Case Study – Vauxhall Way
Luton Borough Council (LBC) commissioned Project Centre (PCL) to undertake a feasibility study on the proposals to widen Vauxhall Way (A505) from a single carriageway, with a single lane in each direction, to a dual carriageway with two vehicle lanes in each direction, off carriageway cycle facilities and improved pedestrian facilities. The proposed highway improvements aim to provide first-rate pedestrian and cycle facilitates, reduce congestion and increase future road network capacity.
Increasing the area of impermeable surface leads to a higher runoff rate in high rainfall. To prevent additional water entering the existing sewer and exceeding its capacity, the additional surface water flow must be retained on site and fed into the sewer network at a low rate.
Additionally, the junction of Hitchin Road / Stopsley Way / Vauxhall Way was at risk of flooding as it lies in an overland flow path. Therefore, any additional storage available would help to reduce the risk of flooding from surface water overflow originating in the catchment upstream of Vauxhall Way.
The main constraints affecting the drainage design was the topology of the site. Any area north west of Hitchin Road / Stopsley Way is at a higher elevation than the carriageway and therefore not suitable for positive drainage features.
The available footprint for features in the corner between Stopsley Way and Vauxhall Way was reduced by existing healthy trees which we wanted to preserve. The main areas available were along Vauxhall Way, on both sides of it. The northern site was needed for a proposed foot and cycle way; therefore, no detention features could be implemented along the surface.
Vauxhall Way has a relatively steep longitudinal fall (away from the junction mentioned before), which is a limiting factor for different SUDS types.
Any available area in the corner of Stopsley Way and Vauxhall Way is used to attenuate runoff from Hitchin Road and Stopsley Way in a detention basin with controlled outflow.
Due to its upstream location, this basin could not be used to catch runoff from Vauxhall Way. Along the steep road, the attempts on the southern and northern site vary.
Along the northern site of Vauxhall Way, all features must be buried to allow space for a foot and cycle path. A series of three geo-cellular storage tanks with a flow control chamber can provide enough storage.
Along the southern site, visible open surface features are possible within the overgrown verge. A swale was designed to convey water downstream and provide water quality treatment. To provide additional detention storage, five concrete check dams with an orifice and overflow section were implemented to cut the swale into six sections. Each section can provide a limited amount of storage and this ensured a suitable utilisation of the storage within the swales. The total amount of detention volume was enhanced with a geo-cellular storage tank implemented downstream of the swale. A sophisticated hydraulic model in MicroDrainage was developed to reflect the physical reality of the integrated drainage and SuDS elements including the swales and the check dams.
The designed drainage features do not only reduce the runoff into the existing sewer, they also have a positive effect on the expected water quality. Both the swale and basin will remove sediments from the runoff as well as a part of oils. A second tier of the water treatment is provided downstream of the basin and swale with the oil and silt separator on both sides of Vauxhall Way just upstream of the geo-cellular storage tanks.
The design has achieved a betterment of 78% on the existing rates and now the site can accommodate up to 100-year return period storms with allowance for climate change.