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

Planning LiaisonEnvironment Agency - North West RegionAppleton House430 Birchwood BoulevardBirchwoodWARRINGTONWA3 7WDFor the attention of Helen TelferOur Ref: RW\2009s0578-S-L005-1.doc19 November 2010Dear Sirs,Wigan Borough Level 2 SFRAThank you for your comments received by email on 15 th November 2010 with regards tothe Wigan Borough Council SFRA. We have included our response, on behalf of theCouncil, addressing each of your points below using the same subheadings as used inyour letter reference SO/2006/000257/SF-01/IS1-L01 dated 11 November 2010.1) Level 2 SFRA ReportDevelopment behind Defences• Current Condition of Flood DefencesUnfortunately, the NFCDD data received for this study does not contain condition ratingsfor flood defences in Wigan. Therefore an existing condition assessment of flood defenceinfrastructure has not been undertaken for this SFRA.• Likely Future Flood Management PolicyFuture maintenance of flood defences and assets should follow the policy as set down inthe relevant Environment Agency flood risk management strategy. Information may alsobe available from the Catchment Flood Management Plan.• Probability and Consequence of Defence Overtopping and FailureThe design standard of the 42 raised defences within the Wigan Borough is typically 40 to50 years (6 have a 40 year design standard. The remaining 36 have a 50 year designstandard). During the 1% AEP design flood event, these formal defences will thereforeonly provide limited protection and are therefore likely to be overtopped or flood frombank level exceedance elsewhere in the catchment. Under the climate change scenario(i.e. 1% AEP flood event + 20% peak flow) more extensive flooding would be expected tooccur.During the 1% AEP design flood event all the defences within Wigan are therefore likelyto be overwhelmed. Using undefended modelling scenarios (as used in this SFRA) willmost accurately represent the extent of flood risk within Wigan and specific breachmodelling would not improve understanding.