SUDS prevent this by dispersing water through the ground, where it can percolate slowly into a watercourse. This water is also cleansed of contaminants such as oil and silt. Stormwater drainage released from SUDS should have no greater impact on the environment than a greenfield site.
SUDS have three key objectives: reducing the amount of water run-off from a site, slowing the run-off to prevent flooding, and treating and cleansing this surface water.
A variety of techniques are used, on their own or in combination, to achieve these objectives. For example, the use of a permeable surface such as porous asphalt or paving allows water to drain directly into the ground. Beds of gravel are used to hold water, delaying discharge into a watercourse. The gravel can also remove some pollutants.
The speed of water flow can be reduced by constructing swales, shallow grass-lined depressions that help slow the flow of water and filter out pollutants. Larger areas may require the use of retention ponds to hold water after a storm for controlled release later.
Solids can settle before the water is discharged into a watercourse. Wetlands such as reed beds provide good pollutant breakdown and settlement of solids. Plants can absorb substances such as nitrates from water, and they can also be an attractive selling point for a development.
There is a big push to promote SUDS. The Environment Agency is keen to introduce them as an alternative to conventional drainage due to their flood-reduction properties. The recently-published Part H of the Building Regulations covering drainage, which comes into effect in April next year, states that rainwater should, if possible, be directed into an infiltration system rather than directly into a watercourse or a sewer. PPG25, which advises on planning for floods, contains a section on SUDS and suggests local authorities encourage their use.
Despite this enthusiasm, a stumbling block prevents the wider adoption of SUDS: maintenance. For example, accumulated silt needs periodic removal, and swales need regular mowing. House Builders Federation technical director David Baker is keen for HBF members to adopt SUDS on new housing developments, but says highways engineers and water authorities are unwilling to adopt them and nervous of the risk of the unfamiliar.
Baker says this puts HBF members off installing SUDS because "people buying houses expect the drainage to be looked after by the local authority or water company – they don't expect to have to look after the drains themselves".
Prosper Paul, regional technical officer for the Environment Agency's Thames region, is trying to form a national framework agreement setting out responsibility to allay fears, particularly those of the water companies. This has been successful in Scotland, where the local authority maintains above-ground, and the water companies below-ground, systems. Paul hopes agreement in England can be reached next year.