4.1 High level multi-disciplinary guidance 45 4.2 Flood and coastal defence policy and strategy 47 4.3 National and regional guidance and policy 50 4.4 Environmental, habitat and biodiversity 56 4.5 Recommendations 60 5. Identification of Assets at Risk from Flooding 62 5.1 Topography and ‘flood cells’ 62 5.2 Review of assets at risk from flooding 63 5.3 Future change scenarios 65 5.4 Definition of proposed valuation methodology for each asset type 66 5.5 Review of data for use in assessing economic impacts of flooding 66 5.6 Recommendations 69 6. Study Boundaries 70 6.1 Temporal 70 PLANNING FOR FLOOD RISK MANAGEMENT IN THE THAMES ESTUARY TECHNICAL SCOPING REPORT - v - CONTENTS CONTINUED 6.2 Management / intervention 71 6.3 Physical Processes / Understanding 72 6.4 Recommendations 72 7. Recommendations 74 8. References 77 Tables Table 2.1 Ranked extreme high waters at London Bridge and Sheerness 6 Table 2.2 Classification of flood defences by immediate hinterland 23 Table 3.1 Environment Agency water level data 27 Table 3.2 Port of London Authority water level data 27 Table 3.3 Tributaries and creeks discharging into the Thames Estuary 29 Table 3.4 Consented discharges into the Thames Estuary in 1995 29 Table 3.5 Consented discharges into tributaries in 1995 30 Table 3.6 Tidal current data 32 Table 3.7 ADP data sets 33 Table 3.8 Defence types comprising the Tidal Walls 40 Table 3.9 Approximate cost of existing tidal defences 41 Table 4.1 North Kent Shoreline Management Policy 47 Table 4.2 South Essex Shoreline Management Policy 48 Table 4.3 Indicative standards of protection (after MAFF 1999) 52 Table 4.4 Planning response to sequential characterisation of flood risk 54 Table 4.5 Summary of Water Level Management Plans 59 Table 5.1 Distribution of assets within the different flood risk compartments 63 Table 5.2 Proposed development within the Thames Gateway ‘Zones of Change’ 65 Figures Figure 2.1 Annual maxima tide levels at Sheerness 6 Figure 2.2 The 1953 surge 12 Figure 2.3 The 1953 and mean spring high water levels 13 Figure 3.1 An example of acoustic backscatter measurement of suspended solids concentration and other associated parameters 37 PLANNING FOR FLOOD RISK MANAGEMENT IN THE THAMES ESTUARY TECHNICAL SCOPING REPORT - vi - CONTENTS CONTINUED Figure 4.1 Hierarchy of plans (after MAFF 2001b) 51 Drawings Drawing 01 Indicative flood risk areas in the Thames Estuary Drawing 02 Thames River basin Drawing 03 Geology Drawing 04 Tidal defence system Drawing 05 Environment Agency boundaries Drawing 06 Local authority boundaries Drawing 07 Population Drawing 08 Hydraulic measures locations Drawing 09 Discharges and outfalls Drawing 10 Wind and wave data Drawing 11 PLA chart extents Drawing 12 Environmental designations Drawing 13 Environmental resources Drawing 14 Archaeology Drawing 15 Extents of policy and strategy studies Drawing 16 Topography Drawing 17 Flood cells Drawing 18 Proposed development areas Drawing 19 Topographic data coverage Drawing 20 Relevant boundaries Appendices Appendix A Planning for Flood Risk Management in the Thames Estuary 83 Appendix B The Outer Thames Estuary 97 Appendix C Combined Sewage Outfalls –