Level 2 Strategic Flood Risk Assessment (.pdf ... - Wigan Council

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6 Cumulative impacts of future development anddrainage designA strategic appraisal of the impact of development within Wigan on downstream flood riskhas been undertaken.6.1 IntroductionCarefully planned development can play a role in reducing the number of properties at directrisk from surface water flooding. The planning system has a key role to play in settingsstandards for sustainable drainage from new developments and ensuring that developmentsare designed to take account of the risk from surface water flooding.6.2 Development Drainage ImpactsDevelopment within upstream local authorities has the potential to adversely affect flood riskwithin Wigan. Likewise, if site drainage is inappropriately designed, development withinWigan itself also has the potential to affect flood risk locally and to the downstream area.This is especially the case for the smaller tributaries of the major rivers and the LeedsLiverpool Canal that are culverted in places and especially sensitive to runoff fromdevelopments.The SFRA has undertaken an assessment of the impacts of development within Wigan onfluvial flood risk both locally and downstream. The SFRA has also considered the additionalimpact of development in the upstream catchments of the River Douglas catchments onfluvial flood risk in Wigan.The management of surface water flooding within Wigan and beyond is a cross boundaryissue. Flooding from canals is also a cross boundary issue, where water overtopping orbreaching from a canal in one local authority could lead to flooding in another. This isdiscussed in relation to the Leeds Liverpool Canal in Wigan in Chapter 3.6.3 Considering downstream impactsDevelopment has the potential to both increase and decrease surface water runoff and henceaffect flood risk downstream. The assumptions of this SFRA are based on the suppositionthat after development surface water would be temporarily attenuated on the respectivedevelopment sites in suitable sustainable drainage systems which mimic natural site drainage(this assumes greenfield rates). The introduction of such systems would attenuate the flowswhich would minimise flood risk. This is a likely scenario under current legislation andEnvironment Agency policy.It is paramount that any new development in the Wigan area incorporates suitable floodstorage measures in outline planning and development to avoid a scenario where afterdevelopment there would be no storage of surface water on the new development sites. Thishas the potential to both increase the rate and volume of surface water runoff into the sewernetwork and local watercourses, increasing flood risk downstream. In the current legislativeand policy environment this scenario is unlikely.The impact of the development on flood risk downstream has been based on a methodologyfor the impact on flood risk during a 1% AEP flood event, considering climate change. FloodEstimation Handbook (FEH) methods were used to calculated flood hydrographs and flows inthe river system.6.4 Wider impactsWhilst development control policies to reduce surface water discharges from newdevelopment could have some benefit locally, development in the wider catchments has an2009s0578 Final SFRA with Environment Agency Comments - L2 Ver5 Wigan.doc 31
important role to play in reducing flood risk in Wigan. This highlights the need for localauthorities both within AGMA and in the wider River Douglas catchment to work together toreduce flood risk through the planning process.2009s0578 Final SFRA with Environment Agency Comments - L2 Ver5 Wigan.doc 32
Page 1 and 2: Wigan Borough Hybrid SFRAVolume I:
Page 3 and 4: JBA OfficeJBA ConsultingSouth BarnB
Page 5 and 6: AcknowledgmentsJBA would like to ac
Page 7 and 8: Understanding flood risk from a pla
Page 9 and 10: 8.3 Planning considerations........
Page 11 and 12: AbbreviationsABDAreas Benefiting fr
Page 13 and 14: The key objectives of a SFRA are to
Page 15 and 16: 2 Flooding from RiversA detailed as
Page 17 and 18: 1. Astely Brook2. Bedford Brook3. W
Page 19 and 20: Unique ID Watercourse Description L
Page 21 and 22: 2.7 Understanding flood risk from a
Page 23 and 24: For example, flooding may affect ke
Page 25 and 26: 3 Flooding from CanalsThe SFRA has
Page 27 and 28: 3.2.2 Canal breach methodologyCanal
Page 29 and 30: 4 Flooding from ReservoirsFollowing
Page 31 and 32: direction in Future Water3, Making
Page 33 and 34: oundaries. Nine key CDAs have been
Page 35 and 36: CDA Sites Site TypeLark Hill, Astle
Page 37: particular hotspots where surface w
Page 41 and 42: In the reverse situation, where flo
Page 43 and 44: 8 Summary of riskA summary of flood
Page 45 and 46: Having followed this procedure it i
Page 47 and 48: 9 Outline Mitigation OptionsChapter
Page 49 and 50: Water-resistant materials for floor
Page 51 and 52: JBA Consulting - Engineers Scientis
Page 53 and 54: JBA Consulting - Engineers Scientis
Page 55 and 56: Table B-1: Flood Risk Balance Sheet
Page 57 and 58: Map Name Map Reference Map Title De
Page 81 and 82: D . Wigan Council - Data RegisterXX
Page 83 and 84: E . SUDs SuitabilityNew SiteReferen
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New SiteReferenceWig 728Wig 824Wig
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New SiteReferenceWig 716Wig 706BSit
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Site Reference Site Location Soil T
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F . Flood Zone 3 Proposed Developme
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G . Environment Agency comments on
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Flood Hazard MappingThe Level 2 rep
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Planning LiaisonEnvironment Agency
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Guide makes further summation diffi
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Wigan Metropolitan Borough CouncilC
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of the extent of any works required
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Planning & Transport StrategyWigan
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Again, avoiding generalities, we ha
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EA comment:Guidance on the preparat
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Please give me a call to discuss wh
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RECORD OF MEETINGStatus:JBA Project
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