Sustainable drainage systems, or SuDS, are designed to diminish the impact of developments by reducing the volume of water that runs off from surfaces in to water courses when it rains.
SuDS also add value by using rainwater as a resource, treating the run-off to reduce pollution as well as enhancing biodiversity and amenity. A key design requirement of SuDS is Interception – the capture and retention of the first 5 mm of every rainfall event, though in practice implementation can be problematic. HR Wallingford has developed a new methodology to help SuDS designers meet the Interception criterion.
Rainfall run-off from paved surfaces, such as car parks and roads, contains a range of pollutants; many of these are from vehicle emissions. The highest concentration of these pollutants tends to be found in run-off from the earliest part of a rain storm. Intercepting just 5 mm of every rain storm – large or small, infrequent or frequent, has positive implications for water quality and quantity. Over 50 per cent of rain events produce less than 5 mm of rainfall, so intercepting just 5 mm from each storm means the majority create no rainfall run-off at all.
Although Interception asks for all storms to comply with retaining the first 5mm, implementing this requirement can be problematic in some circumstances, for instance where infiltration potential is minimal and when particularly wet periods occur. HR Wallingford has been exploring practical applications of SuDS in commercial and industrial car park settings for ACO. This has led to an assessment of how Interception might be achieved in these situations. This specific element has been supported by three major supermarkets.
This study has used a detailed analysis based on a continuous 10 year rainfall data set for a range of SuDS to examine the possibilities for compliance with the Interception criterion. Key conclusions are that:
-interception can be provided even where there is limited space available on a site, and
-a probabilistic approach to compliance, probably as a function of season, is required.
Instead of an absolute prescription for 5 mm retained for every rainfall event, the approach requires a flexible measure of compliance based on a proportion of events meeting the criterion. The study suggests that Interception is viable for most situations using a variety of techniques for 80 per cent of rainfall events in the summer and 50 per cent during the winter. Different levels of compliance can be set and used based on specific requirements of any location.
The next step will be to produce a fully developed tool, which will facilitate convenient assessment by the forthcoming SuDS Approval Bodies (SABs) and those undertaking planning applications.
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Press Release, December 20, 2013