Btcaap011 Strategic Flood Risk
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B R O M L E Y T O W N C E N T R E A R E A A C T IO N P L A N STRATEGIC FLOOD RISK ASSESSMENT London Borough of Bromley Bromley Town Centre Area Action Plan Strategic Flood Risk Assessment November 2008 Halcrow Group Limited Halcrow Group Limited Griffin House 135 High Street Crawley West Sussex RH10 1DQ Tel +44 (0)1293 434500 Fax +44 (0)1293 434599 www.halcrow.com Halcrow Group Limited has prepared this report in accordance with the instructions of their client, London Borough of Bromley, for their sole and specific use. Any other persons who use any information contained herein do so at their own risk. © Halcrow Group Limited 2008 London Borough of Bromley Bromley Town Centre Area Action Plan Strategic Flood Risk Assessment Contents Amendment Record This report has been issued and amended as follows: Issue Revision Description Date Signed 1 1 Draft Jan 08 RA/BV 2 1 Draft Apr 08 RA/BV 3 1 Draft Sept 08 RA/BV 4 1 Final Nov 08 RA/BV Contents 1 Executive Summary 1 2 Background Information 2 2.1 Area Action Plan, Sustainability Appraisal and Preferred Options 2 2.2 Review of the Preferred Options 2 2.3 Proposal Sites 2 2.4 Potential Impacts of Revised Preferred Options 4 3 Requirement for a Level 2 Strategic Flood Risk Assessment 2 4 Relevant Documents 4 4.1 ‘Making Space for W ater’ 4 4.2 Catchment Flood Management Plans 4 4.3 Ravensbourne Policy Unit 6 4.4 Managing flood risk through development 6 4.5 Planning Policy Statement 25 (PPS25) 7 5 Sequential Test and Exception Test 9 5.1 Flood Zones and Flood Maps 9 5.2 The Sequential Test - Overview 10 5.3 Sequential Test for Bromley Town Centre 11 5.4 The Exception Test 13 5.5 Exception Test for Bromley Town Centre 13 5.6 Existing Development and Long-term Future Redevelopment 14 6 Flood Risk and Proposal Sites 17 6.1 Proposal Sites 17 6.2 Groundwater Flooding 17 6.3 Fluvial flooding 18 6.4 Surface W ater Flooding 19 6.5 Residual Fluvial Flood Risk 19 6.6 Sewerage Flooding 19 7 Drainage Strategy 20 7.1 Introduction 20 7.2 The existing drainage system 20 7.3 General Design Requirements 21 7.4 Constraints on drainage design 22 7.5 Source Protection Zones 23 7.6 Underlying geology in the Bromley Town Centre 23 7.7 Contaminated Land 24 7.8 Other constraints on drainage design and SUDS 24 7.9 Drainage design for Bromley Town Centre - Strategic Approach 24 7.10 Drainage design for Bromley Town Centre - Site specific drainage criteria 26 7.11 Outline estimate of SUDS control measures for each Proposed Site 26 8 Flood Warning 29 8.1 Introduction 29 8.2 Flood W arning Codes 29 8.3 Efficiency of Flood W arning Service 30 9 Recommended Policies 31 9.1 Interim Policies 31 9.2 Flooding 31 9.3 River Ravensbourne 31 9.4 Recreational use of the River Ravensbourne 32 9.5 Drainage Design 32 9.6 Development Layout 33 Appendices Appendix A Run-off calculations for drainage design Appendix B Audit Trail Database Figures Figure 1 – Town Centre Opportunity Sites Figure 2 – Thames Catchment: Types of CFMP Flood Plain Figure 3 – Schematic of the PPS25 Flood Zones Figure 4 – Proposed area to be reserved for channel improvements within site K Figure 5 – Study area geology (reproduced from the Environment Agency) Figure 6 - Approximate location of culverts in the study area Figure 7 – Proposed Exception Test measure and off-site attenuation of surface runoff M aps M ap 1 - Flood risk vulnerability: Proposed Town Centre Development Sites M ap 2 – Flood zones and defences M ap 3 – Source Protection Zones Glossary of Terms Area Action Plans – Development Plan Documents that provide a planning framework for areas of change and areas of conservation. Catchment Flood M anagement Plan (CFM P) – A strategic planning tool through which the Environment Agency seeks to work with other key decision-makers within a river catchment, to identify and agree policies for sustainable flood risk management. Development Plan - As set out in Section 38(6) of the Planning and Compulsory Purchase Act (2004), an authority’s development plan consists of the relevant Regional Spatial Strategy (or the Spatial Development Strategy in London) and the Development Plan Documents contained within its Local Development Framework. Development Plan Documents (DPDs) - Spatial planning documents within the Council’s Local Development Framework which set out policies for development and the use of land. Together with the Regional Spatial Strategy they form the development plan for the area. They are subject to independent examination. They are required to include a core strategy and a site allocations document, and may include area action plans if required; other DPDs may also be included, e.g. development control policies. Emergency Planning – Planning for and response to emergencies such as flooding, including consideration of the resilience of emergency infrastructure that will need to operate during flooding. Environment Agency - The leading public body for protecting and improving the environment in England and W ales. Flood management and defence are a statutory responsibility of the Environment Agency; it is consulted by local planning authorities on applications for development in flood risk areas, and also provides advice and support to those proposing developments and undertaking Flood Risk Assessments. The Environment Agency reports to DEFRA. Environment Agency Flood Zones - Nationally consistent delineation of ‘high’ and ‘medium’ flood risk, published on a quarterly basis by the Environment Agency. Flood Estimation Handbook - The latest hydrological approach for the estimate of flood flows in UK. Flood Risk Assessment – A site specific investigation usually carried out by the site developers to be submitted as part of their planning applications. It assess both current flood risk to the site and the impact of development of the site to flood risk in the area. Flood Risk Vulnerability - PPS25 provides a vulnerability classification to assess which uses of land may be appropriate in each flood risk zone. Formal Flood Defence - A structure built and maintained specifically for flood defence purposes. 1nformal Flood Defence - A structure that provides a flood defence function, but has not been built or maintained for this specific purpose (e.g. boundary wall). JFlow - A computer river model based on routeing a flood calculated by Flood Estimation Handbook methodology along a river corridor the levels of which are derived from a Side Aperture Radar (SAR) remote sensed Digital Terrain Model. LiDAR – ‘Light Detection and Ranging’ is an airborne terrain mapping technique which uses a laser to measure the distance between the aircraft and the ground. It therefore provides accurate topographical/contour mapping. Local Development Documents (LDDs) – the collective term for Development Plan Documents and Supplementary Planning Documents. Local Development Framework (LDF) - The name for the portfolio of Local Development Documents. It consists of the Local Development Scheme, a Statement of Community Involvement, Development Plan Documents, Supplementary Planning Documents, and the Annual Monitoring Report. Local Development Scheme (LDS) - Sets out the programme for preparing Local Development Documents. All authorities must submit a Scheme to the Secretary of State for approval within six months of commencement of the 2004 Act (thus all authorities should now have submitted an LDS). LDSs are subject to review. ‘M aking Space for W ater’ (DEFRA 2004) - The Government’s new evolving strategy to manage the risks from flooding and coastal erosion by employing an integrated portfolio of approaches, so as: a) to reduce the threat to people and their property; b) to deliver the greatest environmental, social and economic benefit, consistent with the Government's sustainable development principles, c) to secure efficient and reliable funding mechanisms that deliver the levels of investment required. National Flood Risk Assessment (NaFRA) - Flood risk assessment that uses a risk-based approach to factor in the location, type, condition and effects of flood defences. It calculates the actual likelihood of flooding to areas of land within the floodplain of an extreme flood (0.1% or 1 in 1000 chance in any year), considering the distance from the river or the sea, and factors in the probability that the flood defences will overtop or breach. A variety of scenarios are analysed in order to determine properties at low, moderate and significant flood risk. Planning Policy Statements - The Government has updated its planning advice contained within Planning Policy Guidance Notes (PPGs) with the publication of new style Planning Policy Statements (PPSs), which set out its policy for a range of topics. Previously Developed (Brownfield) Land - Land which is or was occupied by a building (excluding those used for agriculture and forestry). It also includes land within the curtilage of the building, for example a house and its garden would be considered to be previously developed land. Land used for mineral working and not subject to restoration proposals can also be regarded as brownfield land. Regional Spatial Strategy - Sets out the region’s policies in relation to the development and use of land and forms part of the development plan for local planning authorities. Residual Risk - The risk which remains after all risk avoidance, reduction and mitigation measures have been implemented. River Basin M anagement Plan (RBM P) – A strategic tool introduced by the W ater Framework Directive (2000/60/EC) which integrates the management of land and water within a river basin (river catchment or group of catchments). The river basin may cover several political areas. Strategic Environmental Assessment (SEA) - A generic term used to describe environmental assessment as applied to policies, plans and programmes.