Flood Risk Management Review 2014
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Protecting Bridgwater and the Somerset Levels & Moors from Tidal Flooding Flood Risk Management Review November 2014 Environment Agency / Sedgemoor District Council Flood Risk Management Review Protecting Bridgwater and the Somerset Levels & Moors from Tidal Flooding Flood Risk Management Review CONTENTS 1. EXECUTIVE SUMMARY 3 2. AIM OF THE REVIEW 7 3. THE FLOODING CHALLENGE 7 3.1 Tidal and Fluvial Flood Risk ........................................................................................ 7 3.2 Flood Risk Today and in the Future ............................................................................. 9 3.3 Supporting Development and Growth in Somerset ....................................................12 4. FLOOD RISK MANAGEMENT OPTIONS 13 4.1 Options Assessed .........................................................................................................13 4.2 Other Options and Descriptions ..................................................................................16 5. CRITERIA TO BE CONSIDERED IN OPTION ASSESSMENT 17 6. OPTION ASSESSMENT 20 6.1 Flood Risk ...................................................................................................................20 6.2 Economics ...................................................................................................................21 6.3 Environment ................................................................................................................23 6.4 Design and Delivery ....................................................................................................25 7. CONCLUSION OF ASSESSMENT 27 8. THE WAY AHEAD 29 8.1 The Delivery Stages and Programme .........................................................................29 8.2 Cost ..............................................................................................................................30 8.3 Benefits and Funding ..................................................................................................30 8.4 Legislative and Planning Issues .................................................................................31 8.5 Structure Design and Operation .................................................................................31 APPENDICES 33 Appendix A: Frequently Asked Questions 33 Appendix B: Key Features Plans 35 Black & Veatch Limited 1 Environment Agency / Sedgemoor District Council Flood Risk Management Review Details of document preparation and issue: Version Prepared Checked Reviewed Approved Issue date Issue status no. 1 P Morley J Mason F Spaliviero F Spaliviero 27.11.14 Final B&V project no. 122320/1830 Client’s reference no. IMSW002039 Notice: This report was prepared by Black & Veatch Limited (BVL) solely for use by the Environment Agency and Sedgemoor District Council. This report is not addressed to and may not be relied upon by any person or entity other than the Environment Agency and Sedgemoor District Council for any purpose without the prior written permission of BVL. BVL, its directors, employees and affiliated companies accept no responsibility or liability for reliance upon or use of this report (whether or not permitted) other than by the Environment Agency and Sedgemoor District Council for the purposes for which it was originally commissioned and prepared. In producing this report, BVL has relied upon information provided by others. The completeness or accuracy of this information is not guaranteed by BVL. Black & Veatch Limited 2 Environment Agency / Sedgemoor District Council Flood Risk Management Review 1. EXECUTIVE SUMMARY Background The extensive flooding of the Somerset Levels and Moors in 2012 and between December 2013 and March 2014 has brought into sharp focus the flood risk to Bridgwater and the Somerset Levels and Moors. Communities, businesses, agriculture and the environment rely upon effective flood risk management. The sustainable development of Bridgwater in particular relies upon flood risk infrastructure. Reports have previously been developed relating to the options and opportunities for continuing to manage flood risk from the River Parrett to Bridgwater and the surrounding areas: 1 The Sedgemoor District Council Bridgwater Flood Defence Infrastructure Planning report 2009, the findings of which were incorporated into Bridgwater Vision 2009 – a 50 year strategic vision and Framework to bring about transformational change and regeneration of the town; 2 The Environment Agency 2006 Parrett Tidal Flood Defence Technical Review and the Parrett Estuary Flood Risk Management Strategy (both considered the range of options for how to manage the flood risk to Bridgwater). The built up banks of the River Parrett provide defences against tidal flooding coming up the river. These protect approximately 10,000 homes and over 600 businesses in Bridgwater and surrounding villages, plus the low lying land up and downstream of Bridgwater. The 2009 report concluded that the best approach from a flood risk management perspective was to maintain the existing defences, before supplementing them at some point between 2030 and 2050 with a tidal surge barrier constructed somewhere between Chilton Trinity and Dunball. The flooding in early 2014 led a partnership of key authorities to prepare ‘The Somerset Levels and Moors Flood Action Plan’, which was presented to the Secretary of State in March 2014. This identified the critical role a ‘tidal barrier or sluice’ could play in reducing flood risk and was the focus of an initial workshop in June 2014 held by Sedgemoor District Council and the Environment Agency. The Flood Action Plan has a clear objective of building a barrier or sluice at Bridgwater by 2024. Following the June 2014 workshop, several related studies were commissioned: - A review of strategic options for managing flood risk from the tidal River Parrett (this report); - An update to the hydraulic model of the river, incorporating updated cross-sections; - A peer review of the geomorphological impacts of a barrier. Review of options for managing flood risk This review was commissioned to re-examine the previous studies, to determine whether the recent flooding, updated data or other evidence might influence the preferred option or timescale for such an intervention. It is an initial review of existing reports and information; it is not a full business case for investment, which will be produced during the next phase of the project. Black & Veatch Limited 3 Environment Agency / Sedgemoor District Council Flood Risk Management Review It used the latest tide level predictions, flood event data from 2012 and 2013/14, the new Asset Survey carried out during this winter’s floods, the current dredging operation and learning from the 2011 major collapse of West Quay in Bridgwater. The findings that are presented will be confirmed or revised when the updated river model and geomorphology analysis become available early in 2015, and during the future appraisal process. The emerging information is not expected to change the preferred option, but could affect the timing of an intervention. We considered options including raising flood walls and embankments, tidal surge barrier, tidal exclusion sluice, and the Bridgwater Bay Lagoon (a developer-led idea for extracting power from the extreme tidal range in the Severn Estuary). The review assessed these options against four main criteria: flood risk, economics, environment and delivery. Option assessment A tidal surge barrier is designed to close when required to exclude extreme tides. It is comparable to the Thames Barrier in London, which is normally open but closes when necessary. In contrast a tidal exclusion sluice permanently stops tides flowing upstream of the structure. A Bridgwater Bay Lagoon would enclose part of Bridgwater Bay and the Parrett estuary within a sea wall. Potentially Bridgwater Bay Lagoon could be operated to minimise flood risk to the Parrett estuary, although this would be a secondary function, the main function being renewable electricity generation. This would mean there is no need to have a Parrett barrier as well. The 2009 Sedgemoor District Council report identified a preference for a tidal surge barrier. This current review confirms the need for a major intervention and continues to support the option of a tidal surge barrier. This is based on flood risk management requirements only but does align with the previous planning and regeneration preferences that took account of the development opportunities for Bridgwater. The tidal surge barrier is the most appropriate and deliverable structure to build, with less adverse impacts, resulting in lower costs and greater confidence in a successful outcome. The Environment Agency continues to consider this as the most viable option, which concurs with the Council’s approach set out in the Sedgemoor District Council Core Strategy (adopted 2011). The review has identified significant environmental impacts if a tidal exclusion sluice was adopted and there are technical difficulties associated with bank raising. The Bridgwater Bay Lagoon option (where flood risk management may be a secondary benefit) should be kept under review, in case options for building it come forward in the future. Assessment of Current Flood Risk The review has identified that flood defences in Bridgwater are currently offering protection from a tidal flood with around a 1 in 100 (1%) chance of happening in any one year. This standard of protection will continue to decline over time with the combined