WOKING AND SURREY HEATH STRATEGIC FLOOD RISK ASSESSMENT For WOKING BOROUGH COUNCIL and SURREY HEATH BOROUGH COUNCIL MARCH 2007 VOLUME 2: TECHNICAL REPORT (FINAL) Capita Symonds © Capita Symonds Ltd 24-30 Holborn London EC1N 2LX Tel: 02078709300 Fax: 02078709399 Email: [email protected] Woking and Surrey Heath SFRA Woking and Surrey Heath Strategic Flood Risk Assessment Technical Document Final Report REV 0.0 / March 2007 This document is the property of the Woking Borough Council and Surrey Heath Borough Council and its contents are confidential. If you have received it in error, please return it immediately to one of the above named Local Authorities. Job Number: CS/021102 PD: AGJD PM: RG Report status: FINAL REV 0.0 Date of issue: 19/03/07 CSL Main author(s): CB/SC CSL Checked: RG Client Approval: - This report has been prepared by Capita Symonds Limited with all reasonable skill, care and diligence within the terms of the Contract with the Client, incorporating our General Terms and Conditions of Business and taking account of the resources devoted to it by agreement with the client. We disclaim any responsibility to the client and other in respect of any matters outside the scope of the above. This report is confidential to the client and we accept no responsibility of whatsoever nature to third parties to whom this report, or any part thereof, is made known. Any such party relies on the report at their own risk Woking Borough Council and Surrey Heath Borough Council March 2007 Woking and Surrey Heath SFRA PREFACE It is accepted that the technical content of the Woking & Surrey Heath SFRA will need to be reviewed and amended as new information becomes available. Although there is no statutory consultation requirement at this stage the nature of the intended end use for the information makes it appropriate to obtain feedback relating to the report in order to contribute to the overall robustness and credibility of this work. This information will also be an aid when formulating the necessary next steps in engaging those parties who will be involved in the future. It is the responsibility of the reader to be satisfied that they are using the most up to date information and that this has been included within the Woking & Surrey Heath SFRA. Woking Borough Council and Surrey Heath Borough Council March 2007 Woking and Surrey Heath SFRA CONTENTS PREFACE ..................................................................................................................................... II GLOSSARY ................................................................................................................................VII 1. INTRODUCTION ...................................................................................................... 1 2. DOCUMENT REGISTER.......................................................................................... 2 3. ENVIRONMENTAL AND PLANNING CONTEXT ................................................... 3 Background ............................................................................................................. 3 Introduction ............................................................................................................. 3 National Flood Risk Policy..................................................................................... 3 Regional Flood Risk Policy.................................................................................... 5 4. METHODOLOGY ..................................................................................................... 7 Principles................................................................................................................. 7 Flood Risk Assessments ....................................................................................... 8 Long Term Management ........................................................................................ 8 5. CATCHMENT DESCRIPTION ............................................................................... 10 The Catchment ...................................................................................................... 10 Regional Geology ................................................................................................. 11 Watercourses in the Study Area ......................................................................... 11 Topography ........................................................................................................... 12 The Record of Flooding in the Addlestone Catchment .................................... 13 Flooding Processes Within the Catchment ....................................................... 13 Flood Defences in the Study Area ...................................................................... 13 Structures over Watercourses ............................................................................ 14 Drainage and Surface Water................................................................................ 14 6. DATA REVIEW....................................................................................................... 16 7. HYDROLOGICAL ANALYSIS ............................................................................... 18 Catchment Characteristics .................................................................................. 18 Catchment Hydrology........................................................................................... 18 8. HYDRAULIC ANALYSIS........................................................................................ 20 Introduction ........................................................................................................... 20 Approach ............................................................................................................... 20 Woking Borough Council & Surrey Heath Borough Council Page iii March 2007 Woking and Surrey Heath SFRA Flood Model Selection.......................................................................................... 20 Flood Model Development ................................................................................... 21 Flood Model Inflows ............................................................................................. 21 Downstream Boundary......................................................................................... 22 Model Calibration and Sensitivity Testing ......................................................... 22 Hydraulic Modelling to Define Flood Zones....................................................... 23 Hydraulic Modelling to Define Actual Risk......................................................... 24 Hydraulic Modelling to Define Residual Risk .................................................... 24 9. OTHER SOURCES OF FLOODING ...................................................................... 25 Groundwater, Surface Water and Other Sources of Flooding ......................... 25 The Basingstoke Canal ........................................................................................ 27 10. MAPPING OF FLOOD EXTENTS.......................................................................... 30 Floodplain Topographic Survey.......................................................................... 30 Methodology.......................................................................................................... 30 Recommendations................................................................................................ 31 11. UNCERTAINTIES IN FLOOD RISK ASSESSMENT............................................. 32 Generic Risks and Uncertainties......................................................................... 32 12. STRATEGIC FLOOD RISK ASSESSMENT.......................................................... 34 Introduction ........................................................................................................... 34 Justification and Statutory Responsibilities...................................................... 34 Approach ............................................................................................................... 35 STAGE 1 – Flood Zone Review............................................................................ 36 STAGE 2 – Assessment of Actual Risk .............................................................. 38 STAGE 3 – Assessment of Residual Risk .......................................................... 43 STAGE 4 – Assessment of Breach Hazard......................................................... 44 Application of the Woking and Surrey Heath SFRA.......................................... 45 Implementation of the SFRA................................................................................ 48 Emergency Planning ............................................................................................ 48 Summary of Strategic Flood Risk Assessment................................................. 49 13. SFRA GUIDANCE FOR PLANNERS .................................................................... 50 14. CONCLUSIONS ..................................................................................................... 58 Summary of Flood Risk in the Woking and Surrey Heath SFRA Study Area. 58 Outcomes of the Strategic Flood Risk Assessment ......................................... 59 Woking Borough Council & Surrey Heath Borough Council Page iv March 2007 Woking and Surrey Heath SFRA 15. REFERENCES ......................................................................................................