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

More documents

Recommendations

Info

2.4 Areas Benefiting from DefencesThe Environment Agency Flood Map shows that areas of Wallgate and Poolstock in centralWigan benefit from the Douglas defences in a flood with a 1% (1 in 100) chance of occurringin any year. These areas would flood if the defences were not present, but do not floodbecause of the presence of the defence structure. Map number D015-001 in Appendix Cdisplays all Areas Benefiting from Defences in the Wigan area.2.5 Douglas CFMP Account of Historical Flooding"The Douglas catchment has a history of flooding but the most notable events were in August1987 and October 2000 when Croston village flooded affecting around 200 properties. Theflow in 2000 was recorded and estimated to be the equivalent of a 0.5% APE or 1 in 200 yearevent. The Yarrow broke out of its banks upstream and the defences stopped the waterreturning to the river. There have been flood events in Chorley, Wigan and Leyland but recentflood alleviation works have mitigated the level of property damage after more damagingfloods in the past".The main sources of flooding in the Douglas catchment are as follows:"River flooding is mainly from three rivers; the Douglas affects Wigan, the Yarrow affectsCroston and Chorley and the Lostock affects Leyland and Whittle Le Woods. RivingtonReservoirs play an important role in regulating flow. When they are not full they trap waterflow from the upper catchment and when full they slow the water flow. This reduces floodpeaks on the Yarrow and Douglas. In urban areas, culverts and other flow restrictions canmake the flooding worse as flow backs up behind these obstacles and flow out of bank orchannel. Parts of the catchment are tidally influenced; fluvial flood risk can be made worse bytide locking. River water cannot flow out to sea due to a high tide."http://publications.environment-agency.gov.uk/pdf/GENW0309BPTQ-E-E.pdf2.6 Defended Flood Zone 3 areasThe EA maps provide an indication of the likely extent of flooding in the absence of flooddefences. The flood limiting impact of defences can be determined with reference to the"Areas Benefiting from Defences" mapping. In Wigan, existing defences generally assist inmanaging flood water to a 2% or 2.5% AEP event scenario.The EA do not have any detailed mapping that defines areas of functional floodplain (Zone3b). The SFRA includes mapping of potential FZ3b areas (and associated hazard mappingdata) in the absence of defences. Flood mapping of a potential 5% AEP event, in theabsence of defences is included as drawing number D001_010a. All other mapping includesfor the presence of flood defences in accordance with the design maintenance outlined in theEA NFCDD database.Inspection of the defence asset data provided by the EA indicates that the majority ofwatercourses within the Wigan area are defended to some extent in excess of the 5% AEPcriteria. It should be noted that issues such as structural integrity of these defences,variations in crest levels and levels of maintenance have not been considered as part of theSFRA.Areas of low-lying land behind defences, which are defended to a level in excess of the 5%AEP criteria, have been classified as FZ3. The FZ3 categorisation allows the Council toconsidered development proposals within existing defended areas. Residual risks associatedwith proposed development, located behind defences, needs to be considered in detail asthese risks are potentially significant due to the rate and depth of inundation if a defence wereto fail.Inspection of the mapping including "Areas Benefiting from Defences" indicates that themaximum extent of Flood Zone 3 is not influenced, to any significant extent, by the presenceof existing defences.2009s0578 Final SFRA with Environment Agency Comments - L2 Ver5 Wigan.doc 13
2.7 Understanding flood risk from a planning perspectiveThis Level 2 SFRA provides an overview of flood risk from a planning perspective to aid theCouncil when undertaking the Exception Test. The SFRA presents a summary of strategicsites within Flood Zone 3, summarised in Table 2-2 below. A full list of the Council's strategicsites and their flood risk issues can be found in Appendix A .For each of these sites, an outline mitigation strategy has been prepared, which providesadvice on how development could proceed in flood risk areas and be compliant with therequirements of PPS25. The SFRA has assessed the likelihood of strategic developmentsites passing the Exception Test.Table 2-2: Summary of Flood Risk to Development Sites in Flood Zone 3WiganSiteReferenceWatercourse and %FZ3Council CommentWig 161 River Douglas 71%Note: Extent and scale ofresidual risk to bedetermined followingrelease of the EAmodelling for the RiverDouglas FloodAlleviation Scheme.Wig 137 River Douglas 98%Note: Extent and scale ofresidual risk to bedetermined followingrelease of the EAmodelling for the RiverDouglas FloodAlleviation Scheme.Wig 131 River Douglas 82%Note: Extent and scale ofresidual risk to bedetermined followingrelease of the EAmodelling for the RiverDouglas FloodAlleviation Scheme.This location is a major existing developed site in the GreenBelt (Leyland Mill) and is a conversion opportunity.Conversion for residential use would need to be restricted toupper floors unless it can be demonstrated that flood riskcan be effectively mitigated and site access maintainedduring the design flood event.Of the sites remaining Wig 137 is the only significant site(and is already under EA consideration). The EckersleyMills and former bus garage site is considered a keyregeneration site. Large conversion opportunity of existingmajor mills complex with opportunity for new build at theformer bus garage site. Flood defence measures arecurrently under review including consideration of floodcompatible uses on ground floor.Surface level car park at Chapel Lane, gas works and depotoff Darlington Street. More Vulnerable uses, such asresidential, shall be limited to the 18% of the site withinFlood Zone 1. Proposals for residential development inother areas of the site shall be restricted to upper floors,unless it can be demonstrated that flood risk can beeffectively mitigated and site access maintained during thedesign flood event.Wig 694 Millingford 64% There is no proposal for commercial development at thissite but the Council have had previous application enquiriesin the past and, as a town centre site, would strongly prefera (Less Vulnerable) commercial development. Proposeddevelopment would ideally be an extension of the adjacentshopping centre. Commercial opportunity site on 'stilts'atlevel of Gerard Centre, with car parking / servicingunderneath.Wig 047 Chanters Brook 53% Victoria Mill, Conversion for residential use would need tobe restricted to upper floors. Would only be granted if EAsatisfied of flood risk measures. Wig 060 is adjacent landwith potential for new-build to cross-subsidise theconversion. We can assume that only 36% of site Wig 060is available for development.Wig 060 Chanters Brook 64% Assume only 36% (i.e. area outside of Flood Zone 3) is2009s0578 Final SFRA with Environment Agency Comments - L2 Ver5 Wigan.doc 14
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: Unique ID Watercourse Description L
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 and 38: particular hotspots where surface w
Page 39 and 40: important role to play in reducing
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 71 and 72:
Map Name Map Reference Map Title De
Page 73 and 74:
Page 75 and 76:
Page 77 and 78:
Page 79 and 80:
Page 81 and 82:
D . Wigan Council - Data RegisterXX
Page 83 and 84:
E . SUDs SuitabilityNew SiteReferen
Page 85 and 86:
New SiteReferenceWig 354Site Addres
Page 87 and 88:
New SiteReferenceWig 855Wig 857Wig
Page 89 and 90:
Page 91 and 92:
New SiteReferenceWig 716Wig 706BSit
Page 93 and 94:
Page 95 and 96:
Page 97 and 98:
Site Reference Site Location Soil T
Page 99 and 100:
F . Flood Zone 3 Proposed Developme
Page 101 and 102:
G . Environment Agency comments on
Page 103 and 104:
Flood Hazard MappingThe Level 2 rep
Page 105 and 106:
Planning LiaisonEnvironment Agency
Page 107 and 108:
Guide makes further summation diffi
Page 109 and 110:
Wigan Metropolitan Borough CouncilC
Page 111 and 112:
of the extent of any works required
Page 113 and 114:
Planning & Transport StrategyWigan
Page 115 and 116:
Again, avoiding generalities, we ha
Page 117 and 118:
EA comment:Guidance on the preparat
Page 119 and 120:
Please give me a call to discuss wh
Page 121 and 122:
RECORD OF MEETINGStatus:JBA Project
show all

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

Create successful ePaper yourself

Delete template?

Save as template?