Longstanton Park and Ride, Cambridgeshire
Cambridgeshire County Council was required to build a park and ride facility in Longstanton, comprising approximately 18100m² of access roads, car parking bays and external hard standing areas. The surface water system needed to be designed to manage below ground a 1 in 100 year storm event plus 20% allowance for climate change. The outfall from the site was an existing manhole, with an invert level of 7.493mAOD and there was a requirement to limit the surface water runoff entering it from the park and ride development to 10l/s. The ground water level on the site was relatively high at 7.945 and all attenuation tanks were located above this level and encapsulated in impermeable membrane to prevent water infiltrating out of the tanks and into the groundwater. As the ground water table is within 1m of the proposed attenuation tanks, the use of infiltration as a method of dealing with the surface water runoff was considered inappropriate.
The surface water runoff within the car park is collected by pervious paving and stored within either a Permavoid attenuation tanks or the modified granular sub-base. In all but the most intense storm events, the Permavoid provides all the storage, with the modified granular storage providing a conveyance route for the stormwater into the Permavoid. The surface water runoff from the access roads adjacent to the car parks is collected by gullies connected to mini silt/oil separators, which discharge into the Permavoid attenuation tanks beneath the pervious paving, providing run-off attenuation for the access roads. The bus drop off and pick up area is collected by a Permachannel connected a Permavoid attenuation tank.
At the appropriate levels and locations, discharge from the attenuation tanks is limited by flow control devices. The flow control devices within the system are simple orifice plates, prefabricated within silt pit chambers. Each orifice is protected by a substantial removable filter on the upstream side of the orifice. The use of orifice devices is appropriate due to the limited head differential generated within the system, which has a negligible impact on the discharge rates. All chambers are constructed as catchpits with minimum 300mm deep sumps to maximise silt control.
The use of the Permachannel/Permafilter Bio Mat/Permafilter SUDS textile/Pervious Paving/modified granular sub-base/Infiltration ensures the water-quality aspect of the design is very robust, as the system incorporates a minimum of two effective treatment phases along every conveyance route, prior to discharge. Silt/oil loading calculations were provided to demonstrate compliance with PPG3 and CIRIA guidance.
Client: Cambridgeshire County Council
Contractor: BAM Nuttall
Design Engineer: Parsons Brinkerhoff and Arup Consortium
Date: 2009
Key design aspects: The SUDS design is based on the following parameters:
Hard run-off areas: 18100m²
Storm Events: 100 year return period + 20% for climate change
Discharge Rate: 10l/s
Min number of treatment phases: (External Areas) Two
