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home > pervious surfaces > water quality performance
Surface water runoff is a major
cause of pollution in watercourses. Pervious surfaces, together with their
associated substructures, limit the direct discharge of pollutants to receiving
waters and reduce the impacts of pollution.
The pollutants of most concern in highway or car park runoff are:
The majority of pollution is washed off urban surfaces at the beginning of
a rainfall event and this is known as the
first flush. In this regard, sloping roof surfaces make the most significant
contribution to the first discharges within the drainage system, but the later
contributions from the paved surfaces deliver the more significant pollution
contribution
All evidence to date has demonstrated an improvement in water quality by the use of pervious surfaces. Pervious
surfaces can be designed to provide a number of interception mechanisms that
mitigate against the risks posed to controlled waters. Available methods include:
The use of pervious surfaces should also give a benefit to water quality
as a result of reduction in peak flows to receiving waters, which enhances
the settlement and biodegradation of pollutants. Where the outflow is released
to surface waters the reduced peak flow causes less of a short term shock
pollutant load to the receiving waters and allows increased dilution. This
is generally an improvement over traditional systems where the first flush
effect can be pronounced. The delay and treatment provided by pervious systems
can attenuate this effect.
A fully documented qualitative risk assessment of the impacts of a pervious
surface on the receiving waters should always be undertaken and the results
fully communicated to the developers/owners. This should take account of the
sensitivity of the receiving environment, and the likely maintenance regime.