Strategic Flood Risk Assessment Level 1
Total Page:16
File Type:pdf, Size:1020Kb
West Somerset Council & Exmoor National Park Authority Strategic Flood Risk Assessment Level 1 Final Report March 2009 Prepared for: West Somerset Council and Exmoor National Park Authority Level 1 Strategic Flood Risk Assessment Revision Schedule Level 1 Strategic Flood Risk Assessment March 2009 Project Rev Date Details Prepared by Reviewed by Approved by Number 01 January D122558 Draft Level 1 Mark Crussell Dr Rob Sweet Jon Robinson 2009 SFRA Assistant Hydrologist Senior Flood Risk Associate Director Dr Rob Sweet Specialist Senior Flood Risk Specialist 02 March D122558 Final Level 1 Dr Rob Sweet Dr Rob Sweet Jon Robinson 2009 SFRA – Senior Flood Risk Senior Flood Risk Associate Director Incorporating Specialist Specialist ENPA, WSC and EA comments 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 use and reliance of Scott Wilson's client. Scott Wils on accepts no liability for any use of this document other than by its client and only for the purposes, PL1 1LD stated in the document, for which it was prepared and provided. No person other than the client may copy (in whole or in part) use or rely on the conte nts of this document, without the prior written permission of the Company Secretary of Scott Wilson Ltd. Any advice, Tel 01752 676733 opinions, or recommendations within this document should be read and relied upon only in the context of the document as a whole. The conten ts of this document are not to be Fax 01752 676701 construed as providing legal, business or tax advice or opinion. © Scott Wilson Group PLC 2009 www.scottwilson.com West Somerset Council and Exmoor National Park Authority Level 1 Strategic Flood Risk Assessment Table of Contents Abbreviations .......................................................................................... 1 Glossary.................................................................................................. 2 1 Introduction .................................................................................. 3 2 Study Area ................................................................................... 7 3 PPS25 Sequential Test .............................................................. 11 4 Data Collection and Review ....................................................... 18 5 SFRA Flood Mapping ................................................................. 24 6 Assessment of Potential Flood Sources ..................................... 29 7 Flood Risk Management............................................................. 35 8 Drainage of Development Sites.................................................. 39 9 Site Specific FRA Guidance ....................................................... 52 10 Recommendations ..................................................................... 54 11 References................................................................................. 56 12 Figures ....................................................................................... 58 13 Appendices ................................................................................ 59 Appendix A: Sequential & Exception Test ...............................................A Appendix B: Policy Review and Considerations ......................................B Appendix C: Wessex Water Sewer Flooding Data ................................. C Appendix D: Technical Methodology ...................................................... D Appendix E: Broad-scale Flood Risk Review ..........................................E Appendix F: Focused Assessments ........................................................F March 2009 West Somerset Council and Exmoor National Park Authority Level 1 Strategic Flood Risk Assessment Abbreviations Acronym Definition AOD Above Ordnance Datum DG5 Director General 5 DPD Development Plan Documents EA Environment Agency ENPA Exmoor National Park Authority GIS Geographical Information Systems LiDAR Light Detection and Ranging LDF Local Development Framework NFCDD National Flood and Coastal Defence Database PPS25 Planning Policy Statement 25: Development and Flood Risk SA Sustainability Appraisal SFRA Strategic Flood Risk Assessment SFRM Strategic Flood Risk Mapping SuDS Sustainable Drainage Systems WSC West Somerset Council 1 March 2009 West Somerset Council and Exmoor National Park Authority Level 1 Strategic Flood Risk Assessment Glossary Term Definition 1 in 100 year Event that on average will occur once every 100 years. Also expressed as an event event, which has a 1% probability of occurring in any one year. 1 in 200 year Event that on average will occur once every 200 years. Also expressed as an event event, which has a 0.5% probability of occurring in any one year. Flood defence that is designed for an event, which has an annual probability of 1 in 100 year 1%. In events more severe than this the defence would be expected to fail or to design standard allow flooding. This zone comprises of land assessed as having a less than 1 in 1000 annual Flood Zone 1 probability of river of sea flooding in any year (0.1%). This zone comprises land assessed as having between a 1 in 100 year and 1 in 1000 year annual probability of river flooding (1% - 0.1%) or between a 1 in 200 Flood Zone 2 year and a 1 in 1000 year annual probability of sea flooding (0.5% - 0.1%) in any year. This zone comprises land assessed as having a 1 in 100 or greater annual Flood Zone 3a probability of river flooding (>1%) or a 1 in 200 or greater annual probability of flooding from the sea (>0.5%) in any year. This zone comprises land where water has to flow or be stored in times of flood. Flood Zone 3b – SFRAs should identify this Flood Zone (land which would flood with an annual Functional probability of 1 in 20 (5%) or greater in any year or is designed to flood in an Floodplain extreme (0.1%) flood, or at another probability to be agreed between the LPA and the Environment Agency, including water conveyance routes). Infrastructure used to protect an area against floods as floodwalls and Flood defence embankments; they are designed to a specific standard of protection (design standard). Floodplain Area adjacent to river, coast or estuary that is naturally susceptible to flooding. Flood storage A temporary area that stores excess runoff or river flow often ponds or reservoirs. Fluvial flooding Flooding by a river or a watercourse. Broad-scale raster based routing model used to determine flood extents by the JFLOW Environment Agency. The LDF is a suite of documents that will guide spatial planning policies and will Local replace Local Plans. Documents include a Core Strategy which sets out strategic Development policies; Development Control policie s, to guide development and Proposals Maps. Framework (LDF) Other documents may also be included such as Area Plans, Supplementary Planning Documents and Site Allocations. Mitigation An element of development design which may be used to manage flood risk or measure avoid an increase in flood risk elsewhere. Flooding caused when intense rainfall exceeds the capacity of the drainage Overland Flow systems or when, during prolonged periods of wet weather, the soil is so saturated such that it cannot accept any more water. Risk The probability or likelihood of an event occurring. Methods of management practices and control structures that are designed to Sustainable drain surface water in a more sustainable manner than some conventional drainage system techniques. Sustainable Development that meets the needs of the present without compromising the ability development of future generations meeting their own needs. 2 March 2009 West Somerset Council and Exmoor National Park Authority Level 1 Strategic Flood Risk Assessment 1 Introduction Background 1.1 The Planning and Compulsory Purchase Act 2004 (PCPA) (HMSO, 2004) requires Local Planning Authorities (LPAs) to produce Local Development Frameworks (LDFs) that will replace the system of Local, Structure and Unitary Development Plans. LDFs are a portfolio of documents (Local Development Documents (LDDs)) that collectively deliver the spatial planning strategy for the authority area. The PCPA 2004 requires LDDs to undergo a Sustainability Appraisal (SA), which assists LPAs in ensuring that their policies fulfil the principles of sustainability. Strategic Flood Risk Assessments (SFRAs) constitute a component of the SA process and should be used in the review of LDDs or in their production. 1.2 The release of Planning Policy Guidance Note 25: Development and Flood Risk in July 2001 (PPG25) (DTLR, 2001) introduced the responsibility that LPAs have to ensure flood risk is understood and managed effectively using a risk-based approach as an integral part of the planning process. 1.3 PPG25 was superseded by Planning Policy Statement 25: Development and Flood Risk (PPS25) (CLG, 2006) in December 2006. PPS25 re-emphasises the active role LPAs should have in ensuring that flood risk is considered in strategic land use planning, PPS25 states that this should be achieved by the production of a SFRA. In June 2008, a Practice Guide to accompany PPS25 was released following a consultation period. The Practice Guide provides supplementary information, including a suggested approach for the production of SFRAs. Definition of a SFRA 1.4 A SFRA is a tool which provides an overview of the flood risk, from a variety of sources,