Sustainable Drainage Infrastructure for Cambridge Homes Expansion

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Building a sustainable drainage infrastructure for a development providing 1000s of new homes to serve the expanding Cambridge conurbation has delivered important lessons for housebuilders as they prepare for new regulations for SuDS in England and Wales – whilst demonstrating that the solution for every site must be individual.

The SuDS solution at Great Kneighton shows how effective masterplanning, combined with a practical approach to designing effective trains of SuDS from a versatile toolbox of engineered and natural solutions, has delivered a workable solution despite challenging ground conditions.

Following an £8 million infrastructure investment by Countryside Properties on the former farmland, over half of the development remains open space with a generous green corridor running through the site.  Four large SuDS ponds form part of the open space, each providing a different type of amenity and ecology as part of a strategic drainage plan.

Known as Great Kneighton, the development will provide 2,536 homes together with a 120 acre country park and a community square that will host essential facilities and services including a new community / health centre, library, shops, offices, new primary and secondary schools and a transport hub.

From the earliest stages of planning, minimising environmental impact and providing public amenity value was a priority for the former Green Belt land and the resulting site will have extensive mature tree planting, playing fields and community allotments.

Seeking a drainage solution

Consulting Engineers URS developed the drainage strategy and URS’ Tracey Neal is keen to stress that ground conditions and topography meant the drainage solution developed was very unique to the needs of the site:

“Every site is individual and that puts different characteristics and demands on the surface water strategy, “ says Neal.

“Like much of Cambridgeshire, the site is extremely flat.  It has a very high water table and a band of clay with some sands running under much of the site just a metre below ground level.

“Consequently, an assumption of 60% impermeable area for the development parcels was used.  The drainage strategy therefore had little opportunity to use infiltration as part of the solution and there was no option but to discharge the majority of the surface water offsite.”

The Environment Agency set discharge limits to achieve greenfield run off rates of 2 l/s per hectare.  Other important constraints for the drainage infrastructure included the 1 in 100 year storm event parameter with an additional 30% climate change allowance.

A further environmental constraint was the receiving watercourse, which is the carefully protected and historic Hobson’s Brook dating back to 1571.

“Developing a drainage solution was quite a challenge as it is so flat in all directions and the water falls in a very flat gradation,” adds Nigel Borrell, Senior Technical Manager for Countryside Properties.

“We therefore ended up dividing the site into 4 separate stormwater drainage catchments.  The Hobson’s Brook runs through each of the four catchments, with the development all built on one side and green open space on the other with surface water drainage centred on four attenuation ponds, each with its own distinct ecology.

“In these ponds, the area of permanent water covers 1.9 ha.  The largest pond is a public amenity and wetland, one is a dry detention basin, and the others are relatively large with permanent water and shallow margins for controlled flooding.”

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Pond with development in the background.

To control discharge and provide treatment and protection to these ponds, Hydro-Brake® Flow Controls were chosen.  Downstream Defender® hydrodynamic vortex separators are installed at the inlets to the pond from each of the four catchments.  They remove sediments, floating detritus and hydrocarbons, protecting the ponds’ ecology and preventing them from silting up.  Pond 1 also incorporates a sedimentation forebay as additional protection, important during construction phases.

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A Downstream Defender® being installed.

The four large SuDS attenuation ponds are fed from the Downstream Defender® units via syphonic drains (thrust bores) under the Brook.

Treatment train strategy

“The Local Authority required three levels of treatment of the water in the SuDS train.  The first at the individual plot level, rainwater harvesting and detention strips for example, Downstream Defender® units provide the second level and the ponds provide the third,” explains Neal.

As an important provision, it was essential that all of the strategic drainage for the site is being adopted by Anglian Water including the ponds and the Downstream Defender® separators for a regular inspection and maintenance programme.

Early Master Planning was vital

Simon Bunn was leader of the Sustainable Drainage Design Team at Cambridge City Council during the early stages of planning.  He comments: “It was important to set out the needs of the site and the SuDS approach at early Master Plan stage.  The Environment Agency took the early lead and we worked closely with them and the Hobson’s Conduit Trust.  We found the best way to gain acceptance with the developers is through incorporating the SuDS solutions as part of the landscape to enhance the amenity value.”

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