SURFACE WATER MANAGEMENT PLAN DRAIN LONDON LONDON BOROUGH OF WANDSWORTH Quality Management Quality Management DOCUMENT INFORMATION Title: SWMP for the London Borough of Wandsworth Owner: Version: 1.0 Status: Final Project Number: D134785 File Name: DLT2-GP7-SWMP-WANDSWORTH-Report-v1pt0-FINAL.doc REVISION HISTORY Date of Summary of Changes Completed By Version Issue Draft Report (v0.1) updated with Wandsworth Borough Sarah Kelly 14/09/2011 1.0 Council, GLA and Peer Review Comments AUTHOR Name Organisation and Role Sarah Kelly URS Scott Wilson – Project Manager Gemma Hoad URS Scott Wilson APPROVALS Name Title Signature Date Jon Robinson PD of URS Scott Wilson 14/09/2011 Sarah Kelly PM of URS Scott Wilson 14/09/2011 Rob Sweet URS Scott Wilson Reviewer 19/05/2011 Jon Robinson URS Scott Wilson Approver 25/05/2011 DISTRIBUTION Name Organisation and Role Kevin Reid Drain London Project Manager, Greater London Authority RELATED DOCUMENTS Date of Doc Ref Document Title Author Version Issue GP7-PFRA­ PFRA for London Borough of URS Scott Wilson June 2011 1.0 WANDSWORTH Wandsworth Executive Summary Executive Summary This document forms the Surface Water Management Plan (SWMP) for the London Borough of Wandsworth 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 for the London Borough of Wandsworth and includes consideration of flooding from sewers, drains, groundwater and runoff from land, small watercourses and ditches that occurs 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; Phase 1 – Preparation; Phase 2 – Risk Assessment; Phase 3 – Options; and Phase 4 – Implementation and Review. Phase 1 Preparation Phase 1 builds upon work formerly undertaken during Tier 1 of the Drain London Project to collect and review surface water data from key stakeholders and build partnerships between stakeholders responsible for local flood risk management. As part of the Drain London project, the London Borough of Wandsworth has been grouped with the London Boroughs of Lambeth, Merton and Southwark to undertake Tier 2 of the project and work together to understand local flood risk. The London Borough of Wandsworth has begun to establish a broader partnership with neighbouring London Boroughs in south west London, through the establishment of the South West London Strategic Flood Group, in order for these local authorities to pool 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 London Borough of Wandsworth, 13 CDAs have been identified; these are shown in Figure 1. Version 1.0 – Final Report Page i August 2011 Executive Summary The chief mechanisms for flooding in the London Borough of Wandsworth can be broadly divided into the following categories: • River Valleys - Across the study area, the areas particularly susceptible to overland flow are formed by the river valleys of the River Wandle and River Graveney, and / or along narrow corridors associated with topographical valleys which represent the routes of the ‘lost’ rivers of London including the Falcon Brook. • Low Lying Areas - areas such as underpasses, subways and lowered roads beneath railway lines are more susceptible to surface water flooding; • Railway Cuttings - four stretches of mainline railway track (in cuttings) are susceptible to surface water flooding and, if flooded, will impact services into and out of Clapham Junction Railway Station from southwest London and beyond; • 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 Borough); 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 including the areas of Falcon Road, South Balham and Putney. • Fluvial / Tidal (River Wandle) Flood Risk – areas where extensive and deep surface water flooding is likely to be the influence of fluvial / tidal flooding mechanisms (alongside pluvial, groundwater and sewer flooding sources) including Earlsfield, Summerstown, Furzedown and Tooting Graveney. 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 (1% Annual Exceedance Probability, AEP). A review of the results demonstrate that 41,770 residential properties and 3,280 non-residential properties in the London Borough of Wandsworth could be at risk of surface water flooding of greater than 0.03m depth during a 1% AEP rainfall event. Of those, 630 residential properties and 35 non­ residential properties could be at risk of flooding to a depth of greater than 0.5m during the same modelled rainfall event. A review of these statistics coupled with local knowledge of the study area identifies that the following CDAs are at greatest risk of significant flooding (greater than 0.5m deep) from the 1% AEP rainfall event: Table 1 - CDAs at Greatest Risk of Surface Water Flooding in London Borough of Wandsworth Flooded Receptors (>0.03m) Flooded Receptors (>0.5m) CDA Non- Non- Residential Total Residential Total Residential Residential Group7_022 (Clapham Junction) 4,828 405 5,233 181 12 193 Group7_024 (Tooting Bec) 513 20 533 60 0 60 Group7_020 (South Balham) 1,792 117 1,909 58 1 59 Group7_018 (Summerstown) 3,392 218 3,610 50 0 50 Group7_014 (Putney Heath) 586 99 685 17 14 31 Group7_023 (Lavender Hill) 2,925 388 3,313 23 4 27 Within the London Borough of Wandsworth, the greatest number of receptors are at risk from surface water flooding along the route of the ‘lost’ Falcon Brook, which runs north to south through the east of the Borough (Group7_022 (Clapham Junction) CDA). Significant ponding of surface water (>0.5m) is modelled to impact Tooting Bec (Group7_024), Summerstown (Group7_018) and South Balham Version 1.0 – Final Report Page ii August 2011 Executive Summary (Group7_020). Historic surface water flooding records from July 2007 (supported by Wandsworth Borough Council staff knowledge) support the pluvial modelling flooding in the Falcon Road (Clapham Junction area), South Balham and Lavender Hill areas. The Clapham Junction and Tooting Bec CDAs are impacted from upstream surface water flows from the London Borough of Lambeth, and it will therefore be important that the flood risk is managed at a catchment scale by both Councils. A number of CDAs within the London Borough of Wandsworth are cross boundary, and as such will need to be jointly managed to implement the potential options and manage surface water flood risk in these areas. These include: Table 2 – Cross Boundary CDAs in London Borough of Wandsworth CDA Lead Borough Supporting Borough Group7_018 (Summerstown) Wandsworth Merton Group7_022 (Clapham Junction) Wandsworth Lambeth Group7_024 (Tooting Bec) Wandsworth Lambeth Group7_025 (Streatham Common) Lambeth Wandsworth Group7_028 (Nine Elms) Lambeth Wandsworth Phase 3 Options Assessment There are a number of opportunities for measures to be implemented across the Borough to tackle surface water flood risk. 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 London Borough of Wandsworth 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 SuDS and water conservation measures such as water butts and rainwater harvesting technology. In addition, there are Borough-wide opportunities to raise community awareness and improve resilience to flooding. 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 alongside recommendations for further investigation where appropriate. Pluvial modelling undertaken as part of the SWMP has identified that flooding within the London Borough of Wandsworth is heavily influenced by existing and historic river valleys, and impacts a number of regionally important infrastructure assets. To address local flood risk in the London Borough of Wandsworth it is recommended that, in the short to