SURFACE WATER MANAGEMENT PLAN DRAIN LONDON LONDON BOROUGH OF HOUNSLOW Quality Management Quality Management DOCUMENT INFORMATION Title: Surface Water Management Plan for London Borough of Hounslow Owner: Michael Croke Version: v2.0 Status: Final draft Project Number: CS/046913 File Name: L:\Environment\ZWET\CS046913_DrainLondon_Tier2\Reports\Group1_Outp uts\Hounslow\SWMP\DLT2_GP1_Hounslow_SWMP_ Final_Draft_V2.doc REVISION HISTORY Date of Summary of Changes Completed By Version Issue Paul Hlinovsky/Ruth Initial Draft 08/06/2011 1.0 Farrar Final Draft following comments Paul Hlinovsky 21/10/2011 2.0 AUTHOR Name Organisation and Role Paul Hlinovsky Senior Engineer, Capita Symonds Ruth Farrar Engineer, Capita Symonds APPROVALS Name Title Signature Date Ruth Goodall Director, Capita Symonds 21/10/2011 Paul Hlinovsky Senior Engineer, Capita Symonds 21/10/2011 Principal Consultant, Capita Michael Arthur Symonds – Tier 1 lead Kevin Reid Programme Manager DISTRIBUTION Name Organisation and Role Michael Croke Infrastructure Engineer, LB Hounslow Kevin Reid Programme Manager, GLA RELATED DOCUMENTS Date of Doc Ref Document Title Author Version Issue Preliminary Flood Risk Assessment for Capita Symonds 12/07/2011 1.6 London Borough of Hounslow Cover: Burst Thames Water sewer main (2010) Photo: Paul Winter i Acknowledgements Acknowledgements A number of people and organisations outside Hounslow Council have contributed to this Surface Water Management Plan. Their assistance is greatly appreciated, and in particularly inputs and information provided by: The British Geological Survey; British Waterways; Drain London Group 1 Boroughs: o London Borough of Hounslow; o London Borough of Ealing; and o London Borough of Hillingdon. The Environment Agency; The Greater London Authority; London Councils; The London Fire Brigade; Network Rail; Thames Water; Highways Agency; and Transport for London and London Underground. ii Executive Summary Executive Summary This document forms the Surface Water Management Plan (SWMP) for the London Borough (LB) of Hounslow which has been delivered as part of the Tier 2 package of works of the Drain London Project. This document is a plan which outlines the preferred surface water management strategy the LB of Hounslow and includes consideration of flooding from sewers, drains, groundwater and runoff from land, small watercourses and ditches that could occur as a result of heavy rainfall. The SWMP builds upon previous work undertaken at part of the Drain London Tier 1 package of works and has been undertaken following a four phase approach in line with Department of Environment and Rural Affairs (Defra) SWMP technical guidance documentation (2010). These are; Phase 1 – Preparation; Phase 2 – Risk Assessment; Phase 3 – Options; and Phase 4 – Implementation and Review. Phase 1 Preparation Phase 1 builds upon work undertaken during Tier 1 of the Drain London Project. The Tier 1 work involved the collection and review of surface water data from key stakeholders and the building of partnerships between key stakeholders responsible for local flood risk management. It was also decided that London would be delineated into 8 working groups. The LB of Hounslow forms part of Group 1 along with the LB‟s of Ealing and Hillingdon. The LB of Hounslow has begun to establish a broader partnership with the Borough slocated within Group 2 (the LB of Barnet, Brent and Harrow), through the establishment of the North West London Strategic Flood Group, in order for these local authorities to determine best practice and resources to enable each authority to discharge their responsibilities as Lead Local Flood Authority (LLFA) under the Flood and Water Management Act (FWMA) 2010. Phase 2 Risk Assessment As part of Phase 2 Risk Assessment, direct rainfall modelling has been undertaken across the entire Borough for five specified return periods. The results of this modelling have been used to identify Local Flood Risk Zones (LFRZs) where flooding affects houses, businesses and/or infrastructure. Those areas identified to be at more significant risk have been delineated into Critical Drainage Areas (CDAs) representing one or several LFRZs as well as the contributing catchment area and features that influence the predicted flood extent. Within the LB of Hounslow, 12 CDAs have been identified; these are shown in Figure i (overleaf) and Figure 1 within Appendix D. The principal mechanisms for flooding in the LB of Hounslow can be broadly divided into the following categories: River Valleys (current and historical) - Across the study area, the areas particularly susceptible to overland flow are formed by narrow corridors associated with topographical valleys which represent the routes of the „lost‟ watercourses within London; Topographical Low Lying Areas - areas such as underpasses, subways and lowered roads beneath railway lines are more susceptible to surface water flooding; Railway Embankments - discrete surface water flooding locations along the up-stream side of the raised network rail embankment (running roughly west to east through the South of the Borough); iii Executive Summary Topographical Low Points – areas which are at topographical low points throughout the Borough which result in small, discrete areas of deep surface water ponding; and Sewer Flood Risk – areas where extensive and deep surface water flooding is likely to be the influence of sewer flooding mechanisms alongside pluvial and groundwater sources. N Figure i. Critical Drainage Areas within the London Borough of Hounslow. Analysis of the number of properties at risk of flooding has been undertaken for the rainfall event with a 1 in 100 probability of occurrence in any given year. A review of the results demonstrate that 27,274 residential properties and 4,499 non-residential properties in the LB of Hounslow could be at risk of surface water flooding of greater than 0.03m depth (above an assumed 0.1m building threshold) during a 1 in 100 year rainfall event. A review of these statistics coupled with local knowledge of the study area identifies that the following CDAs, located within Table i, are at greatest risk of flooding in terms of the number of receptors at risk – refer to Section 3.8 for CDA details and locations. Table i Critical Drainage Areas at greatest risk in London Borough of Hounslow Commercial / Infrastructure Households Industrial CDA ID Location > 0.5m > 0.5m > 0.5m All All All Deep Deep Deep Group1_029 Flanders Road, Hounslow 0 0 62 0 5 0 Boston Manor Road and Group1_033 1 0 262 2 13 0 Manor Vale in Brentford Bridge Road and Pears Group1_039 6 1 256 5 47 0 Road, Hounslow iv Executive Summary The majority of surface water flooding within the Borough is as a result of topographical low areas and obstructions to natural overland flowpaths. Several rail lines are predicted to be at risk due to the elevations of the finished track being lower than the surrounding areas and within cuttings. There are currently no identified cross boundary CDAs within the LB of Hounslow. Phase 3 Options Assessment There are a number of opportunities for measures to be implemented across the Borough to reduce the impact of surface water flooding. Ongoing maintenance of the drainage network and small scale improvements are already undertaken as part of the operations of the Borough. In addition, opportunities to raise community awareness of the risks and responsibilities for residents should be sought, and LB of Hounslow may wish to consider the implementation of a Communication Plan to assist with this. It is important to recognise that flooding within the Borough is not confined to just the CDAs, and therefore, throughout the Borough there are opportunities for generic measures to be implemented through the establishment of a policy position on issues including the widespread use of water conservation measures such as water butts and rainwater harvesting technology, use of soakaways, permeable paving, Bioretention carpark pods and green roofs. In addition, there are Borough-wide opportunities to raise community awareness. For each of the CDAs identified within the Borough, site-specific measures have been identified that could be considered to help alleviate surface water flooding. These measures were subsequently short listed to identify a potential preferred option for each CDA. Pluvial modelling undertaken as part of the SWMP has identified that flooding within the LB of Hounslow is heavily influenced by existing and historic river valleys, and impacts a number of regionally important infrastructure assets. Section 4 provides information on the preferred surface water flood risk management options and measures which may assist in reducing the surface water flood risk within the Borough. Borough wide, it is recommended that in the short-to-medium term the LB of Hounslow: Engage with residents regarding the flood risk in the Borough, to make them aware of their responsibilities for property drainage (especially in the CDAs) and steps that can be taken to improve flood resilience; Provide an „Information Portal‟ via the LB of Hounslow website, for local flood risk information and measures that can be taken by residents to mitigate surface water flooding to/around their property; Prepare a Communication Plan to effectively communicate and raise awareness of surface water flood risk to different audiences using a clearly defined process for internal and external communication with stakeholders and the public; and Improve maintenance regimes, and target those areas identified to regular flood or known to have blocked gullies. v Executive Summary Phase 4 Implementation & Review Phase 4 establishes a long-term Action Plan for LB of Hounslow to assist in their role under the FWMA 2010 to lead in the management of surface water flood risk across the Borough. The purpose of the Action Plan is to: Outline the actions required to implement the preferred options identified in Phase 3; Identify the partners or stakeholders responsible for implementing the action; Provide an indication of the priority of the actions and a timescale for delivery; and Outline actions required to meet the requirements as the Lead Local Flood Authority (LLFA) under the FWMA 2010.