London Borough of Harrow Council Strategic Flood Risk Assessment (Level 1 SFRA) Volume II – Technical June 2009 Client: London Borough of Harrow Council Project: Strategic Flood Risk Assessment Document Title: Volume II – Technical Report MWH Project Code: 41515318.010101 Document No: FINAL Prepared by Version Date Description/Amendment Checked by Reviewed by (Author) A August 2008 Preliminary Draft Report Jacquie Hannan Gaye McKay Steve Dickie B November 2008 Draft Report Jacquie Hannan Steve Dickie Steve Dickie C April 2009 Final Draft Report Jacquie Hannan Steve Dickie Steve Dickie D June 2009 Final Report Jacquie Hannan Steve Dickie Steve Dickie PLEASE DESTROY ALL SUPERSEDED COPIES OR CLEARLY MARK THEM AS “SUPERSEDED” UNCONTROLLED WHEN PRINTED File Location Date Admin Check by S:\NBE\Flood Risk\Harrow SFRA\Technical\T5 - Reports\Final Report\Harrow SFRA (Final Technical Report).doc OPTIONAL EXTRA IF REQUIRED BY PROJECT MANAGER: Distribution: Name Copy No. London Borough of Harrow Council - Strategic Flood Risk Assessment Volume II – Technical Report 1. EXECUTIVE SUMMARY The London Borough of Harrow is subject to riverine flooding from a number of watercourses, most notably the River Pinn and Wealdstone, Yeading and Edgware Brooks. The Borough also experiences pluvial, surface water and sewer flooding. In accordance with ‘Planning Policy Statement 25 (PPS25): Development and Flood Risk’, this Strategic Flood Risk Assessment (SFRA) provides an assessment of all types of flood risk within the London Borough of Harrow to inform land use planning decisions. Specifically, it provides an overview of: • main sources of flooding; • potential flood risk in the Borough; • the predicted influences of climate change; • identification of options to mitigate the potential flood risk; • the potential flood risk at allocated sites within the Borough; and • relevant planning controls, appropriate sustainable design of developments and mitigation that will be sought in new development and in the retrofitting of existing buildings and extensions. The outcomes of the SFRA will be used as guidance in influencing future Development Plan Document (DPD) policies within the Local Development Framework (LDF). The SFRA should be used by the London Borough of Harrow to inform the site allocations in the Core Strategy (which is currently being developed) and to provide evidence for the Sequential Test to be undertaken for potential development sites. It will also be used by the Council in the Development Control planning process to ensure inappropriate development does not take place in the floodplain. In addition, the SFRA will be a useful tool for developers in determining the suitability of sites for certain types of development. It can also be used by developers and planners as a starting point for progressing site specific Flood Risk Assessments (FRAs), which are required to be submitted with planning applications in areas of flood risk/areas identified as at risk from flooding. Throughout the data collection and review process it has been critical to make best use of the significant amount of information which already exists with respect to flood risk. This information was made available by the Environment Agency, Thames Water, Harrow Drainage team, and other key consultees, and includes: • flood zone mapping; • detailed flood outline mapping; • flood level data; • historical flood information; • details of drainage structures; • Council policy documents; • GIS mapping; • sewer flood risk areas; • groundwater flood risk areas; and • topographic data. MWH UK LTD Page i of 41 London Borough of Harrow Council - Strategic Flood Risk Assessment Volume II – Technical Report Based on data from Council, the Environment Agency and Thames Water, and hydrologic and hydraulic modelling investigations completed as part of this study, the flood risk posed by fluvial, pluvial, sewer and groundwater sources have been mapped. For fluvial flooding, the mapping indicates the location and extent of areas considered to be at low, medium and high probability of fluvial flooding (i.e. PPS25 Flood Zones 1, 2 and 3a) and the Functional Floodplain (i.e. PPS25 Flood Zone 3b). On the basis of flood risk mapping, the potential flood risks associated with each of Council’s Local Development Framework (LDF) allocation sites has been established and guidance has been provided to enable Council to apply the Sequential Test in the future. It is recommended that an integrated approach be adopted within the Borough to address fluvial, surface water and sewer flooding. Based on an understanding of the characteristics of the area, potential flood risk sources and location and relevant local, regional, national and international policy, a number of policy directions have been developed for the London Borough of Harrow. These policy recommendations need to be considered by planners at the strategic level (e.g. when preparing the LDF and DC policy recommendations), site level (e.g. when preparing a FRA or the site layout plan) and/or both levels. Central to these recommendations is the assertion that a planning solution to flood risk management should be sought wherever possible, steering vulnerable development away from areas affected by flooding in accordance with the PPS25 Sequential Test. Where other planning considerations must guide the allocation of sites and the Sequential Test cannot be satisfied, specific recommendations have been provided to assist the Council and the developer to meet the Exception Test. In addition to passing the Exception Test, it would need to be demonstrated that the development is ‘safe’ and reduces flood risk overall. In these cases, a sequential approach should be applied in site design (e.g. siting buildings on higher ground and recreation areas on low-lying grounds). Council should also seek to promote the following through a robust planning and policy framework: • implementation of SUDS measures for new development. • seeking opportunities to make space for water through such measures as ensuring an 8m wide buffer strip alongside watercourses within future development and ensuring preservation of flood flow routes; • flood resilience and resistance measures at a property level; • safeguarding the future operation and function of flood defences and flood- related infrastructure and continue their maintenance regime; • safeguarding sites identified for flood storage and alleviation schemes and protect Greenfield Functional Floodplain; • ensuring developers consult this SFRA prior to the formulation of development proposals and planning applications; • flood awareness in the community and update flood plans in light of the SFRA findings. MWH UK LTD Page ii of 41 London Borough of Harrow Council - Strategic Flood Risk Assessment Volume II – Technical Report The preparation of a Surface Water Management Plan (SWMP) is also recommended. This would provide the Council with a greater understanding of the surface water flood risk within the Borough and assist in developing a coordinated approach to managing surface water drainage and flooding issues. This SWMP should refer to the findings and recommendations of the North Brent Integrated Urban Drainage Pilot Study for the Kenton area. It should identify opportunities to implement SUDS within the Borough and aim to provide cost effective solutions for the areas at greater risk of flooding. The SFRA is comprises in three (3) separate volumes, as follows: • Volume I - Planning & Policy Report - provides a summary of the catchment and flooding background, and details the planning policy framework relevant to this study, methodology used to define the flood risk in the Borough, potential flood risk at proposed site allocations, policy recommendations and guidance for planners and developers in preparing site-specific Flood Risk Assessments. • Volume II - Technical Report - documents the data collection process, catchment and flood characteristics, and hydrologic and hydraulic analyses completed for the study. • Volume III - Mapping - includes the mapping that has been produced for the SFRA. It should be remembered that this SFRA is a living document and should be reviewed annually, or as new flood information becomes available. MWH UK LTD Page iii of 41 London Borough of Harrow Council - Strategic Flood Risk Assessment Volume II – Technical Report Contents Page No 1. EXECUTIVE SUMMARY I 1. INTRODUCTION 4 1.1. Background 4 1.2. Objectives of this Study 5 1.3. Context of the Harrow SFRA 6 1.4. SFRA as a Living Document 6 1.5. Structure of the SFRA Report 7 2. DATA COLLECTION & CONSULTATION 8 2.1. Environment Agency Flood Zone and Flood Outline Mapping 9 2.2. Topographic Data 10 2.3. Historical Flood Data 10 2.4. Surface Water and Sewer Flooding Data 10 2.5. Groundwater Flooding Data 11 2.6. Details of Flood Defence Assets and Flood Structures 11 2.7. Background Documents 11 3. CATCHMENT DESCRIPTION 13 3.1. Overview of London Borough of Harrow 13 3.2. Topography of Borough 13 3.2.1. Geology 13 3.2.2. Drift Geology 14 3.3. History of Flooding in the Catchment 15 3.4. Flood Mechanisms within Harrow 17 3.4.1. River (Fluvial) Flooding 18 3.4.2. Groundwater Flooding 18 3.4.3. Surface Water Flooding 18 3.5. River Catchments in Harrow 19 MWH UK LTD Page i of 41 London Borough of Harrow Council - Strategic Flood Risk Assessment Volume II – Technical Report 3.5.1. Brent River Catchment 20 3.5.2. Pinn River Catchment (part of the Colne Catchment) 21 3.5.3. Crane River Catchment 21 3.6. Reservoirs and Water Supplies (including lakes and ponds) 21 3.7. Flood Defences 23 3.7.1. Silk Stream Flood Alleviation Scheme (Brent Catchment) 24 3.7.2. Oxhey Lane Farm Flood Alleviation Scheme (Pinn Catchment) 25 3.7.3. Hatch End Flood Alleviation Scheme (Pinn Catchment) 25 4. HYDROLOGIC ANALYSIS 27 4.1. Choice of Methodology 27 4.2. Hydrological Modelling 28 4.2.1. Woodridings Stream 28 4.2.2. River Pinn 29 4.2.3. Yeading Brook West (including Elmshot Brook) 29 4.2.4. Smarts Brook 30 4.2.5. Yeading Brook East 30 4.2.6.