Welwyn Hatfield Borough Council Strategic Flood Risk Assessment Final Level 1 Final Report May 2009 Prepared for: Welwyn Hatfield Borough Council Level 1 Strategic Flood Risk Assessment Revision Schedule Welwyn Hatfield Strategic Flood Risk Assessment Rev Date Details Prepared by Reviewed by Approved by 01 16/11/07 WHBC Draft Anna Samuel Stephen Riley Jon Robinson Level 1 Hydrology Specialist Senior Consultant Associate Director SFRA_rev01 Gemma Costin Assistant Flood Risk Specialist Gemma Hoad Assistant Water Scientist 02 03/0708 WHBC FINAL Anna Samuel Stephen Riley Jon Robinson Level 1 Hydrology Specialist Senior Consultant Associate Director SFRA_rev 2 issued 01-08-08 03 08/01/09 WHBC FINAL Anna Samuel Jon Robinson Jon Robinson Level 1 Hydrology Specialist Associate Director Associate Director SFRA_rev 3 issued 08-01- Clare Postlethwaite 09.doc Consultant Water Scientist Gemma Hoad Assistant Water Scientist 04 19/05/09 WHBC FINAL Helen Judd Jon Robinson Jon Robinson SFRA_rev 4 Assistant Hydrologist Associate Director Associate Director Scott Wilson Scott House Alençon Link Basingstoke This document has been prepared in accordance with the scope of Scott Wilson's RG21 7PP appointment with its client and is subject to the terms of that appointment. 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The contents of this document do not provide legal www.scottwilson.com or tax advice or opinion. © Scott Wilson Ltd 2009 Welwyn Hatfield Borough Council Level 1 Strategic Flood Risk Assessment Table of Contents Abbreviations ................................................................................................ i Glossary....................................................................................................... ii Executive Summary .................................................................................... iv Introduction ................................................................................................................................. iv The Welwyn Hatfield Borough Council Study Area...................................................................... iv Level 1 SFRA Methodology......................................................................................................... iv Flooding in the Borough ............................................................................................................... v PPS25 Sequential Test and Exception Test ................................................................................. v Flood Risk Management.............................................................................................................. vi Drainage of Development Sites................................................................................................... vi Policy Recommendations ............................................................................................................ vi Site-Specific FRA Guidance ........................................................................................................ vi Emergency Planning ................................................................................................................... vi Recommendations ......................................................................................................................vii 1 Introduction ....................................................................................... 1 1.1 The Welwyn and Hatfield Borough Council SFRA........................................................... 1 1.2 SFRA Structure .............................................................................................................. 2 1.3 Aim of the SFRA............................................................................................................. 3 1.4 Level 1 SFRA Objectives................................................................................................ 3 1.5 Level 1 SFRA Report Synopsis.......................................................................................4 2 The Welwyn Hatfield Borough Council SFRA Study Area .................5 2.1 Study Area and Land Use............................................................................................... 5 2.2 Local Watercourses........................................................................................................ 5 2.3 Geology.......................................................................................................................... 8 2.4 Hydrogeology ................................................................................................................. 8 2.5 Flood Sources in the Study Area ....................................................................................8 3 Policy Context ................................................................................. 11 3.1 European Policies (EU) ................................................................................................ 11 3.2 National Policies ........................................................................................................... 12 3.3 Regional Policies .......................................................................................................... 14 3.4 Local Policies................................................................................................................ 14 3.5 Environment Agency Policies ....................................................................................... 18 3.6 Other Relevant Policies ................................................................................................ 22 Welwyn Hatfield Borough Council Level 1 Strategic Flood Risk Assessment 4 Data Collection & Review................................................................ 24 4.1 Overview ...................................................................................................................... 24 4.2 Fluvial Flood Data......................................................................................................... 25 4.3 Historic Data/Records................................................................................................... 28 4.4 Flood Defences ............................................................................................................ 29 4.5 Topographic Data ......................................................................................................... 29 4.6 Pluvial Flooding ............................................................................................................ 29 4.7 Groundwater................................................................................................................. 30 4.8 Sewer Flooding............................................................................................................. 31 4.9 Artificial Sources / Infrastructure Failure ....................................................................... 32 4.10 Flood Risk Assessments............................................................................................... 32 5 Strategic Flood Risk Mapping.......................................................... 34 5.1 Requirements of PPS25 ............................................................................................... 34 5.2 GIS Mapping................................................................................................................. 34 5.3 Fluvial Flooding ............................................................................................................ 35 5.4 Sewer Flooding/ Surface Water Flooding...................................................................... 37 5.5 Pluvial Flooding (Overland Flow) .................................................................................. 37 5.6 Groundwater Flooding .................................................................................................. 38 5.7 Artificial Sources........................................................................................................... 39 5.8 Historical Flooding ........................................................................................................ 39 5.9 Residual Risk................................................................................................................ 41 6 Guidance on Applying the PPS25 Sequential Test.......................... 44 6.1 What is the Sequential Test? ........................................................................................ 44 6.2 How should the SFRA be used to apply the Sequential Test? ...................................... 46 6.3 Additional Guidance...................................................................................................... 48 7 Guidance on Applying the PPS25 Exception Test ........................... 50 7.1 What is the Exception Test? ......................................................................................... 50 7.2 Why is there an