SUDS allows building on a bog

The UK's largest sustainable urban drainage system has been ten years in the making. Paul Brocklehurst of Catesby Property Group explains how the system has been effective in making this waterlogged land suitable for development

Catesby Property Group now owns the largest sustainable urban drainage system (SUDS) in the UK, integrated within a 52 hectare mixed-use commercial site. But ten years ago it was a different story.

The land near Balby in Doncaster was acquired for the development of Firstpoint in the late 1990s, but planned works were significantly hindered due to a series of drainage dilemmas. With no public sewers in the area, a waterlogged site and the Potteric Carr Nature Reserve next door, Catesby drafted in the expertise of engineers Hannah Reed & Associates and environmental consultants Penny Anderson Associates to find a feasible and sustainable solution to the surface water drainage.

SUDS were in their infancy when the consultants started looking at the site. And, from a developer's point of view, they were not the most immediately attractive option as there was a potential loss of developable land from a SUDS.

There were also questions regarding the maintenance/adoption of the finished system as statutory water companies do not recognise above ground drainage systems. But, due to the factors that presented themselves, a series of swales and reed beds soon became feasible.

Immediate problems encountered on the site included a high groundwater table and a lack of land drainage. This in turn created a soft and unstable surface to build on and provided no solid base to house foundations.

The site also had a history of high water levels and a flood risk problem. To tackle these issues and dry out the site, large open drainage channels were excavated to provide a means of land drainage. The series of swales were connected by culverts under roads and finally flowed into a reed bed downstream.

The reed beds constructed on the site have a two-fold function. The first is to store excessive surface water runoff and to control the discharge into the division drain. The second is to control any pollution from the site before the water is discharged. The need to store the surface water was a requirement by the local Potteric Carr Independent Drainage Board (IDB), whereby a discharge limit of 1.6l/s/hectare was stipulated as a greenfield runoff. The division drain passes through a Site of Special Scientific Interest area downstream therefore the water quality requirements of any flows into it were very high.

The reed bed controls pollution through a mixture of settlement areas, rafted reed beds and the overall shape of the structure. The rafted reed beds rise and fall with the water level with the roots hanging beneath to trap and absorb pollutants.

For the maintenance of the swales and the reed beds, an agreement was established with the Potteric Carr IDB to manage the drainage infrastructure.

Simon English from Hannah Reed adds: "The first reed bed scheme, developed in 2003, is now well established and is controlling wastewater and run-off at a rate of 1.6l/s/hectare - which compared to a typical greenfield run-off rate of 5l/s/hectare, is pretty impressive. So much so, the Potterick Carr IDB has given it an excellent rating."

The second phase, which has only been in place for 18 months, is aiming to be even more effective. It worked as a SUDS within six to 12 months, but is likely to take up to five years to become properly established.

"Wastewater quality, produced from the SUDS into the division drain, is also remarkable," says English.

"By-pass separators are also in place to separate any oils before entering the swale system, as with all sites where there are car parks, and coupled with the reeds absorbing pollutants, the water is clean enough to feed the existing ecological nature corridors and nearby nature reserve.

"Water voles, rabbits, bats and kingfishers can all be found at the wetland habitat along with various fish. The water is now so clean even swans and coots are regular visitors."

SUDS have provided other benefits. The view has improved for local residents and the occupiers of the offices and industrial units. Walkways have been created in and around the site for public access. And the local infrastructure has been protected by the reduced flood risk.

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