Source control and prevention techniques are designed to counter increased discharge from developed sites, as close to the source as possible and to minimise the volume of water discharged from the site. This offers the benefits of reduced flood risk and improved water quality. It helps to restore underground water resources and maintain flows in surface watercourses during dry weather.

Some of these techniques are familiar to us all. The use of rainwater-butts in gardens to collect the runoff from a roof is an excellent example. The water-butt fills when it rains (the water is collected at/near the place of the rainfall, hence it is collected at source), and the water issued on the garden during dry periods. This principle is applied for all source control techniques. But care is needed with infiltration techniques. In some circumstances, for example on contaminated land, close to water supply boreholes or where the groundwater is vulnerable to pollution, infiltration may not be appropriate. We recommend that developers always seeks the advice of the Environment Agency.

Good site design will maximise the use of areas such as gardens and parklands, which will continue to drain naturally and minimise paved areas. Where paving is required for drives, access roads and car parking areas, permeable materials can reduce the need to collect runoff in drains. Cost savings can be made through the reduction in size, or even elimination, of off-site surface water sewers.

Providing there is no danger of increasing downstream flooding risks, such installations need not be designed to receive very large storms. A system that is designed to accept a twice per year storm before an overflow or bypass takes effect will still have significant environmental benefits. It will greatly reduce the frequency of discharge, provide protection from the pollution resulting from the flushing of pollutants from paved areas (the ‘first flush’) and delay the time of discharge. This will allow time for the flow in the receiving watercourse to increase. In most urban developments, downstream flooding will be a concern and additional storage will need to be provided.

The option of allowing a part of a development site to be flooded under some circumstances should also be considered. It may be acceptable to allow shallow flooding of a car park for short periods once or twice a year rather than building a much larger drainage system to cater for such infrequent events.

With good source control techniques, runoff from new developments need have little impact on the natural movement of water within a catchment.

View sites in Wales with a Source Control system.