SFRA Level 2 Report
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Revision Schedule Sedgemoor District Council Level 2 Strategic Flood Risk Assessment Final Report Rev Date Details Prepared by Reviewed by Approved by 00 October 2008 Draft for Nick Bosanko Dr Rob Sweet Jon Robinson comment Assistant Flood Risk Senior Flood Risk Associate Director consultant Specialist 01 January 2009 Final Report Dr Rob Sweet Paul Curwen Jon Robinson with Stakeholder Senior Flood Risk Principal Engineer Associate Director Comments Specialist Scott Wilson Mayflower House Armada Way This document has been prepared in accordance with the scope of Scott Wilson's appointment with its client and is subject to the terms of that appointment. It is addressed Plymouth to and for the sole and confidential use and reliance of Scott Wilson's client. Scott Wilson accepts no liability for any use of this document other than by its client and only for the PL1 1LD purposes for which it was prepared and provided. 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The contents of this document do not provide legal or tax Fax +44(0)1752 676 701 advice or opinion. © Scott Wilson Ltd 2008 www.scottwilson.com 1 Sedgemoor District Council Level 2 Strategic Flood Risk Assessment Table of Contents 1 Introduction........................................................................................1 1.1 Overview.........................................................................................................................1 1.2 Study Area......................................................................................................................1 1.3 Aim of Level 2 SFRA ......................................................................................................1 1.4 Level 2 SFRA Objectives ................................................................................................1 2 Flood Risk .........................................................................................2 2.1 Overview.........................................................................................................................2 2.2 Tidal Flood Risk..............................................................................................................2 3 Study Context....................................................................................3 4 Results ..............................................................................................4 4.1 Overview.........................................................................................................................4 4.2 Burnham-on-Sea/Highbridge Flood Cell .........................................................................4 4.3 Bridgwater ......................................................................................................................6 5 Mitigation Options............................................................................10 5.1 Purpose of the Costing ................................................................................................. 10 5.2 Types of Flood Defence................................................................................................ 10 5.3 Costing Methodology .................................................................................................... 10 5.4 Potential Mitigation Options .......................................................................................... 11 6 Discussion and Conclusions............................................................13 6.1 Discussion .................................................................................................................... 13 6.2 Conclusions .................................................................................................................. 14 7 References ......................................................................................15 8 Appendices....................................................................................... A Appendix A – Flood Cells ............................................................................ A Appendix B – Modelling Methodology.......................................................... B Appendix C – Burnham-on-Sea/Highbridge Flood Cell Figures ................... C Appendix D – Bridgwater Flood Cell Figures ............................................... D 2 Sedgemoor District Council Level 2 Strategic Flood Risk Assessment 1.3.3 This report should be regarded as a “living document” and periodic reviews should be undertaken to 1 Introduction ensure that, as new data becomes available or new/updated guidance is released, the SFRA reflects this information. 1.1 Overview 1.4 Level 2 SFRA Objectives 1.1.1 Scott Wilson Ltd was commissioned to undertake the Sedgemoor District Council Strategic Flood Risk Assessment (SFRA). This Level 2 SFRA forms part of the evidence base for the Local Development 1.4.1 The aims of this Level 2 SFRA are achieved through the following objectives: Framework and meets the requirements of Planning Policy Statement 25: Development and Flood Risk • (PPS25). Undertake 2-D hydrodynamic modelling for breach and overtopping scenarios of the existing tidal flood defences; 1.1.2 Scott Wilson completed the Sedgemoor District Council Level 1 in August 2008. This has allowed Sedgemoor District Council to determine the variations in flood risk for spatial planning purposes. • Provide depth and hazard mapping for tidal modelling outputs to illustrate the distribution of Where decision-makers are unable to allocate all proposed development and infrastructure in flood risk and to enable a sequential approach to site allocation within flood zones; accordance with the Sequential Test (i.e. to steer development to areas at lowest risk of flooding), it • may be necessary to increase the scope of the Level 1 SFRA to provide information necessary for Provide detailed analysis of modelling results for the 12 PDOs, including information on rate of application of the Exception Test (i.e. Level 2 SFRA). onset of flooding, flood depth and flood hazard; 1.1.3 The Level 1 SFRA identified that Sedgemoor District is significantly constrained by tidal flood risk with • Identify locations where improvements in existing flood defences may be beneficial. approximately 50% located within Flood Zone 3. The draft Regional Spatial Strategy (RSS) has 1.4.2 The depth and hazard mapping form the main content of this report and should be used to inform future identified Bridgwater as a Strategically Significant City/Town (SSCT) and is considered suitable to development opportunities. provide sustainable housing growth. Two main areas were identified through liaison with Sedgemoor District Council and the Environment Agency where further information was required on flood risk to inform strategic planning decisions. 1.1.4 This report presents the findings of the Level 2 SFRA and details the results of an extensive tidal modelling and mapping exercise. This has investigated the residual flood risk from both tidal overtopping and breaching of existing flood defences, to provide further information for application of the Exception Test. Furthermore, the report identifies a number of strategic flood defence options available in locations identified through the modelling and provides indicative costs to implement these. This overview provides the basis for an initial flood defence costs exercise within the study area. 1.1.5 The findings of the Level 1 and Level 2 SFRA reports should be used in tandem for both strategic planning and to inform ongoing development control decisions. 1.2 Study Area 1.2.1 The study area is defined by the administrative boundary of Sedgemoor District Council and is described in detail in the Level 1 SFRA report (Scott Wilson, 2008). The Level 2 SFRA focuses on two flood cells, Burnham-on-Sea/Highbridge and Bridgwater. The extent of these flood cells are provided in Appendix A. 1.2.2 These flood cells have extensive areas of relatively level, low-lying land that is managed through networks of man-made ditches and rhynes. The Exeter to Bristol railway line and the M5 motorway dissect both flood cells. These features are elevated above the surrounding ground level, particularly the M5, with culverts allowing the flow of water beneath. 1.3 Aim of Level 2 SFRA 1.3.1 The aim of this study is to provide supplementary information to the Level 1 SFRA through the ‘increased scope’ referred to in paragraph E6 of PPS25. This will allow Sedgemoor District Council to facilitate the application of the Sequential and Exception Tests through the investigation of tidal flood risk issues and the suitability for development of potential sites with known flooding problems, as identified in the Level 1 SFRA. 1.3.2 The aim of the flood modelling is to simulate flood events to determine and illustrate the potential flood extents, depth and the areas at high, medium and low flood hazard. Detailed analysis of the modelling results for 12 potential development options (PDOs) identified in liaison with Sedgemoor District Council and review of the Issues and Options Report for the Core Strategy (June 2007). The location of each PDO is shown within Appendix A and described further in Section 3. January 2009 1 Sedgemoor District Council