Swales are linear, shallow channels designed to collect and convey rainwater. They also provide pollutant removal and infiltration to some extent. Vegetation and sedimentations removes suspended solids, dissolved pollutants infiltrate with the water into the soil and can so be removed. Three types of swale can be distinguished: Attenuation/conveyance swales, dry swales and wet swales. They each are designed to optimise different aspects of water management. Attenuation/conveyance swales usually do not provide treatment or amenity/ecological benefits, they resemble conventional drainage ditches. Dry swales can be grassed and then are more resembling conventional drainage ditches as well, providing less treatment and flow reduction, or vegetated. Vegetated swales usually feature high grasses and shrubby vegetation, slowing water flow and enabling sedimentation as well as providing more visual and ecological benefits.
Depends on the design of the swale and its surroundings, but swales can provide accessible small greenspaces. This is often in the context of a larger green area and the impact of the swale itself can therefore not be seen separately. (1)
Vegetation of any kind takes up pollutants from the air. Closely mown grass is unlikely to contribute significantly. (14)
Swales can infiltrate 40% of all rainfall events and reduce runoff for an additional 40%, with an overall volume reduction of 50-60% – often low peak discharge or volume control provided by swales. This depends on their design. (1,2,6,9,11,12,13)
Swales have no impact on fluvial flooding.
Evaporation can have positive effects on UHI effect. Little carbon storage possible.(15)
Can function as green corridors and provide habitat to different species. Especially use of native plants and varied vegetation is valuable. (1,8)
Groundwater recharge is usually provided, but care has to be taken to prevent pollution. Water from swale can be discharged into streams and so directly improve low flows – depends on water quality. (1,13)
Swales perform well removing TSS (usually above 65%) and metals but less for nutrients (30-40% or less, with P showing better removal than N). Fine particles are often not captured. Accumulation of pollutants can be a problem. Vegetated swales are sometimes said to perform better.(2,4,5,9,10,13)
Depends on design. Higher growing native vegetation can provide interesting meadow-like appearances. Meandering swales have a more natural look. The design can easily be adapted to suit surroundings. (1, 9)
Can be used as an educational resource, design of the swale should take this into account. Case studies have demonstrated the use of swales as “outdoor classrooms” etc. (1, 9)
Swales are unlikely to contribute much to property value.
Through their impact on reducing and removing surface water runoff, swales can reduce severity of surface water floods.
Considering the Bigger Picture
In the landscape, swales act as connecting elements between other elements of rainwater treatment. While they do provide some storage and treatment, they are best suited to accept runoff from an area – for example, a car park – and lead it into further structures like detention basins or ponds. They can replace conventional pipework in this function. Whether swales can only work as conveyance or also to reduce/treat runoff is determined by the infiltration capacity of the soil. They are ideal for industrial sites as pollution incidents are easily visible. Downstream treatment components should be incorporated.
On the left, you can find an example of how different interventions can be incorporated into the urban landscape.
To provide a comprehensive treatment and management of surface water, swales should be seen within the wider landscape. While they are able to convey runoff, it is important to understand that their ability to take up existing runoff and infiltrate it is limited.
Especially where large quantities of surface runoff are expected, for example where large impermeable areas are located upstream, swales need to be carefully designed to make sure they function as expected. Opportunities for realising swales can be found not only in parks but also roadside verges or similar areas allowing linear structures to be incorporated. Water can further be lead into retention ponds or wetlands to undergo additional treatment or to be stored over a longer period.
10-20£/m2. Medium land take, linear structures allow high adaptability.
Retrofit & high density development possible. Land take limits suitability. Performance depends on the length of the swale in flow direction and vegetation. Hydraulic connectivity must be ensured, not suitable for steep areas or large amounts of storm water and high pollution.
£0.1/a for regular maintenance, marginally higher for remedial or intermittent maintenance. Mowing, litter and debris removal. Clearing of inlets and outlets. May need removal of sediment. Can be included in landscaping costs.
Trade-offs and Potential Dis-services
In peak events, nutrients and metals can be released from the swale and reach watercourses. Correct design and maintenance should work to prevent this.
If maintenance and plant selection is not careful, the swale’s appearance could deteriorate. For swales near roadsides, salt resistant plants should be chosen to be able to survive de-icing in winter.
- Ahiablame, L. M., Engel, B. A. and Chaubey, I. (no date) Effectiveness of Low Impact Development Practices: Literature Review and Suggestions for Future Research.
- Ashley, R. M., Nowell, R., Gersonius, B. and Walker, L. (2011) ‘Surface Water Management and Urban Green Infrastructure’, 44(0), pp. 1–76.
- Berwick, N. and Wade, D. R. (2013) A Critical Review of Urban Diffuse Pollution Control : Methodologies to Identify Sources , Pathways and Mitigation Measures with Multiple Benefits.
- Deletic, A. (2005) ‘Sediment transport in urban runoff over grassed areas’, Journal of Hydrology, 301(1-4), pp. 108–122.
- Ellis, J. B., Shutes, R. B. E. and Revitt, M. D. (2003) Constructed Wetlands and Links with Sustainable Drainage Systems.
- Environment Agency (2015) Cost estimation for SUDS – summary of evidence. Bristol.
- Kazemi, F., Beecham, S. and Gibbs, J. (2011) ‘Streetscape biodiversity and the role of bioretention swales in an Australian urban environment’, Landscape and Urban Planning, 101(2), pp. 139–148.
- Kellagher, R., Martin, P., Jefferies, C., Bray, R., Shaffer, P., Wallingford, H. R., Woods-Ballard, B., Woods Ballard, B., Construction Industry Research and Information Association, Great Britain, Department of Trade and Industry and Environment Agency (2015) The SUDS manual, CIRIA. London.
- Lucke, T., Mohamed, M. and Tindale, N. (2014) ‘Pollutant Removal and Hydraulic Reduction Performance of Field Grassed Swales during Runoff Simulation Experiments’, Water. Multidisciplinary Digital Publishing Institute, 6(7), pp. 1887–1904.
- Pratt, C. J. (2004) Sustainable Drainage. A Review of Published Material on the Performance of Various SUDS Components. Bristol.
- Qin, H., Li, Z. and Fu, G. (2013) ‘The effects of low impact development on urban flooding under different rainfall characteristics.’, Journal of environmental management, 129, pp. 577–85.
- Stagge, J. H., Davis, A. P., Jamil, E. and Kim, H. (2012) ‘Performance of grass swales for improving water quality from highway runoff.’, Water research, 46(20), pp. 6731–42.
- Forest Research (no date) Improving Air Quality.
- Lehmann, S. (2014) ‘Low carbon districts: Mitigating the urban heat island with green roof infrastructure’, City, Culture and Society, 5(1), pp. 1–8.