Designing an effective sustainable drainage system can be a bit of a challenge at the best of times, but it’s ten times worse when you’re trying to come up with something that will work for an area of very flat land, with an underlying seam of clay and a high water table. Add to that a nearby protected watercourse, a requirement to plan for a ‘one in a hundred year’ storm event – with a extra 30% allowance thrown in for climate change – and the whole job gets several orders of magnitude harder.
That was the task facing Consulting Engineers URS, who were brought in by house builders Countryside Properties to come up with the SuDS design for their new Great Kneighton development of a little over 2,500 new homes located on the southern fringe of Cambridge.
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Big Scale Project
The scale of the project inevitably injected its own demands into the mix; in addition to the dwellings themselves, ultimately a community health centre, library, new primary and secondary schools, retail units and offices are also planned. Minimising the environmental impact of the development on what was formerly a Green Belt site assumed a high priority from the outset, and the final result will incorporate a country park of some 120 acres, complete with newly planted, mature trees, sports fields and community allotment space.
All drainage schemes are influenced by the topography of the ground, the nature of the building project itself and the constraints imposed in planning – but for Great Kneighton, those factors were in a league of their own.
No Soak-away Option
The combination of the flat site, a high water table and an impermeable clay band situated only a metre or so below the surface meant that any thoughts of using the ground as part of the drainage scheme and allowing the water to soak-away naturally could be discounted at the outset.
Very quickly it became apparent that the only realistic option would be to discharge the bulk of the surface water away from the site – but that too had its constraints. The Environment Agency set discharge limits of two litres per second per hectare, and to top it all, the receiving waterway – Hobson’s Brook – is itself of significant historical importance and carefully preserved for its role in a scheme to bring water to Cambridge in the early 1600s.
Three-tiered Approach
As part of the planning approval, the Local Authority demanded a three-tiered approach to flood water management to provide protection at the individual plot level, under ‘normal’ discharge conditions and during severe events. Meeting these requirements led the team to devise a cleverly integrated system.
At the level of each individual unit in the development, there is rainwater harvesting to reduce the amount of water needing to be managed in the first place and detention strips to slow the flow of what is left. Hydrodynamic vortex separators – proprietary ‘Downstream Defender’ technology from Hydro International – then provide the second level of protection by screening silt, sediment, floating matter and hydrocarbons from the runoff to protect the receiving water bodies from contamination.
It is the third tier, however, that is perhaps the most striking. The watercourse divides the housing development on one side, from open green space on the other, and the solution the team came up with was to separate the site into four distinct storm-water catchment areas, and use a series of attenuation ponds – each with its own character and function, and fed through thrust bores that run beneath the brook.
Five Acres of Water
The largest of the ponds forms an amenity site for the public, with a wetland designed to promote local biodiversity, two of the others have been sculpted with shallow marginal areas which can be flooded in a controlled way as necessary. Between these three, that amounts to an area of nearly five acres (two hectares) of permanent water onsite. The final ‘pond’ is dry, forming a detention basin and helping to put the necessary slack in the system to cope with excess run-off under extreme conditions.
Integrated SuDS
The end result has been to produce an elegant and comprehensive solution for site drainage that is not only both sustainable and appropriate to the particular needs and constraints of the development, but also one that is uniquely integrated into the amenity value of the site. Fitting perfectly into the rest of the green space of specially created parks, woodlands and wetlands, Great Kneighton’s SuDS design provides a biodiversity boost along with its flood protection role, and minimises the potential impact of the whole building project.
With new SuDS regulations set to come into force, it is a timely demonstration of what can be done to help mainstream house building achieve the demands that it becomes increasingly sustainable, while still delivering the kinds of living environment that people actually want.
As the old saying goes – where there’s a will, there’s a way!
I would like to ask if anyone knows of any way of assessing the effectiveness of water butts and rain gardens in reducing or preventing flooding? In an attempt to prevent periodic flooding, my local community has begun to install water butts and rain gardens around the village catchment area and I am wondering if it is possible to calculate how many we would need to install to make a difference? The area has now flooded three times in the past ten years. The flooding occurs when the chalk stream running through the village is too swollen to pass under the bridge so the water backs up by about 100 metres, flooding about two dozen upstream properties. According to a recent report commissioned by the local authority, the bridge allows 3.3 cumecs to pass under it and causes approximately the same quantity again (over 3 cumecs) to back up. I am wondering, with these figures, if it is possible in any way to estimate the number of water butts and/or rain gardens and/or retention ponds we would need to install to have any impact on the flooding. Many thanks in advance, HS