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Planners and conservationists rarely see eye to eye on many issues – especially water management. But the solution to their resolution is not always found across the negotiating table. It often lies beneath their feet… 

Adorable

Wykery Copse is a fragment of adorable ancient woodland in the English county of Berkshire. Designated a Special Site of Scientific Interest, this home for various rare species of flora and fauna recently became a source of concern to local conservationists. Planners had designed a residential housing estate upstream from the site.

However, on a typical housing development, surface water run-off can increase to 80% of the rainfall compared to a figure of 20% typical of green-field sites. This results in large volumes of stormwater flowing into traditional drainage systems. In the absence of suitable conditions for the use of infiltration drainage, it was evident that this housing estate would require a substantial storage tank. Any disturbance either in water quality through pollution or in water quantity through flooding – could mean disaster for this protected woodland.

A compromise of ideals was out of the question. So how could the conservationists protect their planet and the planners protect their plans?

Largest scheme in UK

The answer came from the underground installation of the largest surface water attenuation scheme in the UK. Over 29,000 individual polypropylene units were assembled to harvest the rainwater on the Jennett’s Park residential estate. They now release their precious contents so slowly that flora and fauna are oblivious to the experience.

These lightweight units were simply clipped together forming two box-like structures. Once encapsulated in a geo-membrane, these underground tanks store up to 5500 m3 of storm water run-off. The water is then slowly released to prevent flooding downstream.

The entire drainage system for this site had to be designed very carefully to prevent flooding, water erosion damage and pollution. Surface water run-off in urban areas may contain small quantities of oil or petrol. An oil interceptor was therefore installed on the estate to eliminate these pollutants before water drains into the woodlands.

Stream of consciousness

Designed to accommodate the drainage requirements of 1,500 houses, these purpose-built attenuation tanks have the capacity to embrace future storm patterns. They will capture, direct and store high volumes of water – associated with extreme weather conditions. Furthermore, their low installation costs are unmatched.

Typically these modern units have a range of benefits from both ecological and economic viewpoints. In this instance, their purpose was to prevent flooding. But they can also be used to mitigate land subsidence and provide water for the garden, washing or flushing toilets.

Upstream at Jennett’s Park, their low maintenance, performance and durability will be appreciated by many future generations of estate managers. Downstream, their invisible contribution to nature will go unnoticed by local conservationists and by the wildlife that inhabit this rather special corner of England’s ‘green and pleasant land’.

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