Environmental Statement volume 4 - Chiltern Evergreen3

The site is on previously-developed land. The land has an existing use as railwayinfrastructure.Given that the bridge footings, wingwalls and abutments are not being altered, itcan be determined that the structure will not obstruct flood flows across thefloodplain nor consume floodplain storage over and above the impact of theexisting structure.Chiltern Railways is able to confirm that these works need only involve an extendeddeck, and that the works will not change the cross sectional area or conveyance capacityof the structure. This commitment includes not increasing the conveyance as this mayhave unforeseen implications in other areas of the floodplain, possibly upstream in theCity of Oxford. This is discussed further within Section 3.6.A surface water runoff calculation for this site has not been provided for thisdevelopment as it is directly over a watercourse, as stated in Section 4.3.6 Summary of AssessmentsA summary of the further assessment requirements for each site, as designs develop ispresented within Table 1. Commitments that are provided for the scheme as a whole aredetailed as follows:Where a Level 3 FRA analysis is required, there is a commitment that the risks offlooding elsewhere will not be exacerbated, as measured by surface water runoff(see Section 4), floodplain storage and obstructing flood flows. The surface waterrunoff attenuation is discussed in more detail within Section 4. Where bridges arewithin the same floodplain, it may be possible to transfer flood storage requiredfrom one structure to another, within this overall commitment.At detailed design stage, the temporary flood risk posed by the constructionprocess will be evaluated and appropriate mitigation measures integrated withthe Code of Construction Practice.The Level 3 FRA analyses will be undertaken using data from the SFRA, assumingthat is completed or the relevant data is available. The SFRA may change thestatus of certain sites and remove the need for further analysis relating tofloodplain storage and obstruction to flood flows at these locations. All works in, over, or within 8 metres ([1]) of watercourses classified as MainRivers, together with culvert and any required impoundment works, will beconducted under appropriate Flood Defence Consents (FDCs) ([2]) and inaccordance with consent conditions.During early discussions with the Environment Agency, the potential for upsizingwatercourse crossings to achieve an increase in conveyance flows was discussed.The Agency confirmed that there were no current conveyance pinch points thatcould usefully be altered. At certain locations where bridges were currentlyundersized, such as Sheepwash Bridge, it was not advisable to increaseconveyance due to the risks associated with exacerbating flooding downstream.Crossings designs have therefore been progressed with a focus upon minimisingthe potential for disturbance to the water environment through the use of existingparapets and openings wherever possible. The ability to achieve a desired([1]) Unless a wider buffer is confirmed for a particular Main River.([2]) As prescribed by the Water Resources Act 1991, Land Drainage Act 1991 and Thames Water Authority Land Drainage Byelaws 1981.35