LONDON BOROUGH of HAMMERSMITH and FULHAM UPDATED SURFACE WATER MANAGEMENT PLAN 47065080 April 2015 I
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London Borough of Hammersmith and Fulham Updated Surface Water Management Plan April 2015 UNITED KINGDOM & IRELAND Prepared for: 47065080 REVISION SCHEDULE Rev Date Details Prepared by Reviewed by Approved by 1 December Draft for client comment Danielle Skilton Emily Craven Jon Robinson 2013 Flood Risk Principal Operations Consultant Consultant Director 2 January Final Draft Danielle Skilton Emily Craven Jon Robinson 2014 Flood Risk Principal Operations Consultant Consultant Director 3 May 2014 Final Danielle Skilton Emily Craven Jon Robinson Flood Risk Principal Operations Consultant Consultant Director 4 June 2014 Final Revision Danielle Skilton Jon Robinson Jon Robinson Flood Risk Operations Operations Consultant Director Director 5 April 2015 Updated with revised Sarah Littlewood Matthew Ince Michael Timmins modelling for client comment Consultant Principal Flood Technical Director Risk Consultant URS Infrastructure & Environment UK Ltd. 6-8 Greencoat Place London SW1P 1PL United Kingdom Telephone: +44(0)20 7798 5000 Fax: +44(0)20 7798 5001 LONDON BOROUGH OF HAMMERSMITH AND FULHAM UPDATED SURFACE WATER MANAGEMENT PLAN 47065080 April 2015 i Limitations Please note that, as of October 2014, AECOM and URS have joined together as one company. Whilst AECOM and URS have become one company, contracting entities (all of which are now wholly owned by AECOM) and lines of communication currently remain the same unless specifically agreed or communicated otherwise. Capita and URS Infrastructure & Environment UK Limited (“Capita URS”) has prepared this Report for the sole use of the Royal Borough of Kensington and Chelsea and the London Borough of Hammersmith and Fulham (“Client”) in accordance with the Agreement under which our services were performed (September 2012). No other warranty, expressed or implied, is made as to the professional advice included in this Report or any other services provided by URS. This Report is confidential and may not be disclosed by the Client nor relied upon by any other party without the prior and express written agreement of URS. 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LONDON BOROUGH OF HAMMERSMITH AND FULHAM UPDATED SURFACE WATER MANAGEMENT PLAN 47065080 April 2015 ii Executive Summary EXECUTIVE SUMMARY This document forms the updated Surface Water Management Plan (SWMP) for the London Borough of Hammersmith and Fulham (LBHF). This document is a plan which outlines the preferred surface water management strategy for the Borough including 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 and Tier 2 packages of work and has been undertaken following a four phase approach (as outlined in the Defra SWMP Technical Guidance, March 2010); 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 and Tier 2 of the Drain London Project to collect and review surface water flood risk data from key stakeholders and build partnerships between stakeholders responsible for local flood risk management. Phase 2 Risk Assessment As part of Phase 2 Risk Assessment, direct rainfall modelling has been undertaken across the entire Borough for eight specified return periods. The results of this modelling have been used to identify flooding hotspots where flooding affects houses, businesses and/or infrastructure. The methodology for defining flooding hotspots specifically in LBHF is defined in Section 3.3. In order to enable a more focused assessment of the surface water flood risk across the Borough, analysis of the modelling results and the number of properties at risk has been undertaken based on the 16 wards within the Borough, as identified in Figure 1. Figure 1 – Maximum surface water flood depth (1% AEP) and ward boundaries The chief mechanisms for flooding in the London Borough of Hammersmith and Fulham (LBHF) can be broadly divided into the following categories: Runoff from higher topography - The areas of greatest flood depths tend to be at the base of the steeper land to the north of the Borough. Localised surface water runoff – Within the central and southern parts of the borough, surface water flooding tends to be a result of localised ponding of surface water. Sewer Flooding – areas where extensive and deep surface water flooding is likely to be influenced by sewer flooding. Where the sewer network has reached capacity and surcharged, this will exacerbate the flood risk in these areas. 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 that pass across the Borough; Railway Embankments - discrete surface water flooding locations along the up-stream side of the raised network rail embankments where water flows are interrupted and ponding can occur. LONDON BOROUGH OF HAMMERSMITH AND FULHAM UPDATED SURFACE WATER MANAGEMENT PLAN 47065080 April 2015 i Executive Summary Analysis of the number of properties predicted to be at risk of surface water 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 identifies that 7,059 residential properties and 889 non-residential properties in the LBHF could be at risk of surface water flooding of greater than 0.1m depth during a 1% AEP rainfall event. Of those, 845 residential properties and 64 non-residential properties could be at risk of flooding to a depth of greater than 0.5m during the same rainfall event. Table 1 provides the rank order of the Wards, based upon the number of properties at risk and the number of Council records of flooding within each Ward. Wards have been given ranked from most at risk (1) to least at risk (16). Table 1 Analysis of surface water risk across LBHF Wards (1% AEP) Ward Ward ID Rank Askew (4) 1 Hammersmith Broadway (6) 2 Addison (7) 3 Town (14) 4 Ravenscourt Park (5) 5 Shepherd’s Bush Green (3) 6 Parsons Green and Walham (15) 7 Munster (12) 8 Wormholt and White City (2) 9 North End (10) 9 Fulham Broadway (13) 11 Fulham Reach (9) 12 Sands End (16) 13 Avonmore and Brook Green (8) 14 College Park and Old Oak (1) 15 Palace Riverside (11) 16 Analysis of this information by Ward indicates that flood risk is greatest in the western central part of the borough, including Askew, Wormholt and White City, Hammersmith Broadway and Addison. There is also a notable risk further south in the Town and Parsons Green and Walham. The Council records of flooding support the findings of the surface water modelling, with significant number of records being reported in Askew, Parsons Green and Walham, Hammersmith Broadway and the Town. Flooding in Wormholt and White City and Askew is influenced by overland flows from the London Borough of Ealing. It is therefore important that the Council work in partnership to manage flood risk at a catchment level in this area. LONDON BOROUGH OF HAMMERSMITH AND FULHAM UPDATED SURFACE WATER MANAGEMENT PLAN 47065080 April 2015 ii Executive Summary Phase 3 Options Assessment There are a number of opportunities for flood risk management measures to be implemented across the Borough to tackle surface water flood risk. In addition, opportunities to raise community awareness of the risks and responsibilities for residents should be sought, and LBHF may wish to consider the implementation of a Communication Plan to assist with this. 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 Wards, site-specific measures have been identified that could be considered to help manage surface water runoff across the Borough. Four options were shortlisted to be incorporated into the baseline modelling.