Articles & Case Studies

Below ground SUDS

Posted: Wednesday 17th September 2008

The North East of England is home to one of the British Army’s largest Barracks with a military and civilian population of over 12000. As part of a long term modernisation and expansion programme of the site, contractors have installed an innovative below-ground, SUDs [Sustainable Urban Drainage] solution to accommodate surface-water run-off for a total of 9400sqm car parking area.

In periods of heavy rainfall, surface water run-off from large areas of impermeable paving can overwhelm drainage systems, leading to flooding. In view of this, groundworks contractor, K Rouse Ltd, chose a combination of conventional channel drainage, feeding into below-ground attenuation tanking designed and installed by SUDS specialists, Stormwater Management Ltd.

The purpose of an attenuation tank is to accept and retain surface water run-off during periods of high intensity rainfall, then manage its discharge into the drainage system at a controlled rate.

At the site, Stormwater Management designed a 576m³ capacity system comprising 2880 of its FlowBlock modular water storage cells.

FlowBlock cells are high strength, lightweight polypropylene units, 1.0x0.5x0.4m with a 95% void ratio. Their modular format allows them to be linked together to form large underground tanks of virtually any predetermined configuration. FlowBlock assemblies are wrapped in either a geotextile or a geomembrane depending on their application and connected to the drainage system.

On site, Stormwater Management installed two tanks 40mx10mx0.8m and 40mx8mx0.8m beneath a car park serving the Barracks. The tank dimensions were calculated using the company’s in-house drainage design software, which is based on the industry standard Wallingford Procedure.

Both tanks were excavated to a depth of 2m deep, graded to line and level, and lined with 1mm thick VFPE impermeable geomembrane. FlowBlock cells were then installed by hand and wrapped in the geomembrane to create a completely sealed system with mechanically welded seams. Flow regulators were installed at the outlet so that discharge rates could be managed to the pre-determined levels.

On completion, the attenuation tank was overlaid with a layer of compacted Mot Type 1, and selected as-dug material with a black top wearing and base course, giving a total construction cover of 1200mm. Surface water drainage was achieved through a network of linear drainage channels.

The Barracks SUD system was installed in early summer 2007 and has performed as specified over its first full year of use.

T: 0845 4210278

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