Assessment of Surface Water Flood Risk 4.1. Introduction
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Appendix 2 - Surface Water Management Plan Surface Water Management Plan for Birmingham Final Report June 2015 Appendix 2 - Surface Water Management Plan Notice This document and its contents have been prepared and are intended solely for Birmingham City Council’s information and use in relation to the Birmingham Surface Water Management Plan. Atkins Ltd assumes no responsibility to any other party in respect of or arising out of or in connection with this document and/or its contents. Document History Job number: 5095758 Document ref: Birmingham Surface Water Management Plan – Intermediate Phase – D1.docx Revision Purpose Description Originated Checked Reviewed Authorised Date Rev 1.0 Draft for Client Comment A Cambridge K Ravnkilde C Nunns R Engledow 17/03/11 (Strategic ) Rev 2.0 Draft for Client Comment A Cox/ K Ravnkilde C Nunns A Rose 01/05/12 (Intermediate) K Whitehouse (BCC) Rev 3.0 Draft for Client Comment K Ravnkilde/ A Cambridge A Rose A Rose 23/12/13 (Detailed, Options and K Whitehouse Action Plan) stages (BCC) added) Rev 4.0 Stakeholder Comments K. Ravnkilde A Cambridge A Rose A Rose 02/05/14 incorporated Rev 5.0 Updated actions K Whitehouse A Cambridge A Rose A Rose 01/06/15 (BCC) Client signoff Client Birmingham City Council Project Birmingham Surface Water Management Plan Document title Final Report Job No. 5095758.039 Copy No. Document reference Appendix 2 - Surface Water Management Plan Table of contents Chapter Pages 1. Introduction 6 1.1. Background 6 1.2. This Report 7 2. Preparation Phase 8 2.1. Introduction 8 2.2. Establishing a Partnership 8 2.3. SWMP Scope 8 2.4. Aims 9 2.5. Objectives 9 2.6. Engagement Plan 10 3. Strategic Assessment – Objectives & Methodology 13 3.1. Introduction 13 3.2. Strategic Assessment Objectives 13 3.3. Approach 13 3.4. Data and Scoring Methods 13 4. Strategic Assessment – Assessment of Surface Water Flood Risk 20 4.1. Introduction 20 4.2. Overview 20 5. Strategic Assessment – Prioritisation 24 5.1. Introduction 24 5.2. Priority Catchments 24 5.3. Next Steps 25 6. Strategic Assessment – Sensitivity Tests 26 6.1. Introduction 26 6.2. NRD Property Points Sensitivity Test 26 6.3. Consequence Sensitivity Test 27 6.4. Probability Sensitivity Test 27 7. Strategic Assessment – Conclusions & Recommendations 28 7.1. Introduction 28 7.2. Conclusions 28 8. Intermediate Assessment –Objectives, Methodology and Data Review 29 8.1. Introduction 29 8.2. Intermediate Assessment Objectives 29 8.3. Approach 29 8.4. Data Review 31 9. Intermediate Assessment – Identification of Local Flood Risk Areas 35 9.1. Introduction 35 9.2. Identification of Local Flood Risk Areas 35 10. Intermediate Assessment – High Level Appraisal and Screening of Flood Risk Areas 38 10.1. Introduction 38 10.2. Approach 38 10.3. High Level Appraisal 38 10.4. Classification of Local Flood Risk Areas 39 11. Intermediate Assessment – Conclusion & Recommendations 42 11.1. Introduction 42 11.2. Conclusions 42 11.3. Recommendations 42 Atkins Birmingham Surface Water Management Plan | Version 5.0 | 3 Appendix 2 - Surface Water Management Plan 12. Detailed Assessment - Approach 43 12.1. Introduction 43 12.2. Detailed Assessment Objectives 43 12.3. Detailed Modelling 43 13. Detailed Assessment - Modelling 45 13.1. Introduction 45 13.2. River Rea Catchment 45 13.3. Hockley Brook catchment 47 13.4. Birchfield catchment 48 13.5. Plants Brook catchment 48 13.6. Modelling Reports 48 14. Options Stage – Approach 50 14.1. Introduction 50 14.2. Options Stage Objectives 50 15. Options Stage - Identifying Measures 51 15.1. Introduction 51 15.2. Final Local Flood Risk Areas 51 15.3. Identifying Measures 52 15.4. Shortlisted Options - Modelling 53 15.5. Shortlisted Options – Site Assessment 54 16. Options Stage - Assessing Modelled Options 55 16.1. Introduction 55 16.2. Modelling Approach 55 16.3. SHLAA Option 55 16.4. Works Options 57 16.5. Cost Benefit Analysis 58 17. Options Stage – Site Assessment 59 17.1. Introduction 59 17.2. Individual Site Assessment 59 17.3. Catchment Wide Actions 67 18. Action Plan 68 18.1. Introduction 68 18.2. Draft Action Plan 68 18.3. Consultation 68 18.4. Final Action Plan 69 18.5. Next Steps 69 19. Bibliography 70 20. Glossary of Terms 71 Appendices 73 Appendix A. Tables 74 A.1. Project Data Register 74 A.2. OS Classes Defined for Categories 75 A.3. Scoring Matrices 76 A.4. Detail of Scoring Matrices 77 A.5. Consequence Sensitivity Testing 78 A.6. Probability Sensitivity Testing 79 A.7. Preliminary Local Flood Risk Areas 80 A.8. High Level Options Appraisal 81 A.9. SWMP Briefing – Consultation 82 A.10. SWMP Action Plan Version 4 83 Appendix B. Drawings and Figures 84 Atkins Birmingham Surface Water Management Plan | Version 5.0 | 4 Appendix 2 - Surface Water Management Plan B.1. Subcatchments Defined for the Strategic SWMP Assessment 84 B.2. Comparison of Address and NRD Property Points 85 B.3. Rainfall Gauge Locations 86 B.4. Priority Subcatchments 87 B.5. Severn Trent Water Schemes 88 B.6. Preliminary Local Flood Risk Areas 89 Atkins Birmingham Surface Water Management Plan | Version 5.0 | 5 Appendix 2 - Surface Water Management Plan 1. Introduction 1.1. Background On 18th August 2009 DEFRA on behalf of the Government announced that Birmingham City Council (BCC) would be allocated £300,000 to produce a Surface Water Management Plan (SWMP) (DEFRA1, 2010). The grant allocation was based on an assessment of the estimated number of properties susceptible to surface water flooding resulting from severe rainfall, with Birmingham having an estimated 22,900 properties and therefore being the highest ranked settlement outside of London. The SWMP process is a framework through which key local partners with responsibility for surface water and drainage in their area work together to understand the causes and effects of surface water flooding and agree the most cost effective way of managing surface water flood risk for the long term. The process of working together as a partnership is designed to encourage the development of innovative solutions and practices. The purpose is to make sustainable urban surface water management decisions that are evidence based, risk based, future proofed and inclusive of stakeholder views and preferences. The SWMP is the culmination of this collaborative process; a description of the level of risk posed and an agreement about who will do what to better manage these risks. A SWMP should establish a long-term action plan to manage surface water in an area and should influence future capital investment, drainage maintenance, public engagement and understanding, land-use planning, emergency planning and future developments. In this context, surface water flooding describes flooding from sewers, drains, small watercourses and ditches that occurs during heavy rainfall in urban areas. It includes: • Pluvial flooding – flooding as a result of high intensity rainfall when water is ponding or flowing over the ground surface (surface runoff) before it enters the underground drainage network or watercourse, or cannot enter it because the network is full to capacity. • Sewer flooding1 – flooding which occurs when the capacity of underground systems is exceeded, resulting in flooding inside and outside of buildings. Normal discharge of sewers and drains through outfalls may be impeded by high water levels in receiving waters. • Flooding from small open-channel and culverted urban watercourses2 which receive most of their flow from inside the urban area. • Overland flows from the urban/rural fringe entering the built-up area, including overland flows from groundwater springs. The framework for undertaking a SWMP study is illustrated through a wheel diagram (Figure 1-1), identifying the four principal phases: Preparation, Risk Assessment, Options, and Implementation and Review. The first three phases involve undertaking the SWMP study, whilst the fourth phase involves producing and implementing the action plan, based on the evidence gained from the SWMP study. It is based on a widely adopted generic approach to evidence and risk based decision making. 1 Consideration of sewer flooding in “dry weather” resulting from blockage, collapse, or pumping station mechanical failure is excluded from SWMPs as this is for the sole concern of the sewerage undertaker 2 Interactions with larger rivers and tidal waters can be important mechanisms controlling surface water flooding Atkins Birmingham Surface Water Management Plan | Version 5.0 | 6 Appendix 2 - Surface Water Management Plan Figure 1-1 – Surface Water Management Plan Wheel 1.2. This Report This report describes the work that has been undertaken to complete the SWMP for Birmingham and covers the four phases identified by the SWMP Guidance (DEFRA2, 2010) (hereafter referred to as “the guidance”): preparation, risk assessment, options and implementation and review, however the review element will continue for a number of years following the development and implementation of the action plan. Following the completion of each stage this report will be updated to summarise the work undertaken and provide the results/findings of that stage. The purpose of this approach is to ensure that the findings from all phases are reported in a single document; this report is therefore a ‘live’ document until the final stage of work has been completed. Atkins Birmingham Surface Water Management Plan | Version 5.0 | 7 Appendix 2 - Surface Water Management Plan 2. Preparation Phase 2.1. Introduction This phase of the SWMP focused on preparing and scoping the requirements of the study and is split into three main parts: • Identifying the need for a SWMP study (which was already identified by DEFRA) • Establishing a Partnership, and • Scoping the SWMP study. 2.2. Establishing a Partnership The guidance identifies that a partnership approach is the most efficient way to co-ordinate flood risk management activities given the complex nature of surface water flooding.