Waltham Forest Council Level 2 Strategic Flood Risk Assessment Final Report May 2011 Prepared for Waltham Forest Council Level 2 Strategic Flood Risk Assessment Revision Schedule Level 2 Strategic Flood Risk Assessment May 2011 Rev Date Details Prepared by Reviewed by Approved by 01 October 2010 Draft Report for Emily Blanco Elizabeth Gent Elizabeth Gent Client Comment Senior Flood Risk Specialist Principal Consultant Principal Consultant Tom Edwards Jon Robinson Senior Consultant Technical Director 02 December Final Report Tom Edwards Elizabeth Gent Jon Robinson 2010 incorporating Senior Consultant Principal Consultant Technical Director client and EA comments 03 March 2011 Final Report Tom Edwards Jon Robinson Jon Robinson incorporating Senior Consultant Technical Director Technical Director further comments 04 May 2011 Final Report Tom Edwards Jon Robinson Jon Robinson with minor Senior Consultant Technical Director Technical Director amendments This document has been prepared in accordance with the scope of Scott Scott Wilson Wilson's appointment with its client and is subject to the terms of that 6-8 Greencoat Place appointment. 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The contents of this document do not provide legal or tax advice or opinion. © Scott Wilson [Ltd] 2011 www.scottwilson.com Waltham Forest Council Level 2 Strategic Flood Risk Assessment Executive Summary Scott Wilson has been commissioned to prepare a Level 2 Strategic Flood Risk Assessment (SFRA) on behalf of Waltham Forest Council. This assessment builds on the findings of the North London Level 1 SFRA1 and provides more detailed information regarding fluvial flood risk associated with the River Lee system along the western margin of the borough. The purpose of this increased scope Level 2 SFRA is to enable the application of the PPS25 Exception Test and to contribute to the evidence base for Waltham Forest’s Local Development Documents (LDDs), with particular reference to the Area Action Plans (AAPs) for the Blackhorse Lane Area and Northern Olympic Fringe Area. This study has utilised available hydraulic modelling information to produce mapping outputs which define the flood extents and provide further detail regarding the likely depth of flooding and flood hazard classification during a range of flood events. The mapping outputs should be used to refine and update the PPS25 Sequential Test process. If, as anticipated, this confirms that some development is required within flood risk areas to provide wider sustainability benefits to Waltham Forest then the outputs can be used to inform the PPS25 Exception Test. The Flood Depth and Flood Hazard mapping should be used to apply a sequential planning approach, whereby the most vulnerable development types are located within the lowest flood risk areas. In order to ensure that future development proposed within flood risk areas is safe, and does not increase flood risk in adjacent areas, a number of development control measures will need to be enforced, such as, the location and layout of different type of development according to vulnerability; setting of appropriate finished floor levels; provision of safe access and egress; the use of appropriate flood resistant and resilient construction techniques in accordance with expected flood depths; provision of safe refuge above the flood level; and identification of suitable evacuation procedures in the event of a flood. Specific guidance and recommendations with respect to flood risk mitigation measures for future development are provided within this report which should be addressed as part of a site specific Flood Risk Assessment to support the planning application. Information on when a Flood Risk Assessment is required, the varying levels of Flood Risk Assessments, and the minimum requirements of an FRA have also been outlined. A walkover of the main flood defence network has been completed to provide an indication of the current standard of flood protection. This suggests that the standard of protection associated with parts of the Ching Brook and Dagenham Brook is less than 1 in 20 years. The River Lee Flood Relief Channel offers a slightly improved standard of protection, estimated as approximately 1 in 50 years however these standards highlight the actual risk of flooding which is present within Waltham Forest. Additional investment in flood defence infrastructure will be required in the future to prevent the standard of protection reducing further in the face of increasing urbanisation, climate change and an ageing defence network. It is recommended that the areas highlighted as having the lowest standard of protection should initially be targeted for future improvements, and detailed studies will be required develop potential options and evaluate impacts. It is considered unlikely that construction of new, large scale flood defence infrastructure provides a sustainable flood risk management approach for Waltham Forest, as such measures can often relocate flooding problems to adjacent areas. It is therefore recommended that the regeneration plans throughout 1 Mouchel (August 2008) North London Strategic Flood Risk Assessment D133184 May 2011 i Waltham Forest Council Level 2 Strategic Flood Risk Assessment the study area should be viewed as a mechanism through which reduction in the level of flood risk can be achieved. This will require incorporation of flood storage and other on site mitigation such as use of sustainable drainage techniques to provide attenuation and reduce surface water runoff. D133184 May 2011 ii Waltham Forest Council Level 2 Strategic Flood Risk Assessment Table of Contents Acronyms & Abbreviations .......................................................................................................................... v Glossary ........................................................................................................................................................ vi 1 Introduction .................................................................................................................................... 1 1.1 Background ...................................................................................................................................... 1 1.2 Aims & Objectives ............................................................................................................................ 1 1.3 Methodology ..................................................................................................................................... 2 1.4 Previous Flood Risk Studies ............................................................................................................ 2 2 Study Area Characteristics ........................................................................................................... 4 2.1 Overview .......................................................................................................................................... 4 2.2 Topography ...................................................................................................................................... 5 2.3 River Network ................................................................................................................................... 5 2.4 Reservoir Network ............................................................................................................................ 7 3 Flood Risk in Waltham Forest ...................................................................................................... 9 3.1 Fluvial Flooding ................................................................................................................................ 9 3.2 Surface Water Flooding ................................................................................................................. 15 3.3 Groundwater Flooding.................................................................................................................... 16 3.4 Sewer Flooding .............................................................................................................................. 17 3.5 Artificial Sources ............................................................................................................................ 18 4 Specific Area Considerations ..................................................................................................... 19 4.1 Overview ........................................................................................................................................ 19 4.2 Sequential Approach ...................................................................................................................... 19 4.3 Key Flooding Mechanism ............................................................................................................... 19 4.4 Blackhorse