9713 Planning document covers 22/10/08 14:01 Page 1 24 and Local25 Development Framework SeptemberTechnical Background Report Strategic Flood Risk Assessment 2008 10am – 4pm Halfords, Hayes Bridge Retail Park, Uxbridge Road, Hayes UB4 0RH Prepared for the London Borough of Hillingdon by www.hillingdon.gov.uk London Borough of Hillingdon Strategic Flood Risk Assessment – Level 1 Report Revision Schedule Strategic Flood Risk Assessment Level 1 November 2008 Rev Date Details Prepared by Reviewed by Approved by 00 August Draft Heather Rich Jon Robinson 2007 Strategic Flood Risk Specialist Associate Director Flood Risk Assessment 01 November Final Level 1 Heather Rich Liz Williams Jon Robinson 2007 Strategic Flood Risk Specialist Senior Consultant Associate Director Flood Risk Assessment 02 January Final Level 1 Liz Williams Heather Rich Jon Robinson 2008 Report Rev 1 Senior Consultant Flood Risk Specialist Associate Director 03 February Final Level 1 Heather Rich Jon Robinson 2008 Report Rev 2 Flood Risk Specialist Associate Director 04 April 2008 Final Level 1 Heather Rich Liz Williams Jon Robinson Report Rev 3 Flood Risk Specialist Senior Consultant Associate Director 04 November Final Level 1 Eleanor Cole Liz Williams Jon Robinson 2008 Report Rev Assistant Senior Consultant Associate Director 4-Hillingdon Hydrologist Comments Scott Wilson Ltd., 6-8 Greencoat Place London SW1P 1PL Tel: +44 (0)20 7798 5200 Fax: +44 (0)20 7798 5001 www.scottwilson.com London Borough of Hillingdon Strategic Flood Risk Assessment – Level 1 Report Abbreviations Acronym Definition ABDs Areas Benefiting from Defences AONB Area of Outstanding Natural Beauty CFMP Catchment Flood Management Plan CLG Communities and Local Government DEM Digital Elevation Model DPD Development Plan Documents DTLR UK Department of Transport, Local Government and Regions EA Environment Agency for England and Wales FRA Flood Risk Assessment FRIS Flood Reconnaissance Information System GIS Geographical Information Systems IDB Internal Drainage Board LBH The London Borough of Hillingdon LDDs Local Development Documents LDF Local Development Framework LDS Local Development Scheme LiDAR Light Detection and Ranging LPA Local Planning Authority LPD Local Planning Documents mOD Meters Ordnance Datum MoD Ministry of Defence ODPM Office of the Deputy Prime Minister PBA Peter Brett Associates PCPA 2004 Planning and Compulsory Purchase Act 2004 PPG25 Planning Policy Guidance Note 25: Development and Flood Risk PPS25 Planning Policy Statement 25: Development and Flood Risk RFRA Regional Flood Risk Appraisal RPG Regional Planning Guidance London Borough of Hillingdon Strategic Flood Risk Assessment – Level 1 Report RSS Regional Spatial Strategy SAR Synthetic Aperture Radar SAC Special Area of Conservation SA Sustainability Appraisal SFRA Strategic Flood Risk Assessment SPA Special Protection Area SPG Supplementary Planning Guidance SSSI Site of Special Scientific Interest SuDS Sustainable Drainage Systems London Borough of Hillingdon Strategic Flood Risk Assessment – Level 1 Report Glossary Term Definition A source of groundwater comprising water-bearing rock, sand or Aquifer gravel capable of yielding significant quantities of water. Catchment A high-level planning strategy through which the Environment Flood Agency works with their key decision makers within a river catchment Management to identify and agree policies to secure the long-term sustainable Plan management of flood risk. Culvert A channel or pipe that carries water below the level of the ground. Infrastructure used to protect an area against floods as floodwalls Flood defence and embankments; they are designed to a specific standard of protection (design standard). Area adjacent to river, coast or estuary that is naturally susceptible to Flood plain flooding. A temporary area that stores excess runoff or river flow often ponds Flood storage or reservoirs. Fluvial flooding Flooding by a river or a watercourse. Height of flood defence crest level (or building level) above designed Freeboard water level. Water that is in the ground, this is usually referring to water in the Groundwater saturated zone below the water table. Inundation Flooding. The core of the updated planning system (introduced by the Planning Local and Compulsory Purchase Act 2004). The LDF comprises the Local Development Development Documents, including the development plan Framework documents that expand on policies and provide greater detail. The (LDF) development plan includes a core strategy, site allocations and a proposals map. Local Planning Body that is responsible for controlling planning and development Authority through the planning system. Mitigation An element of development design which may be used to manage measure flood risk or avoid an increase in flood risk elsewhere. Flooding caused when intense rainfall exceeds the capacity of the Overland Flow drainage systems or when, during prolonged periods of wet weather, the soil is so saturated such that it cannot accept any more water. Pluvial Flooding associated with rainfall Risk The probability or likelihood of an event occurring. Sewer flooding Flooding caused by a blockage or overflowing in a sewer or urban drainage system. London Borough of Hillingdon Strategic Flood Risk Assessment – Level 1 Report Sustainable Methods of management practices and control structures that are drainage designed to drain surface water in a more sustainable manner than system some conventional techniques. Development that meets the needs of the present without Sustainable compromising the ability of future generations meeting their own development needs. Event that on average will occur once every 100 years. Also 1 in 100 year expressed as an event, which has a 1% probability of occurring in event any one year. 1 in 100 year Flood defence that is designed for an event, which has an annual design probability of 1%. In events more severe than this the defence would standard be expected to fail or to allow flooding. London Borough of Hillingdon Strategic Flood Risk Assessment – Level 1 Report Table of Contents Abbreviations............................................................................................ 2 Glossary ................................................................................................... 4 1 Introduction.................................................................................... 1 1.1 Hillingdon’s Profile......................................................................................................3 1.2 Aim .............................................................................................................................7 1.3 The Strategic Flood Risk Assessment Objectives......................................................8 1.4 The SFRA Structure ...................................................................................................8 2 Policy Context ............................................................................. 11 2.1 National Policies.......................................................................................................11 2.2 Regional Policies......................................................................................................12 2.3 Local Policies ...........................................................................................................18 2.4 Environment Agency Policies...................................................................................20 2.5 Neighbouring Strategic Flood Risk Assessment Status ...........................................22 2.6 Other Relevant Policies............................................................................................23 3 Data Collection and Methodology................................................ 24 3.1 Review of Data Collection Process ..........................................................................24 3.2 Data Collection .........................................................................................................25 3.3 Fluvial Flooding ........................................................................................................26 3.4 Sewer Flooding ........................................................................................................35 3.5 Overland Flow ..........................................................................................................36 3.6 Groundwater Flooding..............................................................................................37 3.7 Artificial Sources.......................................................................................................38 3.8 Additional Mapping...................................................................................................38 4 Flood Risk in Hillingdon............................................................... 40 4.1 Fluvial Flooding ........................................................................................................40 4.2 Sewer Flooding ........................................................................................................46 4.3 Overland Flow ..........................................................................................................46 4.4 Groundwater Flooding..............................................................................................47 4.5 Artificial Sources.......................................................................................................47 4.6 Residual Risks..........................................................................................................48