Chelmsford Borough Council Strategic Flood Risk Assessment Appendix B Chelmsford Supplementary Report Report April 2008 Prepared for: Chelmsford Borough Council D115326 Strategic Flood Risk Assessment Revision Schedule Mid Essex Strategic Flood Risk Assessment April 2008 Rev Date Details Prepared by Reviewed by Approved by 01 June 07 Draft Heather Rich Peter Mansell Damon O’Brien Flood Risk Specialist Principal Engineer Technical Director 02 July 07 Final Draft Heather Rich Liz Williams Damon O’Brien Flood Risk Specialist Senior Consultant Technical Director 03 October 07 Final Eleanor Cole Liz Williams Jon Robinson Graduate Hydrologist Senior Consultant Associate Director 04 April 08 Final Eleanor Cole Liz Williams Jon Robinson Graduate Hydrologist Senior Consultant Associate Director Scott Wilson 6-8 Greencoat Place, London, SW1P 1PL Tel: 020 7798 5200 Fax: 020 7798 5001 www.scottwilson.com Chelmsford Borough Council D115326 Strategic Flood Risk Assessment PREFACE Purpose: The purpose of this report is to provide Strategic Flood Risk Assessment (SFRA) information specific to the Chelmsford Borough Council area. It outlines the main flood risks posed to the council area and how the long-term management fluvial flood sources could be addressed by the incorporation of Flood Storage Areas. The flood risks to potential sites have been identified on both greenfield and brownfield sites. Flood Hazard Mapping for South Woodham Ferrers along the River Crouch has been undertaken based on breach simulation modelling. Flood hazard and depth mapping has been provided for the climate change scenarios for the River Chelmer, using recent model information completed by Halcrow Ltd as part of this study. This information should be used in conjunction with guidance in the SFRA to apply the Sequential Test (Planning Policy Statement (PPS) 25: Development and Flood Risk, Communities and Local Government). It is presumed throughout this report that the reader is familiar with the contents of the SFRA main report (Mid Essex Strategic Flood Risk Assessment, Scott Wilson) and the methodologies presented therein. Objective The SFRA objective is to aid the partner Boroughs in their development process through the application of the Sequential Test as required by PPS25. It assesses the flood risks posed to the region and outlines the main hazard zones in order to further aid the development planning process. The objective of this report is to assess the flood risks posed to the Chelmsford Borough Council area by breaches in the tidal defences and fluvial flood sources and to assist the Council in applying the Sequential Test. Limitations The SFRA approach was outlined in the Inception Report & Stage 2- Brief & Specification (Scott Wilson, November 2006). All methodologies, including breach parameters, have been agreed with the Environment Agency in accordance with the current best practice at the time of completion. This report details the flood issues posed to the local authority of Chelmsford Borough Council, it does not include information for neighbouring council areas such as Uttlesford, which forms part of the River Chelmer catchment area. Chelmsford Borough Council D115326 Strategic Flood Risk Assessment Table of Contents 1 Non-Technical Summary.................................................................... 1 1.1 SFRA Background.......................................................................................................... 1 1.2 SFRA Planning Objectives ............................................................................................ 1 1.3 SFRA Report Layout ...................................................................................................... 2 1.4 Chelmsford Borough Council Considerations ............................................................ 2 1.5 Way Forward................................................................................................................... 5 1.6 A Living Document......................................................................................................... 5 2 Introduction and Background.............................................................. 6 2.2 Scope and Objectives .................................................................................................... 6 2.3 Report Structure............................................................................................................. 7 3 Level 1 Assessment ........................................................................... 8 3.1 Level 1 SFRA – Study Area, Flood Source Review and Data Review........................ 8 3.2 Chelmsford Borough Council Area .............................................................................. 9 3.3 Sources of Flooding..................................................................................................... 12 3.4 Climate Change ............................................................................................................ 13 3.5 Historic Flooding.......................................................................................................... 15 4 Level 2 SFRA ................................................................................... 17 4.2 What is the Exception Test? ....................................................................................... 18 4.3 What is Required to Pass the Exception Test? ......................................................... 18 5 Fluvial Flooding Sources .................................................................. 19 5.1 Sources .........................................................................................................................19 5.2 Fluvial Level 2 Study.................................................................................................... 24 6 Arterial Drainage Network Flooding Sources ................................... 26 6.1 Sources .........................................................................................................................26 6.2 Arterial Drainage Network Level 2 Study ................................................................... 27 7 Tidal Flooding Sources..................................................................... 29 7.1 Sources .........................................................................................................................29 7.2 Tidal Level 2 Study....................................................................................................... 31 8 Other Sources of Flooding................................................................ 36 8.1 Overland Flow............................................................................................................... 36 8.2 Surface Water ............................................................................................................... 37 8.3 Strategic Spatial Flood Source Summary.................................................................. 38 Chelmsford Borough Council D115326 Strategic Flood Risk Assessment 9 North Chelmsford Area Action Plan (NCAAP).................................. 41 10 Planning and Development Advice to Chelmsford Borough............. 42 10.2 Flood Risk ..................................................................................................................... 43 10.3 Sustainable Drainage Systems ................................................................................... 43 10.4 Flood Mitigation............................................................................................................ 49 10.5 Water Environment ...................................................................................................... 50 11 References ....................................................................................... 52 Annex A – Flood Alleviation Scheme Appraisal .......................................... 53 Background ................................................................................................................................ 53 Chelmsford Flood Alleviation Viability Study (2007) .............................................................. 53 Flood Storage Areas & Breach Scenarios ............................................................................... 54 Interim development period for Chelmsford ........................................................................... 56 FSA Appraisal............................................................................................................................. 57 Annex B – Halcrow Methodology for River Chelmer Modelling Updates ............................................................................................ 63 Introduction ................................................................................................................................ 63 Overview of Hydraulic Model .................................................................................................... 63 Methods and Assumptions ....................................................................................................... 64 Outputs ....................................................................................................................................... 65 Chelmsford Borough Council D115326 Strategic Flood Risk Assessment 1 Non-Technical Summary 1.1 SFRA Background 1.1.1 Scott Wilson Ltd was commissioned by the Mid Essex Area Liaison group (MEAL) to undertake a ‘Stage 2’ Strategic Flood Risk Assessment (SFRA) of Mid Essex.