Environmental Statement volume 4 - Chiltern Evergreen3

2. The topographic model appears to be incorrect as flooding does not followexisting channels (e.g. immediately upstream of the A41 embankment adjacentto the railway).Given this uncertainty, it is recommended that a Level 3 FRA may be required, subject toa review at detailed design of the clarifications released within the forthcoming SFRA todetermine the impact of the extended platforms on floodplain flows. This is importantgiven the fact that the platforms extend beyond the Pingle Brook culvert: this issue willbe investigated further under a Level 3 FRA. Without mitigation in terms of flood storageand correct culvert design the development will potentially:decrease the flood storage volume of the area; andobstruct flood flows.A commitment has been made by the project that appropriate mitigation will be providedfor demonstrable loss of floodplain storage and obstruction to flood flows, in agreementwith the Environment Agency.Within the scope of the current work, a surface water runoff assessment is required,which is undertaken in Section 4. Given the extensive use of brownfield land for thisdevelopment, it is considered that there is significant potential for seeking reductions insurface water runoff from the current brownfield areas using SUDS as part of theredevelopment.Any SUDS incorporated will need to be of suitable type to minimise risks associated withany potential mobilisation of historic contamination and to reduce the potential for the‘washing out’ of open features by flood waters.3.5.3 AP4 – A41 OverbridgeThe development site lies within Flood Zone 3; a Sequential Test is therefore required.There are no reasonably available sites for this particular development elsewhere, giventhat the A41 and railway intersect at this site.The Exception Test must be satisfied, as the development is in Flood Zone 3.The site is on previously-developed land. The land has an existing use as railwayinfrastructure.A Level 3 FRA is required to demonstrate that the development will neitherobstruct flood flows across the floodplain nor consume floodplain storage; if notthen this work must quantify the mitigation required.Given that this work only entails lowering the track and widening the span of the bridgeto accommodate a third siding track, it might be the case that when detailed designs arefinalised that a re-assessment of the site may satisfy the Exception Test criteria withoutthe need for a detailed Level 3 FRA.There is uncertainty in the accuracy of the EA Flood Map in the area of the Pingle Brookand the A41 embankment. Evidence for this is explained above and this should be.subject to a review at the detailed design stage in light of the clarifications releasedwithin the forthcoming SFRA update.At present there is no requirement for surface water runoff calculations to beundertaken, as their will be neither structural changes nor a change in footprint as aconsequence of the development.30