Springfield Retail Park, Spalding

The Springfields Outlet Shopping and Garden Festival development is located in Spalding, Lincolnshire. The entire site covers some 12 ha with 5.2 ha comprising buildings and hard paved areas that are drained directly into the SUDS system. Flood protection against a 1 in 100 year return period storm was required. Working in partnership with SEL Environmental and the Environmental Protection Group, a conceptual SUDS design was developed as an alternative to a conventional stormwater drainage and storage scheme.

Elevated groundwater levels precluded the use of infiltration drainage systems. The discrete, shallow attenuation storage capabilities of the cellular Permavoid units with passive flow controls were best suited to resolving the site-specific issues. The hydrology of the system was modelled using InfoWorks software to test compliance with the required design standards.

The model was also used to optimise the required storage volumes and to check flood routeing during extreme rainfall events. Envisaged construction problems with the traditional design included the depth and size of pipework and storage, exacerbated by a high water table and cohesive silty soils.

For these reasons, the scheme was re-designed as a series of shallow storage tanks with shallow interconnecting pipework, which was installed to a maximum depth of less than 2m with the vast majority less than 1.5m deep, with the storage tanks generally less than 0.60m deep. Pipe sizes were kept to a minimum, averaging less than 225mm diameter. The car park utilises impervious paving (asphalt). The stormwater drainage is achieved by proprietary drainage kerbs (Permakerb). The Permakerb performs several key functions including stilling the sheet flow to encourage controlled deposition of silt and effluent interception & separation at source. This is achieved using a patented, oil-interception technology. This unique interceptor technology has been independently trialled and validated by Coventry University and has shown to be capable of replacing a class 1 oil-interceptor within SUDS schemes.

The high strength interlocking properties of the Permavoid geocellular units also functions as a subbase replacement and reduced the amount of imported granular material required. As a direct result of the construction benefits arising from the Permavoid SUDS scheme, the client realised substantial financial savings. Although this case study concerns a new-build project, the systems described are appropriate for existing developments, where retro-fit SUDS can provide substantial benefits.

Client:

Contractor: Interserve Projects Ltd

Design Engineer: EPG Ltd

Permavoid distributor: S.E.L. Environmental

Date: 2003

Key design aspects: The SUDS design is based on the following parameters:

Area of Permavoid: 8000 sq.m

Permavoid units: 32000 units

Length of Permakerb/channel: 523 lin.m

Area of Pavement: 30000 sq.m