Strategic Flood Risk Assessment of West Sussex VOLUME II TECHNICAL REPORT January 2010 successful people successful projects successful performance Issue box The West Sussex Strategic Flood Risk Assessment (SFRA) is a 'live' document. The current version is 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 document are listed in the table below. Version Issue Date Issued by Issued to Amendments from previous version Draftv1 15/04/2007 Capita West Sussex County Council N/A Symonds Ltd Draft 2 07/06/2007 Capita Discussed with West Sussex West Sussex County Council Symonds Ltd County Council at meeting of general comments same date Final 1 25/01/2008 Capita West Sussex County Council Restructure as per WSCC and Symonds Ltd Environment Agency advice Update 1 12/05/2009 Capita West Sussex County Council Updated with new/ additional Draft v1 Symonds Ltd and Environment Agency flood data Final 26/01/2010 Capita West Sussex County Council West Sussex County Council Update 1 Symonds Ltd and Environment Agency general comments Capita Symonds House Wood Street East Grinstead West Sussex RH19 1UU Tel: 01342 327 161 Fax: 01342 315 927 Project manager: Kristy Chandler Project director: Scott Ferguson www.capitasymonds.co.uk Document management system Job Number: CS/36568 PD: SF PM: KLC Report status: Final Update 1 Date of issue: 26/01/2010 Main author(s): ER/KLC Checked: SF Approved: SF Contents Foreword ................................................................................................................... a 1. Introduction................................................................................................... 1-1 2. Flood defences and assets.......................................................................... 2-1 3. Flood warning and emergency plans.......................................................... 3-1 4. Flooding from rivers..................................................................................... 4-1 5. Flooding from the sea .................................................................................. 5-1 6. Flooding from land ....................................................................................... 6-1 7. Flooding from groundwater......................................................................... 7-1 8. Flooding from sewers .................................................................................. 8-1 9. Flooding from artificial sources .................................................................. 9-1 Glossary and notation............................................................................................... i Contents Foreword ................................................................................................................... a 1. Introduction................................................................................................... 1-1 2. Flood defences and assets.......................................................................... 2-1 3. Flood warning and emergency plans.......................................................... 3-1 4. Flooding from rivers..................................................................................... 4-1 5. Flooding from the sea .................................................................................. 5-1 6. Flooding from land ....................................................................................... 6-1 7. Flooding from groundwater......................................................................... 7-1 8. Flooding from sewers .................................................................................. 8-1 9. Flooding from artificial sources .................................................................. 9-1 Glossary and notation............................................................................................... i Annex A - Maps Background maps Overview Map O - Overview of West Sussex County, including existing sites and sites of search Management Map M1 - Areas covered by Catchment Flood Management Plans (CFMPs) and Shoreline Management Plans (SMP) Map M2 - Existing flood management policies across West Sussex Map M3 - Location of existing hydraulic model extents across West Sussex Flood defences and flood warning Map D - System Asset Management Plan areas and Flood Defences Map W - Environment Agency flood warning areas Sequential Test Flooding from rivers (fluvial) Map F1-F - Fluvial Flood Zones (no defences) Map C2-F - Fluvial Flood Zones with climate change (no defences) Flooding from the sea (tidal) Map F1-T - Sea flood zones (flooding without flood defences) Map C2-T - Extent of flooding from rivers with allowance for climate change Flooding from land Map L - Areas prone to flooding from land Flooding from groundwater Map G - Land more prone to groundwater flooding Flooding from sewers Map S - Historic incidents of sewer flooding Flooding from artificial sources Map R - Location of potential artificial sources of flooding Exception Test Flooding from rivers (fluvial) Map A1-F - Actual and residual risk of flooding from rivers (extent) Map A2-F - Depth of actual flooding from rivers Map C1-F - Extent of flooding from rivers with allowance for climate change Flooding from the sea (tidal) Map A1-T - Extent of actual flooding from the sea Map A2-T - Depth of actual flooding from the sea Map C1-T - Extent of flooding from rivers with allowance for climate change Map B - Locations where breach assessments may be required Foreword Foreword West Sussex County Council (WSCC) is required to prepare a Strategic Flood Risk Assessment (SFRA) to support the development of their Minerals and Waste Development Framework (MWDF). In doing so, they will be able to demonstrate that they have applied a risk-based sequential approach in preparing the framework, as required by Planning Policy Statement 25: Development and Flood Risk (PPS25). The main objective of the 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 development frameworks; and • that identifies what further investigations may be required in flood risk assessments for specific development proposals. A Level 1 SFRA has been undertaken over the whole of the county. Additional information, that would normally be included in a Level 2 SFRA, has been supplied at key locations. The approach for undertaking the SFRA is consistent with relevant planning guidance and has been discussed with the Environment Agency and WSCC throughout development. The SFRA is primarily based on existing information, however some new modelling and analyses have been undertaken where existing information was not available. These additional analyses were presented to the Environment Agency for discussion and agreement during the scoping stage. As it is not practical to undertake a highly detailed assessment across the whole of the county, a balance has been made between accuracy of results and cost effectiveness. The SFRA is presented in a number of documents: • VOLUME I – decision support document • VOLUME II – technical report and maps • VOLUME III – management guide • VOLUME IV – sites of search (Part A - Minerals sites, Part B - Waste sites) The SFRA is a live document which is intended to be updated as new information and guidance become 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. West Sussex SFRA (January 2010) a Volume II - Technical Report Introduction 1. Introduction West Sussex SFRA (January 2010) 1-1 Volume II - Technical Report Introduction Purpose of this report 1.1 Strategic Flood Risk Assessments (SFRAs) provide flood risk information which can inform a range of activities, such as land use planning, emergency planning, development control and the development of specific flood risk management policy. 1.2 This West Sussex SFRA has been developed to inform the county wide Minerals and Waste Development Framework (MWDF). 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. For this reason, it is crucial that there is transparency in the data and methods used in the assessment. 1.3 This report represents Volume II of the SFRA, containing all of the technical information and methods used in the assessment of flood risk across West Sussex. It includes information on the sources and reliability of data, methods used in the assessment, discussions regarding uncertainty, and key assumptions made. 1.4 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. Volume I contains a summary of flood risk across West Sussex. The maps generated during the assessment are provided in Annex A. 1.5 The user is referred to Volumes I and III for guidance on how to interpret