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  • South Somerset District Council Strategic Flood Risk Assessment Level 1 SFRA - Final Report August 2008 Volume I Halcrow Group Limited

  • Halcrow Group Limited Burderop Park, Swindon, Wiltshire, SN4 0QD Tel +44 (0)1793 812479 Fax +44 (0)1793 812089 www.halcrow.com Halcrow Group Limited has prepared this report in accordance with the brief from South Somerset District Council, for their sole and specific use. Any other persons who use any information contained herein do so at their own risk. © Halcrow Group Limited 2008

    South Somerset District Council Strategic Flood Risk Assessment Level 1 SFRA - Final Report August 2008 Volume I Halcrow Group Limited

  • Halcrow Group Limited Burderop Park, Swindon, Wiltshire, SN4 0QD Tel +44 (0)1793 812479 Fax +44 (0)1793 812089 www.halcrow.com

  • Halcrow Group Limited Burderop Park, Swindon, Wiltshire, SN4 0QD Tel +44 (0)1793 812479 Fax +44 (0)1793 812089 www.halcrow.com

    South Somerset District Council Strategic Flood Risk Assessment Level 1 SFRA - Final Report

    Contents Amendment Record This report has been issued and amended as follows: Issue Revision Description Date Signed 1 0 Final report - draft 29 February 2008 2 3

    1 2

    Final report– draft Final report - draft

    22 May 2008 11 July 2008

    4 3 Final report 27 August 2008

    D Kensett A Corner A McConkey M Barker P Crozier D Wilson PS Rayner

  • South Somerset District Council Strategic Flood Risk Assessment

    Table of Contents

    Executive Summary (separate document) Abbreviations

    1 Introduction.................................................................................................... 1 2 The South Somerset SFRA Area .................................................................. 2 2.1 Overview....................................................................................................................... 2 2.2 South Somerset .............................................................................................................. 2 2.3 South Somerset Rivers..................................................................................................... 3 2.4 Flood Risks.................................................................................................................... 4 2.5 Geology ........................................................................................................................ 4

    3 SFRA Approach & Methodology .................................................................. 6 3.1 Overview....................................................................................................................... 6 3.2 Purpose of this SFRA...................................................................................................... 6 3.3 Outcomes of the SFRA Process........................................................................................ 6 3.4 The Sequential Test......................................................................................................... 8 3.5 The Exception Test ........................................................................................................ 8 3.6 Planning response matrix ............................................................................................... 10 3.7 Level 1 SFRA methodology ........................................................................................... 10 3.8 The need for Level 2 SFRA............................................................................................ 11 3.9 SFRA User Guide......................................................................................................... 12

    4 Planning policy framework......................................................................... 14 4.1 Overview..................................................................................................................... 14 4.2 Planning Policy Framework............................................................................................ 14 4.3 National Planning Policy................................................................................................ 14 4.4 Regional Planning Policy................................................................................................ 16 4.5 Local planning policy .................................................................................................... 18 4.6 Summary ..................................................................................................................... 18

    5 Data sources ................................................................................................ 19 5.1 Overview..................................................................................................................... 19 5.2 Consultation process ..................................................................................................... 19 5.3 Environment Agency Flood Zone maps .......................................................................... 20 5.4 Detailed hydraulic modelling .......................................................................................... 20 5.5 Localised flooding......................................................................................................... 20

    6 Assessment of potential causes of flooding............................................. 21 6.1 Overview..................................................................................................................... 21 6.2 Sources of flooding ....................................................................................................... 21 6.3 Probability of flooding................................................................................................... 21 6.4 SFRA flood maps ......................................................................................................... 22 6.5 River flooding – fluvial .................................................................................................. 23 6.6 Groundwater flooding ................................................................................................... 27 6.7 Surface water flooding - land drainage ............................................................................. 32 6.8 Sewer flooding ............................................................................................................. 33 6.9 Reservoirs.................................................................................................................... 36 6.10 Other artificial water retaining structures.......................................................................... 38 6.11 Critical services/infrastructure within Flood Zones ........................................................... 38

  • South Somerset District Council Strategic Flood Risk Assessment

    7 Assessment of flood risk management practices .................................... 39 7.1 Overview..................................................................................................................... 39 7.2 Flood risk management responsibilities............................................................................ 39 7.3 Existing flood defences ................................................................................................. 39 7.4 Flood warning areas ...................................................................................................... 43

    8 Assessment of the capacity for the use of SUDS..................................... 45 8.1 Overview..................................................................................................................... 45 8.2 Types of SUDS systems................................................................................................. 46 8.3 SUDS at the planning stage ............................................................................................ 46 8.4 Application of SUDS within South Somerset .................................................................... 46 8.5 Effective application of SUDS techniques ........................................................................ 48 8.6 Constraints on discharges to ground................................................................................ 48 8.7 The role of groundwater & aquifer bodies in sustainable drainage........................................ 48

    9 Flood risk & climate change....................................................................... 50 9.1 Overview..................................................................................................................... 50 9.2 Flood risk & climate change ........................................................................................... 50 9.3 Integrated urban drainage .............................................................................................. 51 9.4 Potential increase in flood risk caused by future development ............................................. 51

    10 Strategic Land-Use Planning...................................................................... 53 10.1 Overview..................................................................................................................... 53 10.2 Flood risk management policy ........................................................................................ 53 10.3 Strategic flood risk management studies ........................................................................... 53 10.4 Gr