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

3 Flooding from CanalsThe SFRA has undertaken a strategic assessment of breach from the Leeds - LiverpoolCanal and Bridgewater Canal.3.1 IntroductionThe Leeds and Liverpool Canal (LLC) is fed by gravity from Foulridge reservoir, which islocated at the head of the canal system. The LLC flows from Chorley to Wigan in onecontinuous pound (a stretch of canal between two locks) before dropping, at Top Lock, intoWigan via a series of 22 lock gates. At the bottom of the valley the canal splits with the leftbranch flowing eastward towards Leigh (Leigh Branch) and the right flowing westward toLiverpool.The LLC is an important link with other waterways. The Rufford Branch connects the RiverDouglas and the River Ribble (via the Ribble Link) to the Lancaster Canal. The BridgewaterCanal runs east in one continuous 43 mile long pound from Leigh to Ellesmere Port. Only asmall section of this pound is located within the Wigan borough as the Leeds and Liverpoolcanal effectively becomes the Bridgewater canal to the east of Leigh.The interactions of the canal and the main rivers are integral to the understanding of flood riskin the area. British Waterways is a key stakeholder in the management of canals and hasbeen consulted throughout the SFRA process.As shown on the associated canal breach mapping, as listed in Appendix C, we havereviewed a number of potential breach failures along the Leeds Liverpool Canal through theWigan borough. Canal breach assessments have been identified based on areas of raisedembankments and do not take the structural integrity of the embankment into account orquantify a risk of failure. Flooding may occur at any location along the canal system wherethere is a raised embankment. Canal breach analysis is therefore indicative and digital plansonly have been submitted as part of this SFRA.3.2 Flood risk from canalsCanals do not generally pose a direct flood risk as they are a regulated water body. Theresidual risk from canals tends to be associated with lower probability events such asovertopping and embankment failure.The residual risk associated with canals is more difficult to determine as it depends on anumber of factors including, for example, the source and magnitude of surface water runoffinto the canal, the size of the canal, construction materials and level of maintenance.The probability of a breach is managed by continued maintenance. High embankments areknown as Principal Embankments and British Waterways have more stringent managementregimes in these areas.No attempt is made in this SFRA to assess the probability of failure other than noting thatsuch events are very rare. However, in accordance with PPS25, all sources of flooding needto be considered. If a breach event were to occur then the consequences, to people andproperty, could be high. In order to understand the possible impacts, a series of breachmodels have been generated for this SFRA. It should be noted that the canal breachlocations have been identified based on areas of the canal that includes raisedembankments. The mapping is intended to provide an indication of the likely impact ofselected failure scenarios. It is not intended that inundation mapping provides acomprehensive analysis of all failure scenarios and further site specific analysis will need tobe considered at all sites located within the vicinity of a canal system. Developers should beaware that any site that is at or below canal bank level may be subject to canal flooding andthis should be taken into account when building resilience into low level properties.2009s0578 Final SFRA with Environment Agency Comments - L2 Ver5 Wigan.doc 18