SUDS are structures built to manage the rainwater runoff from a developed area in a way which will take into account water quantity, water quality and amenity value for that area. Depending on the type of SUDS facility chosen, they can offer attenuation and storage of flows, act as a treatment system and also provide wildlife habitat capable of contributing significant amenity value to the local community. But why do we really need SUDS?

Problems with conventional drainage systems
In undeveloped areas, rainwater will seep into the ground and slowly find its way into watercourses and groundwater systems as part of the natural hydrological cycle. However, in developed areas this is not possible due to the increase in impermeable surfaces, from what was soil and vegetation to roads, roofs, car parks, etc. The water must therefore be drained away by some other means to avoid flooding. In conventional drainage systems, rainwater is either carried away by a system of gullies and surface water pipes and discharged directly to a watercourse, or drains into what is known as a combined sewerage system – where foul drainage and surface water drainage share the same pipe – and is carried to a wastewater treatment plant.

In separate surface water systems, this can cause problems due to both the quantity and the quality of the rainwater draining away. During storms, large volumes of rainwater quickly reach the watercourse via the network of pipes and flows peak higher and sooner than in an undeveloped catchment – water can back up in pipes and in severe storms, rivers get dangerously high. The rainwater can also pick up pollutants from road surfaces, yards etc. such as oil, sediment, metals and salts and carry these directly to the watercourse.

In the case of combined sewerage systems, high rainfall can place additional hydraulic pressure on the system, causing combined sewer overflows (CSOs) to spill to watercourses more frequently and impacting upon the performance of wastewater treatment plants.

Benefits of SUDS
A well designed SUDS can relieve these problems and in some cases can also provide further environmental and social benefits. SUDS work by slowing down the transport of rainwater often by mimicking the natural drainage process which, in turn, will allow settlement of sediments, filtration and in some cases biodegradation. SUDS can vary greatly, depending on requirements and location.

There are four general methods of control:

• filter strips and swales
• filter drains and permeable surfaces
• infiltration devices
• basins, ponds and wetlands.

A basic overview of SUDS can be found in the Sustainable Urban Drainage Scottish Working Party (SUDSWP) booklet:
Sustainable Urban Drainage Systems – Setting the Scene in Scotland

Further information about the types of SUDS and the successful planning, selection and design of SUDS can be found at the following sites:

• http://sudsnet.abertay.ac.uk/
• http://www.ciria.org/suds/
• www.sepa.org.uk/dpi/suds/index.htm
• http://www.scotland.gov.uk/Topics/Environment/Water
  /15561/3913.

For examples of SUDS wetlands, click on the links below:

• The Caw Burn Project
• Elrick SUDS wetland.