Updated SFRA 2012 Watford Borough Council Strategic Flood Risk Assessment Volume I Mar 2012 Update based on the SFRA for Watford Borough Council et al 1 [Dec 2007, Halcrow Group Limited 2] And on additional information on fluvial flooding from the Environment Agency based on the Strategic Modelling Study for the Upper Colne Area from 2011 [Halcrow Group Limited] Dacorum Borough Council St Albans City & District Council Three Rivers District Council Watford Borough Council Strategic Flood Risk Assessment Volume I 1 Original title: Dacorum Borough Council, St Albans City & District Council ,Three Rivers District Council, Watford Borough Council: Strategic Flood Risk Assessment, Volume I 2 2007 Report: from Halcrow Group Limited Windsor House Queensgate Britannia Road Waltham Cross Hertfordshire EN8 7NX Tel +44 (0)1992 707100 Fax +44 (0)1992 760594 www.halcrow.com Halcrow Group Limited has prepared this report in accordance with the instructions of their client, Dacorum Borough Council St Albans City & District Council Three Rivers District Council Watford Borough Council, for their sole and specific use. Any other persons who use any information contained herein do so at their own risk. Contents Amendment Record This report has been issued and amended as follows: Issue Revision Description Date Signed 1 0 Draft Final Report June 07 PLW, Halcrow 1 1 Draft Final Report August 07 PLW, Halcrow 2 0 Final Report August 07 PLW, Halcrow 1 Updated Report Feb 2012 PK, Watford Borough Council Contents The Updated SFRA 2012 Error! Bookmark not defined. Foreword 1 Executive Summary 4 1 Introduction 7 1.1 Overview 7 1.2 Study Area 7 2 SFRA Approach & Methodology 12 2.1 SFRA Aims 12 2.2 Outcomes of the SFRA Process 12 2.3 The Sequential Test 13 2.4 The Exception Test 15 2.5 Level 1 SFRA Methodology 16 2.6 Need for Level 2 SFRA 17 3 Policy Framework 19 3.1 Overview 19 3.2 Planning Policy Framework 19 3.3 National Policy 19 3.4 Regional Policy 21 3.5 Local Policy 23 3.6 Possible Funding Mechanisms 28 3.7 Summary 29 4 Data Collection and Review 30 4.1 Overview 30 4.2 Consultation Process 30 4.3 Environment Agency Flood Zone Maps 32 4.4 Detailed Hydraulic Modelling 33 4.5 Historical Flooding 34 4.6 Flood Defences 53 4.7 Topographical Data 56 4.8 Site-Specific Flood Risk Assessments - Developers 57 4.9 Flood Risk Management Strategies - Environment Agency 57 5 Strategic Flood Risk Mapping 63 5.1 Overview 63 5.2 Surface Water (Land Drainage) and Groundwater Flooding 66 5.3 Sewer Flooding 66 5.4 Climate Change 67 5.5 Residual Risk 69 5.6 Review of Indicative Sites 70 6 Flood Risk Management Policy Recommendations 71 6.1 Overview 71 6.2 Future Flood Alleviation Schemes 71 6.3 Policy Recommendations 72 6.4 Guidance to Developers 79 6.5 Sustainable Urban Drainage Systems 84 7 Flood Warning and Emergency Planning 88 7.1 Existing Flood Warning System 88 7.2 Future Improvements to Flood Warning 89 7.3 Emergency Planning Recommendations 89 8 Recommendations 91 8.1 Site Allocation Process 91 8.2 Additional Studies 91 8.3 Council Policy 92 8.4 Emergency Planning 92 8.5 Future Updates to the SFRA 93 9 Glossary 94 10 References 96 11 Appendices 97 Appendix A – PPS25 Overview 98 Appendix B – Data Register 99 Appendix C – Communication Plan 100 Appendix D – Model Review 101 Appendix E – Fluvial Flooding History 102 Appendix F – Sewer Flooding History 103 Appendix G – Indicative Sites 104 11 Addendum 104 Addendum - SFRA 2012 addendum with update on fluvial maps attached Tables Table 4-1 Estimated magnitudes/return period of the flood events in Wheathampstead 36 Table 4-2 Estimated return periods of flood events in Batford 36 Table 4-3 Environment Agency records of groundwater flooding 38 Table 4-4 Thames Water sewer flooding records over the last 10 years 51 Table 5-1 Flood Zone 3b (Functional Floodplain) mapping 64 Table 5-2 Flood Zone 3a (high probability) mapping 65 Table 5-3 Flood Zone 2 (medium probability) mapping 65 Table 5-4 Climate change guidance (from PPS25) 67 Table 5-5 Climate change mapping ( Future = 2025 – 2115, Present = 2007) 68 Table 5-6 Flood Zone classification of indicative sites 70 Figures Figure 2-1 Application of the Sequential Test – Source: Development and Flood Risk: A Practice Guide Companion ‘Living Draft’ 14 Figure 6-1 Guidance for developers for individual planning applications 80 The Updated SFRA 2012 Following upgrades to the flood defences on the River Colne and subsequent Regional Flood Risk Modelling by the Environment Agency new information became available for an update to the Strategic Flood Risk Assessment [SFRA] published in 2007. Beginning April 2012 Watford Borough Council decided to take these changes into account for an update of the flood plain boundaries for both rivers, River Colne and River Gade. This decision will be reflected in the Updated SFRA 2012 and the Council- wide available web map 3regarding ‘Flood History and Flood Risk’, based on the Corporate GIS database. This update had already been foreseen by the consultants that prepared the SFRA published in 2007: Recommendation For Future Updates from SFRA 2007: (p91 para. 8.5 ) ‘The SFRA should be retained as a ‘living’ document and reviewed on a regular basis in light of better flood risk information and emerging policy guidance. It is recommended that outputs from ‘The Upper Colne Strategic Flood Risk Mapping (Environment Agency 2008)’ is used to update future versions of the SFRA report and associated maps.’ Background At present based, on Watford Borough Council’s Strategic Flood Risk Assessment (2007), the Functional Floodplain (FRZ 3b) is based on a 1 in 20 year event for most areas that fall within the Gade catchment in Watford. However, due to historical flooding issues and continued flood risk in the Watford area, it was decided within the SFRA process, to reduce the risk of further development exacerbating flood risk in the Colne area, That means that the Upper River Colne and Mimmshall Brook area were modelled equivalent to a 1:50 year flood event, based on the 1 in 50 year approach used Doc No D4512 Rev 0: Date: August 2007, updated May 2012 1 in the Upper Colne Flood Risk Management Strategy (2005). Since 2005, this area including other tributaries, such as parts of River Gade has been re-modelled and updated flood risk information has become available. Why it needs updating The Upper Colne Strategic Flood Risk Mapping (Upper Colne SFRM, Environment Agency 2008) provides refined PPS25 Flood Zones for the Upper River Colne, and the rest of the Watford area. This Regional Flood Risk Modelling update was done for several reasons: • previous modelling for the Watford Borough Council area was piecemeal [i.e. stemming from different FRAs and modelling approaches]; • new flood defences have been built since the 2005 modelling for the Upper Colne Flood Risk Management Strategy and SFRA; and • the River Colne in the Watford area has very limited area of available flood plain and is seen as a strategic section of the River Colne in regard to flood management (it provides an areas for the storage/holding of flood waters which would otherwise move downstream to London and cause flooding there). There has been an upgrade to the flood defences in this area which is reflected in the Upper Colne SFRM 2008 study. This led to the modelling for this area being redone. The revised maps show differences in the flood risk boundaries for this area. However, as updates to Flood Risk Zones 2 and 3 boundaries are published regularly by the Environment Agency [EA] via Geostore and these updates to GIS flood maps had already been regular practice since about 2007/8. These maps are basically ‘living maps’ and related documents are known as ‘living documents’. Changes from the 2012 Update However, the extent of Flood Risk Zone 3b (the functional floodplain), in which uses are most restricted, is a local authority decision, based on data and options provided by the Environment Agency and had not been part of previous ‘living maps’ updates. Watford Borough decided to use a model from the EA Upper Colne SFRM Study to produce a defended 1 in 50 year outline for the River Colne and a defended 1 in 20 year Doc No D4512 Rev 0: Date: August 2007, updated May 2012 2 outline for the River Gade within the local authority’s area. This floodplain definition accounts as a technical update as it had previously been used. Further effects of the above change have been explained in the ‘Updated SFRA 2012’ addendum [in the back of this document including an indicative map showing FRZ 2, 3a and 3b for fluvial flooding]. Foreword The purpose of this Strategic Flood Risk Assessment (SFRA) study is to assess and map all forms of flood risk from groundwater, surface water, sewer and river sources, taking into account future climate change predictions, and use this as an evidence base to locate future development primarily in low flood risk areas. The outputs from the SFRA will help the Four Council’s to prepare sustainable policies for the long-term management of flood risk and improve existing emergency planning procedures. Issues of emergency planning, disaster recovery and climate change have been topping the news headlines this summer as a result of the devastating flooding which occurred in the Midlands, North East and Thames Region, following record levels of rainfall (worst affected areas were Sheffield, Hull, Gloucestershire and Oxfordshire and Berkshire). By comparison, the level of flood risk is lower within the study area, particularly along the chalk tributaries (Rivers Ver, Gade, Bulbourne and Chess).