Environmental Statement volume 4 - Chiltern Evergreen3

Greenfield runoff rates;Current Brownfield runoff volumes; andPost development runoff volumes.These runoff volumes are indicative of the storage that would need to beaccommodated by the Scheme’s SUDS design, which will be progressed within adetailed design Drainage Strategy, to reduce runoff to greenfield rates. Asreported within Section 1.2, and discussed further within Section 4.3 below, theachievement of runoff rates as close to greenfield rates as practicable at both thecatchment level and across the overall infrastructure installed is a commitment ofthe Scheme.The magnitude of greenfield runoff rates for a given event is dominated by thepermeability of soils and underlying substrates. The variation in rates presentedwithin Table 3 shows a clear progression from the high production of runoff forAPs located on impermeable clay soils through to lower production rates for APslocated on the Thames corridor alluvium and colluviums and the free drainingsoils over limestones in the lower Langford Brook.Relative to the size of the receiving sub-catchment the greatest changes in thecumulative impermeable extent are within the Langford Brook (recognising thecatchments are nested, i.e. the Langford Brook is a tributary of the River Ray,which in turn is a tributary of the River Cherwell; the River Cherwell is a tributaryof the River Thames). The largest post development extents (approximately 0.5hectares or greater), and consequently greatest post development runoff volumesare associated with:AP2 – Tubbs Lane Footbridge;AP3 – Bicester Town Station;AP6 – Elm Tree Farm/Langford Lane Overbridge;AP8 – Holts Farm Overbridge;AP11 and 11a – Islip Station and equestrian bridge;AP10 – Oddington Overbridge;AP14 – Gosford and Water Eaton; andAP19 – Oxford Station.Of these, the station APs have significant pre-development impermeable extentsand hence offer the greatest opportunities for betterment of current surface waterrunoff rates through the use of appropriate SUDS. The impermeable extentsassociated with the overbridges are primarily associated with the access roadsand, as discussed, the runoff from these will be reduced to greenfield ratesthrough the use of suitable SUDS, such as swales.The impermeable extents for the remaining APs (primarily footbridges) arenegligible. They have been included within the analysis for completeness and toallow the total runoff volumes for the scheme to be assessed.40