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

In the reverse situation, where floodwaters from a river enter the canal network, the effectsare likely to be two-fold. Firstly, the canal may be able to convey the flood waters away fromthe interaction site and possibly outside of the expected fluvial flood extent. This excess floodwater may then spill from the canal resulting in flood risk, possibly some distance from theriver. Secondly, the canal may provide additional flood storage, as well as conveying someflow. The result may well be a reduced flood extent along the river downstream of theinteraction.For this study possible interaction locations between rivers and canals have been identifiedusing a GIS desk-based approach. The Environment Agency's Flood Zone 2 and a canalhazard zone (produced from breach and overtopping modelling scenarios) for the SFRA havebeen used. Local knowledge and data from OS mapping were also used to provideadditional information where possible.Table 7-1 summarises locations within the study limits where interactions betweenwatercourse and the Leeds Liverpool and Bridgewater Canals are considered possible. Anyfuture studies in these areas should consider how these interactions may affect theirobjectives.Table 7-1: Canal River InteractionsWatercourseNameBedfordBrookAstley BrookWhiteheadBrookWestleighBrookCommonLane BrookHey BrookInce BrookPoolstockBrookSummaryThe Bridgewater Canal passes over Bedford Brook in an aqueduct. Should thisaqueduct or adjacent embankments overtop or breach then flood water from thecanal would flow into Bedford Brook and into Hooten Gardens, Leigh and out acrossthe natural floodplain (farmland). Flow from the river into the canal is not consideredpossible due to the elevated position of the canal.It is possible that overtopping or breach of the Bridgewater Canal would result inadditional water flowing into Astley Brook, resulting in the downstream flooding ofMoss Side. It is not considered possible for flow from Astley Brook to enter the canalat this location because of the elevation difference.It is possible that overtopping or breach of the Bridgewater Canal at Whitehead wouldresult in additional water flowing into the Brook. It is not considered possible for flowfrom Whitehead Brook to enter the canal at this location because of the elevationdifference.It is possible that breach of the Leeds Liverpool Canal at Firs Lane would result inadditional water flowing across the site and into Westleigh Brook, potentiallyincreasing flood risk at Pennington. It is not considered possible for flow fromWestleigh Brook to enter the canal at this location as the canal runs along a raisedembankment.In the event of the raised embankment of the Leeds Liverpool Canal breachingfloodwaters would enter the brook, adding to the flow through The Flash atPennington Flash Country Park. It is not considered possible for flow from CommonLane Brook to enter the canal at this location because of the elevation difference.It is possible that breach of the Leeds Liverpool Canal around the confluence withHey Brook would result in additional water flowing into the river and PenningtonFlash. It is not considered possible for flow from Hey Brook to enter the canal at thislocation because of the elevation difference. There has been past flooding of HeyBrook in Abram in 1944 reported as a leakage leading to embankment failure.Excess flows along the Leeds Liverpool Canal spill into Ince Brook to the west ofWorsley Mesnes. It is also considered possible for flow from Ince Brook to enter thecanal at this location due to the low elevation difference. Both of these scenarioswould lead to the flooding of Scotsman's and Pearson's Flash.It is possible that overtopping or breach of the Leeds Liverpool Canal at Poolstockwould result in additional water flowing into Poolstock Brook which leads to the RiverDouglas. There has been past overtopping at Poolstock Lock in April 2009, reported2009s0578 Final SFRA with Environment Agency Comments - L2 Ver5 Wigan.doc 34