Environmental Statement volume 4 - Chiltern Evergreen3

The existing EA Flood Map of the area is likely to be unreliable given that Flood Zones 2and 3 do not follow the path of the river. The site lies mostly in Zone 1, but extends outinto Zone 2 and 3. This will be investigated further under a Level 3 FRA at detaileddesign. Without mitigation in terms of flood storage and correct culvert design thedevelopment may 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. As previously confirmed the provision of land for thispurpose, based upon a reasonable worst case approach, has been included within themaximum land take, the LOD and LLAU, for the Scheme.At the detailed design stage, an assessment of the loss of flood storage and the requiredsizing for the culvert will be undertaken using a methodology to be approved by theEnvironment Agency. The culvert will be sized to ensure that the conveyance capacity ofthe current culvert is not reduced. If necessary, lost floodplain storage will be mitigatedby creating flood storage through scrapes or storage ponds.As stated above, a surface water runoff assessment is required, but covering theimpermeable bridge structure only. It is suggested that the construction of theembankment provides the opportunity for minimising any runoff that might occur duringvery intense storm events. It is therefore recommended that swales are provided alongthe foot of the embankment to attenuate topographically-driven runoff.3.5.2 AP3 – Bicester Town StationThe Bicester Town development site lies almost entirely within Flood Zone 1, but withthe very south-western edge lying in Flood Zone 3; the Sequential Test has thereforebeen applied. There are no reasonably available sites for this particular developmentelsewhere, given that the station exists at this site.The Exception Test must also 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 the fact that the work will entail refurbishing an existing series of buildings,implications for increasing flood risk are thought to be minimal, as the size of thedevelopment is not increasing from the existing land-take areas bar the drop off areathat is located entirely within Flood Zone 1. This location offers some potential forincorporating SUDS within the redevelopment in order to decrease the overall flood riskfor the site. To this end, a significant reduction towards the greenfield runoff rate shouldbe the aim of the incorporation of SUDS into the final station designs.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 twofold:1. The existing A41 embankment does not appear to constrict flood flows in areaswhere this would be expected; and29