Strategic Flood Risk Assessment Tonbridge and Malling Borough Council Stage 2 Report
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Strategic Flood Risk Assessment Mott MacDonald Tonbridge and Malling Borough Council Stage 2 Report Tonbridge and Malling Borough Council Gibson Drive Kings Hill West Malling Kent ME19 4LZ Strategic Flood Risk Assessment Tonbridge and Malling Borough Council Stage 2 Report August 2006 227741/01/A - August 2006/ P:\Cambridge\Demeter - Daedalus\WEM\PROJECTS\227741_Tonbridge-Malling_SFRA\F_Reports\Stage_2\Final_figures\Stage 2 Report_TMBC_SFRA.doc/DS Strategic Flood Risk Assessment Mott MacDonald Tonbridge and Malling Borough Council Stage 2 Report Strategic Flood Risk Assessment Tonbridge and Malling Borough Council Stage 2 Report This document has been prepared for the titled project or named part thereof and should not be relied upon or used for any other project without an independent check being carried out as to its suitability and prior written authority of Mott MacDonald being obtained. Mott MacDonald accepts no responsibility or liability for the consequence of this document being used for a purpose other than the purposes for which it was commissioned. Any person using or relying on the document for such other purpose agrees, and will by such use or reliance be taken to confirm his agreement to indemnify Mott MacDonald for all loss or damage resulting therefrom. Mott MacDonald accepts no responsibility or liability for this document to any party other than the person by whom it was commissioned. To the extent that this report is based on information supplied by other parties, Mott MacDonald accepts no liability for any loss or damage suffered by the client, whether contractual or tortious, stemming from any conclusions based on data supplied by parties other than Mott MacDonald and used by Mott MacDonald in preparing this report. 227741/01/A - August 2006/ P:\Cambridge\Demeter - Daedalus\WEM\PROJECTS\227741_Tonbridge-Malling_SFRA\F_Reports\Stage_2\Final_figures\Stage 2 Report_TMBC_SFRA.doc/DS Strategic Flood Risk Assessment Mott MacDonald Tonbridge and Malling Borough Council Stage 2 Report Glossary AOD Above Ordnance Datum DSM Digital Surface Model DTM Digital Terrain Model EA Environment Agency EFO Extreme Flood Outline FEH Flood Estimation Handbook FRA Flood Risk Assessment FSR Flood Studies Report FW Foul Water GIS Geographical Information System IFM Indicative Flood Map IFSAR Interferometric Synthetic Aperture Radar LAMP Local Asset Management Plan LiDAR Light Detection and Ranging NFCDD National Flood and Coastal Defence Database OS Ordnance Survey PPG25 Planning Policy Guidance Note 25 PPS25 Planning Policy Statement 25 QMED Median Annual Maximum Flood (m³/s) SAR Synthetic Aperture Radar SFRA Strategic Flood Risk Assessment SoP Standard of Protection SuDS Sustainable Drainage Systems SW Surface Water T&MBC Tonbridge and Malling Borough Council WwTW Wastewater Treatment Works Acknowledgement: All data provided by the Environment Agency which has been used in the compilation of this report remains the property of the Environment Agency and must not be relied on or used for any other project without the prior written authority of the Environment Agency being obtained. i 227741/01/A - August 2006/S-1 of 4 P:\Cambridge\Demeter - Daedalus\WEM\PROJECTS\227741_Tonbridge-Malling_SFRA\F_Reports\Stage_2\Final_figures\Stage 2 Report_TMBC_SFRA.doc/DS Strategic Flood Risk Assessment Mott MacDonald Tonbridge and Malling Borough Council Stage 2 Report Contents List 1 Introduction 1-1 1.1 Study Objectives 1-1 1.2 Government Avice on Flood Risk 1-1 1.3 Primary Purpose of the SFRA 1-2 1.4 SFRA Stages 1-2 2 SFRA Background Information 2-5 2.1 Data Collection and Verification 2-5 2.2 Interface with other Flood Risk Products 2-5 2.2.1 Differences between Agency Flood Zone Maps and SFRA Maps 2-5 2.2.2 The Extreme Flood Outline 2-6 2.3 Project Outputs 2-6 2.4 Flood Zones - General Criteria 2-8 2.4.1 Zone 2 / Zone 3 Boundary 2-8 2.4.2 Zone 1 / Zone 2 Boundary 2-9 2.4.3 Zone 3c – Functional Floodplain 2-9 2.4.4 Flood Risk Assessments for Individual Development Proposals 2-9 2.5 Climate Change 2-10 3 Ground Surface Data 3-11 3.1 Photogrammetry Data 3-11 3.2 Ground Survey Data 3-11 3.3 Aerial Photography 3-12 4 Flood Defences 4-15 4.1 General 4-15 4.2 Leigh Barrier Operation 4-16 4.2.1 Automatic Control 4-16 4.2.2 Leigh Barrier Operating Procedure 4-16 5 Main River Data 5-18 5.1 Medway Catchment 5-18 5.2 Other Watercourses 5-19 5.3 Gas Works Stream 5-19 5.4 Botany Stream 5-19 5.5 Mill Stream 5-19 5.6 Hilden Brook and Hawden Stream 5-19 5.7 Hydraulic Modelling 5-20 5.8 River Medway Flood Risk Mapping, Phase 3 Study, 2004 5-21 5.8.1 Model Approach 5-21 5.8.2 Calibration 5-21 ii 227741/01/A - August 2006/S-2 of 4 P:\Cambridge\Demeter - Daedalus\WEM\PROJECTS\227741_Tonbridge-Malling_SFRA\F_Reports\Stage_2\Final_figures\Stage 2 Report_TMBC_SFRA.doc/DS Strategic Flood Risk Assessment Mott MacDonald Tonbridge and Malling Borough Council Stage 2 Report 5.8.3 Flood Mapping 5-22 5.9 Modifications to River Medway, Phase 3 Study, 2006 5-23 5.9.1 Model Approach 5-23 5.9.2 Leigh Barrier Inflow 5-24 5.9.3 Calibration 5-25 5.9.4 Interpretation in the vicinity of Control Structures 5-27 5.9.5 Flood Mapping 5-30 5.10 Medway SFRA 5-31 5.10.1 Modelling Approach 5-31 5.10.2 Boundary Conditions 5-31 5.10.3 Calibration 5-31 5.10.4 Flood extent 5-32 6 Tonbridge 6-33 6.1 Flood Risk Areas - Tonbridge Town. 6-34 6.1.1 Tonbridge Flow Paths 6-35 6.2 Potential Re-Development in Tonbridge 6-36 6.3 Flood Risk Zones - West of Tonbridge 6-37 7 Aylesford 7-39 8 Limitations 8-40 9 Flood Risk Management, Mitigation and Enhancement Measures 9-41 9.1 Introduction 9-41 9.2 Aims of Management, Mitigation and Enhancement Measures 9-41 9.2.1 Generic Risk Management Measures 9-42 Appendices Appendix A References 10-47 Appendix B Sequential Test 10-48 Appendix C Tonbridge Figures 10-51 Appendix D Aylesford Figures 10-62 Appendix E Modelling limitations 10-66 iii 227741/01/A - August 2006/S-3 of 4 P:\Cambridge\Demeter - Daedalus\WEM\PROJECTS\227741_Tonbridge-Malling_SFRA\F_Reports\Stage_2\Final_figures\Stage 2 Report_TMBC_SFRA.doc/DS Strategic Flood Risk Assessment Mott MacDonald Tonbridge and Malling Borough Council Stage 2 Report List of Figures Figure 1.1: Constraints 1-3 Figure 1.2: Location Plan 1-4 Figure 3.1: Area covered by Photogrammetry 3-13 Figure 3.2: Survey Location Plan (Tonbridge) 3-14 Figure 4.1: Flood Defences 4-17 Figure 5.1: Calibration of Model at Town Lock Tonbridge: Oct 2000 event 5-26 Figure 5.2: Schematic of flows path through Tonbridge from the Medway Model Phase 3 5-28 Figure 5.3: Schematic to Illustrate Flood Flow Paths of the revised model through Tonbridge. 5-29 Figure 6.1: Flood Extent on 13 October 2000 (looking north-west from east Tonbridge) 6-38 Figure 6.2: Flood Extent on 13 October 2000 (looking north-west) 6-38 Figure C.1: Tonbridge Centre: Defended 100-year outline 10-52 Figure C.2: Tonbridge Centre: Defended 1 in 100 year outline : Graduated risk zones 10-53 Figure C.3: Tonbridge Centre: Defended 1 in 10-year outline with climate change: Functional Floodplain 10-54 Figure C.4: Tonbridge Centre: Defended 1 in 100-year outline with climate change 10-55 Figure C.5: Tonbridge Centre: Flood Zone 3 and Flood Zone 2 10-56 Figure C.6: Tonbridge Development Quarters 10-57 Figure C.7: West Tonbridge – Defended 1 in 100-year outline 10-58 Figure C.8: West Tonbridge: Defended 1 in 100-year outline with Graduated flood risk 10-59 Figure C.9: West Tonbridge: Defended 1 in 10 year outline including climate change 10-60 Figure C.10: West Tonbridge: Flood Zone 3 and Flood Zone 2 10-61 Figure D.1: Aylesford: Defended 1 in 200-year and 1 in 1000-year outlines 10-63 Figure D.2: Aylesford: Defended 1 in 200-year outline with climate change 10-64 Figure D.3: Aylesford: Flood Zone 3 and Flood Zone 2 10-65 Table Table 2.1: Flood Risk Zones in PPG 25...............................................................................................2-8 iv 227741/01/A - August 2006/S-4 of 4 P:\Cambridge\Demeter - Daedalus\WEM\PROJECTS\227741_Tonbridge-Malling_SFRA\F_Reports\Stage_2\Final_figures\Stage 2 Report_TMBC_SFRA.doc/DS Strategic Flood Risk Assessment Mott MacDonald Tonbridge and Malling Borough Council Stage 2 Report 1 Introduction 1.1 Study Objectives The Environment Agency (EA) and Tonbridge and Malling Borough Council (T&MBC) have jointly commissioned Mott MacDonald to carry out a Strategic Flood Risk Assessment, (SFRA). The focus of the study areas was selected after the application of a sequential test as described in Planning Policy Guidance Note 25: Development and Flood Risk (PPG25)1. The sequential test used a multi-tiered approach to inform the draft future spatial strategy2 for Tonbridge and Malling Borough, see Appendix B. The key planning constraints identified as part of the sequential test can be seen in Figure 1.2 The outcome of the sequential test found that virtually all of the future residential growth, which is anticipated to take place within the Borough during the next 15 years, outside of Tonbridge central area, will be in locations at no risk from flooding. However, in order to deliver key national, regional and local sustainable development objectives, particularly in relation to town centres and to respond to the outputs of local baseline studies, the test found that for some required development no reasonable options are available in Flood Zones 1 or 2.