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

Douglas CFMP and Mersey Estuary CFMP with regard to maintenance and upgrade.Supporting material for the schedule to be provided by the consultant will include walkoversurvey records, as-build records where available, a directory of photos, and locationplans”.We feel that this information is important to inform future development within the boroughof Wigan. Currently the SFRA does not achieve this objective of the brief.The EA’s comments here are incorrect as we have included consideration of the defencedesign standards in our analysis. We have also included all available details, includinglocation plans and asset information in the SFRA.As explained our initial response, the EA have chosen not to provide the NFCDDcondition rating or any as-built records for their defences. The EA must already havesignificant elements of this information as part of the River Douglas FAS.It is not the purpose of an SFRA to generate a new defence database, this after all is theEA’s responsibility. As discussed in our previous letter, the key outcome of our analysisis that the proposed development sites in Wigan are not afforded protection from discreetand continuous defences. This combined with the generally low standard of defencesinfluences significantly the mechanism of flooding at your identified sites. Theimplications of this and our approach to assessing risk should be the key messages thatthe EA focus on.Whilst we could provide a “directory of photos” these would obviously be taken atselected locations. We are not sure what value this would add to the SFRA.Detailed assessment, possibly including structural analysis, will be required at detailedFRA stage to confirm the level of protection during lower return periods if defences aresubsequently to be relied upon. However, evaluation will need to be based on the EA’slatest modelling for the Douglas FSA when this becomes available.EA comment:Likely Future Flood Management PolicyJBA state that “information may also be available from the Catchment Flood ManagementPlan”. We would again refer the Council to the above paragraph (2.7 a) from the briefwhich expects the Level 2 Report to contain links / reference to the relevant parts of boththe Douglas CFMP and Mersey Estuary CFMP.The links to the CFMPs have been established in the SFRA and we do not intend toinclude further duplication. Although the CFMPs, SFRA, PFRA and SWMP are discretedocuments, the recommendations of each will need to be taken into account by theCouncil.EA comment:Probability and Consequence of Defence Overtopping and FailureWe note the assumption made by JBA in the assessment of breach modeling and thatthe defence within the borough are likely to be overwhelmed. Whilst it would have beenuseful to look at breach scenarios at lower return periods, we accept that the use of the1% AEP flood event + 20% peak flow is in line with paragraph 3.63 of the Practice Guideand we would not want to pursue this issue further.2009s0578-S-L007-1.doc 2/710 December 2010www.jbaconsulting.co.uk