Strategic Flood Risk Assessment of Bath and North East Somerset VOLUME I Technical Report April 2008
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Strategic Flood Risk Assessment of Bath and North East Somerset VOLUME I Technical Report April 2008 s u c c e s s f u l p e o p l e s u c c e s s f u l p r o j e c t s s u c c e s s f u l p e r f o r m a n c e Contents Foreword .................................................................................................................... i 1. Introduction................................................................................................... 1-1 2. Strategic assessment................................................................................... 2-1 3. Environment and planning context............................................................. 3-1 4. Flood defences and assets.......................................................................... 4-1 5. Flood warning and emergency planning .................................................... 5-1 6. Flooding from rivers..................................................................................... 6-1 7. Flooding from the sea .................................................................................. 7-1 8. Flooding from land (surface water)............................................................. 8-1 9. Flooding from groundwater......................................................................... 9-1 10. Flooding from sewers ................................................................................ 10-1 11. Flooding from artificial sources ................................................................ 11-1 12. Glossary and notation................................................................................ 12-1 Annex A - Maps Overview maps Map O Overview of B&NES Map T1 Sources of topographic data Map T2 Topography Map G1 Soils Map G2 Geology Map E1 to E6 Examples of non-flooding constraints to development in B&NES Map H(A) and H(B) Recorded historic incidents of flooding Map M1 Catchment Flood Management Plan policies Map M2 Approaches to river flood modelling in B&NES Map N New potential development sites Map X Existing land use Flood defences and flood warning Map D1 Draft Environment Agency 'System Asset Management Plan' units Map D2 Flood defences in B&NES Map W Environment Agency flood warning areas Flooding from rivers (fluvial) Map F Flood zones (As per Table D1 in PPS25) Map A1 Extent of actual flooding from rivers (with flood defences) Map A2 Depth of actual flooding from rivers (with flood defences) Map A3 Velocity of actual flooding from rivers (with flood defences) Map C Extent of flooding from rivers with allowance for climate change Flooding from land Map L1 Areas prone to flooding from land Map L2a to L2f Suitability of different sustainable drainage systems Flooding from groundwater Map G3 Land more prone to groundwater flooding Flooding from sewers Map S1 Historic incidents of sewer flooding Map S2 Areas with higher incidents of sewer flooding Flooding from artificial sources Map R Location of potential artificial sources of flooding Foreword Foreword Bath and North East Somerset (B&NES) Council are required to prepare a Strategic Flood Risk Assessment (SFRA) to support the production of their Local Development Framework (LDF). The SFRA creates a strategic framework for the consideration of flood risk when making planning decisions. It has been developed in accordance with Planning Policy Statement 25: Development and Flood Risk (PPS25), as well as additional guidance provided by the Environment Agency. The guidance provided in PPS25 requires local authorities and those responsible for development decisions to demonstrate that they have applied a risk-based, sequential approach in preparing development plans and considered flooding through the application of a Sequential Test. Failure to demonstrate that such a Test has been undertaken potentially leaves planning decisions and land allocations open to challenge during the planning process. The underlying objective of the risk-based sequential allocation of land is to reduce the exposure of new development to flooding and reduce the reliance on long-term maintenance of built flood defences. Within areas at risk from flooding, it is expected that development proposals will contribute to a reduction of flood risk. A SFRA is essential in enabling a strategic and proactive approach to be applied to flood risk management. The assessment allows us to understand current flood risk on a wide-spatial scale and how this is likely to change in the future. The main objective of the B&NES SFRA is to provide flood information; • so that an evidence-based and risk-based sequential approach can be adopted when making planning decisions, in line with PPS25 • that is strategic in that it covers a wide-spatial area and looks at flood risk today and in the future • that supports sustainability appraisals of the local level documents (including the LDF) • that identifies what further investigations may be required in detailed flood risk assessments (FRAs) for specific development proposals. The SFRA is presented in a number of documents; • Non technical summary • Volume I – technical report and flood maps • Volume II – user guide • Volume III – management guide Volume III – management guide The SFRA is a live document which is intended to be updated as new information and guidance becomes available. The outcomes and conclusions of the SFRA may not be valid in the event of future changes. It is the responsibility of the user to ensure they are using the best available information when making a land planning decision. B&NES SFRA (April 2008) i Volume I - Technical Report Introduction 1. Introduction The B&NES SFRA is a 'live' document. The current version has been developed using the best information and concepts available at the time. As new information and concepts become available the document will be updated and so it is the responsibility of the reader to be satisfied that they are using the most up-to-date information and that the SFRA accounts for this information. All revisions to this summary document are listed in the table. Version Issue Date Issued by Issued to Final 23/04//2008 Capita B&NES, EA Symonds Ltd B&NES SFRA (April 2008) 1-1 Volume I - Technical Report Introduction Purpose of this report This Bath and North East Somerset (B&NES) SFRA has been developed to inform the Local Development Framework (LDF). The SFRA must be robust and be evidence-based so that it does not leave planning decisions and land allocations open to challenge through the land use planning process. It is crucial that there is transparency in the data and methods used in the assessment. This volume of the Bath & North East Somerset Strategic Flood Risk Assessment is the Technical Report (or evidence base) This report represents Volume I of the SFRA, and is the evidence base of Flood Risk in B&NES. This document contains all of the technical information and methods used in the assessment of flood risk across the study area. It includes information on the sources and reliability of data, methods used in the assessment, discussions regarding uncertainty, and key assumptions made. To ensure that the technical information is easily updated when new assessments are undertaken in the future, the six ‘sources of flooding’ have been reported in stand alone chapters. Chapter 2 provides a summary of flood risk across the B&NES District. The maps generated during the assessment are provided in Annex A to this document. The user is referred to Volumes II and III for guidance on how to interpret the information in this technical report and how to update the SFRA following improvements in data or changes in guidance. The SFRA is based on a range of data from different sources and of various degrees of certainty. It is the responsibility of the user to consider the source and certainty of the data when referring to the flood risk summaries and flood maps. Overview of B&NES District Location B&NES district covers a 350km² area south east of Bristol in South West England. Map O in Annex A shows the district boundary and main rivers. The largest urban area is the historic and world heritage city of Bath, which lies in the north east corner of the B&NES district. Bath is famous for its naturally occurring hot springs and the land surrounding the city is designated as an Area of Outstanding Natural Beauty (AONB). The towns of Keynsham and Midsomer Norton also fall within the district. The remaining area is predominantly rural, featuring medieval enclosed fields and Avon forest. Chew Valley Reservoir lies in the south west corner and is an important water resources reservoir. It is designated as a Site of Special Scientific Interest (SSSI) and a Special Protection Area (SPA). The population of B&NES has been slowly but steadily growing during recent decades and at the 2001 census stood at 169 040. About half the population lives in the city of Bath with the remaining living in Keynsham, Midsomer Norton and Radstock. There are also many villages and hamlets spread across 47 rural parishes which accommodate a substantial rural population. Map X in Annex A shows the existing land use in B&NES. The rural landscape of the District is striking and diverse. Around a quarter of the rural areas lie within designated Areas of Outstanding Natural Beauty (AONB). Thirty villages in B&NES are designated as Conservation Areas, to preserve and enhance their existing character. Strung along the foothills of the Mendips are the villages of Ubley, Compton Martin and East and West Harptree. To the north of the Mendips is the undulating valley of the River Chew within which lie Chew Valley and Blagdon Lakes and a network of small villages. The largest of these is the historic village of Chew Magna which acts as a community focus for villages and hamlets. Maps E1, E2, E3 and E5 provide an indication of environmental constraints in B&NES. B&NES SFRA (April 2008) 1-2 Volume I - Technical Report Introduction Topography The topography of B&NES is shown in Map T2 in Annex A. The Lower Avon valley in the north east corner of B&NES is the lowest lying area of land and slopes gently westward towards the Severn Estuary. The remaining area is fairly steep and generally slopes in a north easterly direction to meet the Lower Avon.