Published by The Environment Agency:
A Guide to SUDS (Sustainable Urban Drainage System)
Surface Water Run-offs
Development can harm our water resources if a traditional approach to drainage is adopted. Removing water from the site as quickly as possible causes a range of impacts:
Increased downstream flood risk as a result of the run-off from roofs and paved areas. This also causes sudden rises in flow rates and water levels in local watercourses.
Rainwater diverted to piped systems reduces the amount of water soaking into the ground. As a result, ground water levels fall and dry weather water flows in watercourses are reduced;
Surface water run-off can contain contaminants such as oil, organic matter and toxic metals. Although often at low levels, cumulatively they can result in poor water quality in rivers and streams, adversely affecting biodiversity and amenity value. After heavy rain, the first flush of water through the drainage system is often highly polluting; As a result, many urban watercourses are lifeless and unattractive, and are often hidden in culverts under the ground. Some pollution arising from urban run-off may be unavoidable, and water treatment at every outfall is impractical.
Sustainable Drainage Systems (SUDS) can significantly reduce the harm to our water resources, and improve the quality of our built environments, by moderating flows and filtering run-off.
Towards Sustainable Drainage Systems
Sustainable Drainage Systems (SUDS) are designed with three objectives in mind:
• to control the quantity and rate of run-off from a development
• to improve the quality of the run-off
• to enhance the nature conservation, landscape and amenity value of the site and its surroundings
SUDS deal with run-off as close to its source as possible and balance all three objectives, rather than focussing only on flood prevention. Implementing SUDS contributes significantly towards achieving sustainable development. In recognition of this, Local Plans increasingly state that all applicants should, in the first instance, incorporate SUDS into development proposals.
SUDS – the benefits
Implementing SUDS may lead to cost savings, for example, by avoiding or reducing the need for:
• gully pots
• surface water sewers
• piped connections to distant outfalls
SUDS can be cost-effectively designed to work with retained natural features such as ditches or ponds, and to form an integral part of hard and soft landscaped areas. In this way, they can contribute towards
an attractive scheme that enhances the nature conservation and amenity value of the development, while also making the best use of the valuable water resource.
SUDS and the planning process
SUDS include tried-and-tested techniques that are already being implemented on a range of projects in the United Kingdom and Europe. They incorporate cost-effective techniques that are applicable to any development scheme. These range from small developments to major residential, leisure, commercial or industrial operations with large areas of hardstanding and roof. They can also be successfully retro-fitted to existing developments.
Planning Policy Statement 25 for England on Development and Flood Risk emphasises the role of SUDS and introduces a general presumption that they will be used. SUDS are being incorporated in other planning policies as they are revised. As with other key considerations in the planning process – transport, landscape, heritage and nature conservation – incorporating SUDS needs to be considered early in the site evaluation and planning process, as well as at the detailed design stage.
Many planning authorities will expect planning applications, whether outline or detailed, to demonstrate how a more sustainable approach to drainage is to be incorporated into development proposals, and for detailed design information to be submitted at the appropriate stage. Planning authorities may use planning conditions to secure the implementation of SUDS.
Adoption and future maintenance
The arrangements for adoption and future maintenance of the system should be considered during the early stages of design. This is likely to influence the design just as much as technical considerations. It is recommended that maintenance should be the responsibility of a publicly accountable body. This will often call for the payment of a commuted sum or a legal agreement, possibly backed up by the deposit of a financial bond. The adopting organisation will probably wish to approve the design before construction.
Sustainable Drainage System (SUDS)
Sustainable drainage is a design philosophy that uses a range of techniques to manage surface water as close to its source as possible. To produce a workable and effective scheme, SUDS must be incorporated into the development at the earliest site-planning stage.
The need for surface water drains and off-site sewers can be reduced or eliminated where run-off is encouraged to flow through porous pavements made from materials like concrete blocks, crushed stone or porous asphalt. Depending on the ground conditions, the water may infiltrate directly into the subsoil or be stored in an underground reservoir (for example, a crushed stone layer) before slowly soaking into the ground. If infiltration is not possible or appropriate (for example, because of ground contamination), an impermeable membrane can be used with an overflow to keep the pavement free from water in all conditions. Pollutant removal occurs either within the surfacing or sub-base material itself, or by the filtering action of the reservoir or subsoil.