Wyre Forest District Council Level 2 Strategic Flood Risk Assessment
Total Page:16
File Type:pdf, Size:1020Kb
Wyre Forest District Council Level 2 Strategic Flood Risk Assessment Wyre Forest District Council February 2010 Final Report 9T6121 A COMPANY OF HASKONING UK LTD. COASTAL & RIVERS th 5 Floor Radcliffe House Blenheim Court Solihull B91 2AA United Kingdom +44 (0)121 7096532 Telephone [email protected] E-mail www.royalhaskoning.co.uk Internet Document title Wyre Forest District Council Level 2 Strategic Flood Risk Assessment Document short title Level 2 SFRA Status Final Report Date February 2010 Project name Level 2 SFRA Project number 9T6121 Client Wyre Forest District Council Reference 9T6121/R/303693/Birm Drafted by Laura Sanderson Checked by Mike Stringer Date/initials check …………………. …………………. Approved by Mike Stringer Date/initials approval …………………. …………………. Level 2 SFRA - i - 9T6121/R/303693/Birm Final Report February 2010 CONTENTS Page 1 INTRODUCTION 1 1.1 Commission Award 1 1.2 Background 1 1.3 Study Area 3 1.4 Prospective Development in the Study Area 4 1.5 Scope 6 1.6 Data Used 9 2 UPDATES TO LEVEL 1 RESULTS 11 2.1 Potential Development Sites 11 2.2 Fluvial Flood Risk, Climate Change, Flood Risk Management Infrastructure and Flood Warning 11 2.2.1 Kidderminster 11 2.2.2 Stourport-On-Severn 15 2.2.3 Bewdley 17 2.2.4 Cookley and Rural Wyre Forest 17 2.3 Summary 18 3 FLOOD RISK IN KIDDERMINSTER AND SURROUNDING AREA 19 3.1 Study Area 19 3.2 Overview of Flood Risk 19 3.3 Flood Risk from the River Stour through Kidderminster 20 3.3.1 Flood Defence Infrastructure 20 3.3.2 Methodology 23 3.3.3 Breach Analysis 24 3.3.4 Flood Hazard Analysis 26 3.3.5 Rapid Inundation 29 3.3.6 Pluvial Flooding 29 3.4 Flood Risk from Minor Watercourses and the River Stour Outside Kidderminster 30 3.4.1 General 30 3.4.2 The River Stour outside Kidderminster 30 3.4.3 Drakelow/Hors Brook 35 3.4.4 Blakedown Brook 36 3.4.5 Hoo Brook 38 4 FLOOD RISK IN STOURPORT-ON-SEVERN 41 4.1 Study Area 41 4.2 Overview of Flood Risk 41 4.3 Flood Risk from River Severn 41 4.3.1 Flood Defence Infrastructure 41 4.3.2 Methodology 41 4.3.3 Pluvial Flooding 42 5 FLOOD RISK IN BEWDLEY 45 5.1 Study Area 45 5.2 Overview of Flood Risk 45 Level 2 SFRA - i - 9T6121/R/303693/Birm Final Report February 2010 5.3 Flood Risk from River Severn 45 5.3.1 Flood Defence Infrastructure 45 5.3.2 Methodology 46 5.3.3 Breach Analysis and Rapid Inundation Zones 46 5.3.4 Flood Hazard Analysis 47 5.3.5 Rapid inundation analysis 48 6 GUIDANCE 51 7 CONCLUSIONS AND RECOMMENDATIONS 53 7.1 Development Sites in Zone 1 – Low Probability 53 7.2 Development Sites in Zone 2 – Medium Probability 55 7.3 Development Sites in Zone 3a – High Probability 57 7.4 Development Sites in Zone 3b – The Functional Floodplain 60 APPENDICES 1 Figures 2 Wyre Forest District NFCDD 3 Hoo Brook Hydrology and Hydraulics 4 Guidance 5 Flood Risk Vulnerability Classification 6 Technical Note 7 EA Sign off Letter FIGURES 1 Flood Zones and Development Sites within Wyre Forest District 2 Flood Zones and Development Sites in Kidderminster and Surrounding Area 3 Kidderminster and Surrounding Area Low Grade Flood Defence Infrastructure 4a Kidderminster Flood Depth and Hazard 1% AEP - Dam Overtopping 4b Kidderminster Flood Depth and Hazard 1% AEP Plus Climate Change – Dam Overtopping 4c Kidderminster Flood Depth and Hazard 0.1% AEP – Dam Overtopping 4d Kidderminster Flood Depth and Hazard 0.1% AEP Plus Climate Change – Dam Overtopping 5 Kidderminster Breach Locations 6a Kidderminster Flood Depth and Hazard 1%AEP – Breach 1 6b Kidderminster Flood Depth and Hazard 1%AEP – Breach 2 6c Kidderminster Flood Depth and Hazard 1%AEP – Breach 3 6d Kidderminster Flood Depth and Hazard 1%AEP – Breach 4 6e Kidderminster Flood Depth and Hazard 1%AEP – Breach 5 7a Kidderminster Flood Depth and Hazard 1%AEP with Climate Change - Breach 1 7b Kidderminster Flood Depth and Hazard 1%AEP with Climate Change – Breach 2 7c Kidderminster Flood Depth and Hazard 1%AEP with Climate Change – Breach 3 7d Kidderminster Flood Depth and Hazard 1%AEP with Climate Change – Breach 4 Level 2 SFRA - ii - 9T6121/R/303693/Birm Final Report February 2010 7e Kidderminster Flood Depth and Hazard 1%AEP with Climate Change – Breach 5 8a Kidderminster Flood Depth and Hazard 0.1%AEP – Breach 1 8b Kidderminster Flood Depth and Hazard 0.1%AEP – Breach 2 8c Kidderminster Flood Depth and Hazard 0.1%AEP – Breach 3 8d Kidderminster Flood Depth and Hazard 0.1%AEP – Breach 4 8e Kidderminster Flood Depth and Hazard 0.1%AEP – Breach 5 9 Kidderminster Flood Depth and Hazard Dam Breach 10 Kidderminster Flood Depth and Hazard Worst Case Scenario Breaching 11 Kidderminster Pluvial Flood Depth and Hazard 12 Development Site SH12 and D37/SH29 and River Stour Flood Zones 13 Topography of sites SH12 and D37/SH29 14 Development Site SH13 and River Stour Flood Zones 15 Wolverley Flood Extents 16 Development Sites D20 and D33 with River Stour Flood Zones 17 Topography at D20 and D33 18 Wilden Flood Extents 19 Development Site SH21 and River Stour Flood Zones 20 Blakedown Brook Flood Extents 21 Development Site D22 and River Stour Flood Zones 22 Hoo Brook Flood Extents 23 Flood Zones and Development Sites in Stourport-on-Severn 24 Stourport-on-Severn Pluvial Flood Depth and Hazard 25 Flood Zones and Development Sites in Bewdley 26 Bewdley Breach Locations 28a Bewdley Flood Depth and Hazard 1% AEP – Breach A 28b Bewdley Flood Depth and Hazard 1% AEP with Climate Change – Breach A 28c Bewdley Flood Depth and Hazard 0.1% AEP – Breach A 29a Bewdley Flood Depth and Hazard 1% AEP – Breach B 29b Bewdley Flood Depth and Hazard 1% AEP with Climate Change – Breach B 29c Bewdley Flood Depth and Hazard 0.1% AEP – Breach B 30 Bewdley Flood Depth and Hazard Worst Case Scenario Breaching Level 2 SFRA - iii - 9T6121/R/303693/Birm Final Report February 2010 Level 2 SFRA - iv - 9T6121/R/303693/Birm Final Report February 2010 EXECUTIVE SUMMARY Study Objectives This Level 2 Strategic Flood Risk Assessment (SFRA) for the Wyre Forest District Council (hereafter, ‘the Council’) considers the detailed nature of flood hazard across the District by taking into account the presence of flood risk management measures and has been undertaken with a principal purpose to facilitate application of the Sequential and Exception Tests. The key objectives of the study are to: • Review the Flood Zones presented in the Level 1 SFRA, in particular the Functional Floodplain (Flood Zone 3b); • Review flood defence infrastructure, including its present condition, maintenance and upgrading, consequences of overtopping or failure and the response to climate change; • Model flood risk across the Flood Zones, including the identification of rapid inundation zones, risk to people behind defences and the effect of increased runoff from developments on flood risk; and • Analyse site specific flood risk. In addition guidance notes are provided for the execution of the Exception Test, the preparation of FRAs, Emergency Planning Measures and Dealing with Surface Water Drainage. Outputs This report focuses on development sites within Flood Zones 2 and 3, or sites that pose potential risk to un-modelled ordinary watercourses. A 1D hydraulic model was constructed of the Hoo Brook in order to address misalignments of the Flood Zones. TUFLOW 2D models were constructed to review breach scenarios in Kidderminster and Bewdley. Pluvial analysis was carried out for Kidderminster and Stourport. The Kidderminster dam was also reviewed and analysed for overtopping. A detailed methodology of the Hoo Brook modelling is presented in Appendix 3, with results presented in the form of maps. Outputs from the TUFLOW models are presented in maps and colour–coded tables within the main body of the report. Due to the nature of the project requirements, all models within the study were constructed as new using data obtained from site investigations in conjunction with topographic data derived from LiDAR. Data Sources The data used within this SFRA was documented within the Level 1 SFRA and updated with the addition of new development sites provided after the completion of the Level 1. Supplementary data was collected during site visits undertaken at the end of August 2008 and May 2009. Co-operation This SFRA was carried out for the Council with the co-operation and support of the Environment Agency. Level 2 SFRA - i - 9T6121/R/303693/Birm Final Report February 2010 Level 2 SFRA - ii - 9T6121/R/303693/Birm Final Report February 2010 GLOSSARY Brownfield site Any land or site that has been previously developed. Catchment The area contributing flow or runoff to a particular point on a watercourse. Climate change Long-term variations in global temperature and weather patterns both natural and as a result of human activity, primarily greenhouse gas emissions. Culvert Covered channel or pipe that forms a watercourse below ground level. Design flood level The maximum estimated water level during the design event. Development The carrying out of building, engineering, mining or other operations in, on, over or under land or the making of any material change in the use of any buildings or other land. Enmained Watercourse designated as a Main River Environment Agency Government Agency charged with the protection of the environment Exception Test The final process of the PPS25 Sequential Test (TIERS 3 & 4). It is required when a development application is made for a site within Flood Zones 2 & 3 and no other site of lower flood risk is available.