SUDS Infrastructure for Major Homes Development

Aerial Image of Clay Farm Development

Construction of a sustainable drainage infrastructure for the new Clay Farm Housing Development in Cambridge is currently underway.  The overall SUDS strategy for the 109 hectare site is based on a series of four ponds with infiltration and settling basins located across the site.

Project Outline

Construction of a sustainable drainage infrastructure for the new Clay Farm Housing Development in Cambridge is currently underway.  The overall SUDS strategy for the 109 hectare site is based on a series of four ponds with infiltration and settling basins located across the site.

The scheme demonstrates the versatility of the SUDS toolbox, and how engineered and semi-engineered solutions can be combined to mimic natural drainage processes.

The development on former Green Belt land by Countryside Properties (UK) Limited is planned to have a maximum of 2300 homes and just under half (49 hectares) is designated as strategic open space in the Green Corridor.

Local Authority Leadership

Simon Bunn has led the Cambridge City Sustainable Drainage Design Team for two and a half years, and has been working in drainage design for 17 years, including a spell as a consulting engineer. He has extensive experience in drainage and SUDS for developments, and Cambridge introduced its own SUDS design guide in 2009, Cambridge Sustainable Drainage Design and Adoption Guide, with extensive public support; see www.engineeringnaturesway.co.uk/resource/cambridge-city-council-design-guide/

Background

“Cambridgeshire is a flat county and the City of Cambridge is intimately associated with water. Apart from the River Cam which winds its way through the Backs behind the Colleges, there are numerous backwaters and ponds. Hobson’s Brook, into which the Clay Farm Development discharges, makes its way through the centre of Cambridge to the Cam, feeding local water amenities as it goes – and even the Master’s swimming pool at one College.

“Work started in Spring 2010 at Clay Farm which is about 4km South of the City of Cambridge.  The first building started in Winter 2010 and completion of all development is anticipated in 2020. There is a mixture of approximately 60% houses and 40% apartments.

The Role of the Local Authority

“At master plan stage, it was important to set out the needs of the site. In particular, given the topography and local drainage, the discharge limit to Hobson’s Brook is a very constrained 2 litres per second per hectare.

“In Clay Farm, we had an opportunity to extend the SUDS strategies adopted in the City to a major development; all the more important because the discharge ends up in the City. The Environment Agency took the early lead and we worked closely with them and the Hobson’s Conduit Trust particularly with the initial model development.

“Amenity value is a recognised selling asset now for any development, and the area is Green Belt which is reflected in the master plan and design guidelines. We found the best way to gain acceptance is through incorporating the SUDS solutions as part of the landscape to enhance the amenity value.

The solution

“Although the name Clay Farm suggests a former brickworks and heavy clay, in fact, there is a mixture of clays and sands at Clay Farm.  The water table is generally about a metre below ground level and infiltration rates vary across the site, and can be reasonably high in places.

“The strategy for the Clay Farm site is based on a series of three large ponds – one of which is almost a lake – with infiltration and settling basins located across the site. Downstream Defender® hydrodynamic separators are used to remove sediments, floatables and any hydrocarbons before they reach the ponds. Discharge from the ponds into Hobson’s Brook is controlled via Hydro-Brake® Flow Control Devices.

“Developing the ecological/amenity role of the ponds is an important aspect. In particular, the largest of the ponds is to be a wetland for wading (and other) birds such as Lapwing and Golden Plover, compensating for the loss of winter farmland habitat. The area of permanent water covers 1.9ha and the total ‘bird mitigation area’ covers 5ha. In time this will become a major amenity for local people who will be able to enjoy the bird life from special viewing areas.

“There is potentially up to seven or eight parts of the site, with a different developer/building contractor on each. Further attenuation is incorporated at each site as it is developed. As well as the design code for the construction, the SUDS design code must also be followed to ensure consistency of approach.

Key Lessons

Site by site approach

  • Each separate development has to be considered on a site by site basis; there is no ‘one size SUDS fits all’ as the soil and topography vary considerably.
  • As well as ponds and basins, there are swales and permeable pavement where ground conditions are suitable.
  • The approach demonstrates the versatility of the SUDS toolbox, and how you can combine engineered and semi-engineered solutions to mimic natural processes.

Selling the ideas to developers

  • The earlier you start talking to the developers the better; there is some variability in their understanding and acceptance of the worth of SUDS. This means getting the architects, engineers and landscapers together.
  • Ponds are, after all, open space; open space provision is integral to the amenity quality of the site, with multi-use benefits for the ponds – leisure, ecology and water quality through the encouragement of reed beds.

Now read Simon Bunn’s Blog

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