Pioneering SUDS in Oxfordshire

Photo of the Witney Housing Development

Plans to increase housing provision in the picturesque historic market town of Witney, 10 miles west of Oxford are set out in the Oxfordshire Structure Plan which makes provision for some 3,000 dwellings up to the period 2016.

One such site, on the former playing fields of Henry Box school, was identified by Oxfordshire County Council as a location to build affordable homes.

This is one of a series of case study examples shared by Local Authorities on Engineering Nature’s Way to outline the experiences of local authorities in introducing SUDS principles in their areas.


Project Outline

Plans to increase housing provision in the picturesque historic market town of Witney, 10 miles west of Oxford are set out in the Oxfordshire Structure Plan which makes provision for some 3,000 dwellings up to the period 2016.

One such site, on the former playing fields of Henry Box school, was identified by Oxfordshire County Council as a location to build affordable homes.

Witney is nestled in the valley of the River Windrush, which forms an attractive green corridor through the town.  But it also means there is a wide floodplain south of the town, and flooding problems occur periodically, most notably during the summer of 2007.

The one-hectare Henry Box site was almost entirely within the flood plain and is virtually flat with a gradient of 1: 1000. Planning permission for the development was granted on the proviso that site level should be maintained and not raised as it had already been on neighbouring industrial land.  Stormwater had to be managed and contained within the site, controlled at source.

Local Authority Leadership

Gordon Hunt has led the drainage team for Oxfordshire County Council for over 20 years.  Gordon has long been an enthusiastic proponent of SUDS and at forefront of introducing SUDS features within Oxfordshire.  Oxfordshire have kept ahead of expected legislation on SUDS and often implemented SUDS-compliant schemes before any legal necessity to do so.

Background

We worked closely the Environment Agency and the West Oxfordshire District council as the planning authority from the beginning, ensuring that stormwater drainage for the site as a whole was approved at planning stage.    At this time, this was quite an achievement.   The challenging site conditions made the site unsuitable for many conventional drainage solutions – and perfect for the application of sustainable drainage techniques.  It was the perfect site for SUDS.

The Environment Agency and the council both insisted on using source control methods, meaning all surface water had to be contained within the site, and could not simply be conveyed or pumped off site for someone else to worry about.   The Environment Agency required discharge levels of 9.5 litres per second, even though the site would generate 154 litres per second.  This meant a large amount of water would need to be stored or infiltrated.   But natural infiltration was not an option in the ground conditions.   In fact, when top layer of peat was removed, the site flooded like a swimming pool!

The overriding factor was the large flow of groundwater across the site at around a metre under the surface.  The water table was only 400mm to 700mm below the surface.  Drainage needed to be kept as close to the surface as possible.

The Role of the Local Authority

“The Witney scheme got underway in the early 2000s and was in many ways a pioneering example of what can be done on a challenging site using SUDS techniques.

“I was closely involved from the beginning.  We worked with the EA,  West Oxfordshire District Council as the planning authority and the developers’ consulting engineer WS Atkins from a very early stage in evaluating the options for the site.  We were also able to co-ordinate a close partnership approach with Sovereign Housing Association, which bought the land from the County Council. It equally was important to consider the needs of future residents and come up with cost-effective solutions.  As the site was to be affordable homes, a cost-effective solution was essential.

“Working with WS Atkins, it was therefore important to examine the full range of options and develop a design for the whole site. But infiltration methods such as permeable paving, soakaways, swales and ditches were not suitable by themselves to drain the site effectively.    A sealed system with gullies was not an option, as this would have meant raising the very level site to summits, leading to potential ponding, and meaning the discharge would need to be pumped away from the site.

“In particular, the site had to be allowed to flood.  Since all the house floors were suspended, holes were placed in all walls, so flood water could flow throughout the site, across roads and in and out of houses and gardens.  There could be no raised traffic calming, no fixed wall garages – only car ports and garden walls had to have holes near the bottom so the water flow is not restricted.

The solution for Witney

“The design solution for the Witney centred on a kerbside drainage infrastructure across the whole development with water from roads and houses draining via the kerbside system into geocellular online storage.  ACO Technologies worked closely with the council to design the drainage system.   Downpipes from the houses are even directed under the footway and connected to the kerbside system to help flush the system.

“Hydro Stormcell® was then specified to provide 130m3 of online storage.    The water is then discharged gradually from the storage system using penstock valves into three swales, situated at the edges of the development within a 20m development exclusion zone.

“We have measured flows and monitored the swales carefully across the development.  The integrated SUDS approach is all working effectively.

Key Lessons

“This, and other schemes in Oxfordshire, has helped us to get a head start with SUDS in many ways.  With the Flood and Water Management Act, it’s an exciting time for Oxfordshire’s drainage team.

“Our approach is very much ‘hands on’ working closely with all stakeholders involved to make a real difference in controlling surface water in the county using sustainable, low-maintenance practices.  We are already well prepared for our official role as Lead Local Flood Authority and to establish SABs.  In fact, we have been implementing many provisions of the Flood and Water Management Act, well ahead of legislation.

Working with landscape architects

  • “It’s essential for the local authority to be closely involved from the master plan stage for any development. From the outset, it’s important to involve landscape architects as well as the property developers and infrastructure engineers.
  • “Developers’ initial thoughts on landscaping all too often were towards putting in plenty of trees and removing swales and ponds. We must ensure that, wherever possible, we introduce both quality and quantity control measures and help educate all parties on the reasoning behind what they may see as interference.

Planning for maintenance

  • “We have also learnt the importance of planning for maintenance and low lifetime costs, which now affects our choice of SUDS techniques in both engineered and wholly natural methods.

Developer relationships

  • “In dealing with developers, we find that the large developers have a much clearer appreciation of the benefits of SUDS than they used to have.   They have come to appreciate that SUDS is not more expensive and often cheaper than conventional options. This applies to both installation and maintenance; smart SUDS is the objective.
  • “Small developers often still need persuasion and help; they can be suspicious of SUDS engineering and automatically think it is going to cost more. The small builder likes to think that buried ironwork in the road is the answer to discharge requirements; they must be educated that shunting as much water to the storm sewer as possible does not solve the problem.

Challenges for the future

“Across the county, adoption of private and non-highway drainage is the latest challenge, and will mean we have to undertake careful investigation of every site.”

Flood Plain: It had to be SUDS: Read Gordon Hunt’s Blog about this development:

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