Environmental Statement volume 4 - Chiltern Evergreen3

commitments to implement drainage solutions once development designs have beenfinalised.For the sites lying in Flood Zone 1, there is no requirement for a Level 3 FRA to beundertaken beyond design level drainage planning.These sites are: AP2 – Tubbs Lane Footbridge; AP8 – Holts Farm Overbridge; AP9 – Oddington Footbridge No. 5; AP10 – Oddington Overbridge; AP11 and 11a – Islip Station and Equestrian Bridge; AP13 – Water Eaton No. 5 Overbridge; AP14 – Water Eaton Parkway; AP15 – Gosford and Water Eaton Footbridge No. 10; AP16 – Wolvercot Tunnel and First Turn Overbridge; and AP19 – Oxford Station.3.5 Sites Lying with Flood Zones 2 or 33.5.1 AP1 – Bicester ChordThe Bicester Chord crosses Flood Zone 3, and the Sequential Test has therefore beenapplied. There are no reasonably available sites for this particular developmentelsewhere, given that these two railway lines only intersect at this location.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 been designated forrailway use under current Local Development Framework (LDF) interim policy.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.It should be noted that planning permission has been given on appeal for a residentialdevelopment adjacent to Gavray Drive, immediately downstream of the proposedlocation for the Chord. This neighbouring development had initially been rejected on thebasis of the flood risk grounds. For the initial planning refusal, it was not demonstratednumerically that surface water runoff could be attenuated by relevant SUDS built intothe development.Given that the Langford Brook is already culverted through the site, it is unlikely that,given appropriate mitigation (flood storage and surface water runoff), there would be asignificant increase in flood risk due to this aspect of the scheme. This will beinvestigated further under the Level 3 FRA.The Langford Brook culvert passing through the existing Birmingham to London line is tobe extended by between 7.5 m from a current length of approximately 14 m toincorporate the new Chord extension. This culvert extension will be sized to ensure thatthe conveyance capacity is not reduced by the extension.The embankment is to be formed primarily from permeable aggregates. Runoff fromthese can be considered to take place at the greenfield rate. There is no need foradditional drainage provision to capture runoff over and above the greenfield runoff. Thefootbridge is classified as impermeable for runoff calculations.28