The cost of flooding to the national economy in England and Wales is estimated to be £270 million a year. Following the Pitt review of the 2007 summer floods, local authorities are responsible for coordinating flood management.
Sustainable urban drainage systems (SUDS) are an increasingly important part of our green infrastructure. SUDS minimise surface water run-off and flood risks in an environmentally friendly way by mimicking natural water systems such as ponds, wetlands, swales and basins. SUDS can involve various ‘green’ engineering options such as infiltration trenches and filter drains, in order to slow water flow rate to reduce flood risk, whilst managing pollutants on site. SUDS offer neighbourhoods multiple benefits, including attractive planting features, and increased biodiversity whilst helping to ensure adaptation to climate change.
SUDS are also applicable to brownfield sites and techniques can be adapted to deal with a lack of space, poor soil infiltration, soil contamination and enhance and maintain biodiversity and wildlife.
The primary functions of trees, vegetation and soils are to aid in water interception, storage and infiltration while increasing evapotranspiration potential. Not all sites have the potential for open green spaces and trees, especially in highly urbanised areas where soil conditions restrict the amount of urban canopy cover. Floods usually occur during and after major storm events after canopy storage has been exceeded and although trees reduce runoff they are not effective at flood control. In contrast, tree planting on floodplains upstream of urban areas can significantly reduce flood risk.
SUDS can comprise of one or more structures to manage surface water runoff. A combination of techniques using the ‘management train principle’ helps alleviate the pressures on a drainage system. These will often incorporate traditional underground drainage systems. Some SUDS techniques involve vegetation and water storage (ponds) encouraging green space in urban areas whilst other techniques are engineered solutions below ground level. The SUDS involving green space include controlling the water at source through transpiration in trees and vegetation, green roofs, infiltration trenches and filter drains, swales and basins, and ponds and wetlands. Green space provision will need to be considered alongside increased storage thus utilising sustainable drainage techniques.
Public perception surveys have highlighted several recommendations for the design and management of SUDS that address public acceptability:
- Ponds should be made as “natural” in appearance as possible
- Marginal vegetation and planting adjacent to SUDS is important and should include native species
- Shore slopes should be gentle
- Natural barriers (e.g. planting) should be introduced to help manage perceived safety risks
- Deep water warning signs should be used
- Benches should be introduced
- Picnic tables, walkways and children’s play areas should be considered
- Land based wildlife and aquatic species, including fish, should be encouraged to colonise the system and its marginal areas.
Operation and maintenance:
- Litter and silt removal programmes should be given a high priority
- Clearing of inlets and outlets should be regularly undertaken
- Management of marginal vegetation should be regularly undertaken
- Pre-purchase information on local drainage and SUDS proposals should be provided to householders
- Educational campaigns should be set up for local community groups
- Interpretation boards should be introduced around SUDS.
Aspects of technical design that should be addressed include:
- Determine the suitability of SUDS for different soils
- Allow sufficient landtake for SUDS when planning the site layout (typically 5-7% of the site area but this can be significantly less if source control techniques are used)
- Minimise impermeable areas and encourage as much infiltration as possible
- Avoid pipes where possible by using swales or open ditches
- Design SUDS for wildlife and amenity (e.g. by providing ponds and wetlands)
- Use porous surfacing for driveways and parking areas or allow run-off to shed to adjacent land
- Use sheet flow to direct water into infiltration
- Use trenches, filter drains, filter strips and swales rather than gulleys and pipes
- Use water butts to attenuate roof drainage.
Oxfordshire County Council
As a result of a history of flooding in the are Oxfordshire County Council has made sustainable urban drainage systems (SUDS) an integral part of the planning process in developments throughout the county. Developments have featured a range of alternatives to traditional drainage, such as balancing ponds, wetlands and swales. Each development aims to make sustainable drainage into an attractive feature that can also bring additional benefits such as increasing biodiversity or introducing traffic calming measures. For further information see Put to the test by Oxfordshire’s floods.
Further case studies
- Sustainable drainage systems in Malmo, Sweden. New storm water management system in Ekostaden Augustenborg, Malmö(PDF-118K).
- Sustainable drainage systems. Bristol Business Park sustainable drainage system for preventing water runoff and flooding of nearby areas(PDF-139K).
Further advice on planning and designing SUDS can be obtained from ‘A Dos and Don’ts Guide for Planning and Designing Sustainable Urban Drainage Systems (SUDS)’ from The Scottish Environment Protection Agency.
There are a variety of resources available from CABE on sustainable urban drainage.
Ciria have provided an assessment of the Social Impacts of SUDS in the UK(PDF-1000K).