Swales

Code: 
U04
Sector: 
Urban
Other sector(s): 
Agriculture
The complete description of the NWRM: 
Summary: 

Swales are broad, shallow, linear vegetated channels which can store or convey surface water (reducing runoff rates and volumes) and remove pollutants. They can be used as conveyance features to pass the runoff to the next stage of the SuDS treatment train and can be designed to promote infiltration where soil and groundwater conditions allow. Three kinds of swale give different surface water management capabilities:

  • Standard conveyance swale – Generally used to convey runoff from the drainage catchment to another stage of a SuDS train.  They may be lined or un-lined, depending on the suitability for infiltration.
  • Enhanced dry swale – Includes an underdrain filter bed of soil beneath the vegetated conveyance channel to accommodate extra treatment and conveyance capacity above that of the standard swale.  The underdrain leaves the main channel dry except for larger runoff events, and will prevent channels becoming waterlogged where the swale is situated on gentler slopes. A lining can also be incorporated into the underdrain if infiltration to underlying ground is not appropriate.
  • Wet swale - Where prolonged treatment processes are required for the storm runoff, the swale’s conveyance channel can be encouraged to maintain marshy conditions by using liners to control infiltration, or by siting in an area with high water table.

The promotion of settling is enhanced by the use of dense vegetation, usually grass, which promotes low flow velocities to trap particulate pollutants.  In addition, check dams or berms can be installed across the swale channel to promote settling and infiltration. As a result, swales are effective in improving water quality of runoff, by removing sediment and particulate pollutants. In wet swales, the effectiveness is further enhanced by providing permanent wetland conditions on the base of the swale.

Swales are applicable to a wide range of situations. They are typically located next to roads, where they replace conventional gullies and drainage pipe systems, but examples can also be seen of swales being located in landscaped areas, adjacent to car parks, alongside fields, and in other open spaces. They are ideal for use as drainage systems on industrial sites because any pollution that occurs is visible and can be dealt with before it causes damage to the receiving watercourse. 

Illustration(s): 

Swale

Source: Andras Kis’ presentation, NWRM Workshop 1

 

Possible benefits with level: 
Benefits Levelsort descending
PO9 - Take adequate and co-ordinated measures to reduce flood risks High
BP2 - Slow runoff High
ES8 - Erosion/sediment control Low
PO2 - Improving status of physico-chemical quality elements Low
PO4 - Improving chemical status and priority substances Low
PO5 - Improving quantitative status Low
PO12 - More sustainable agriculture and forestry Low
BP14 - Create terrestrial habitats Low
ES1 - Water storage Low
ES3 - Natural biomass production Low
PO8 - Prevent groundwater status deterioration Low
BP7 - Increase soil water retention Low
BP8 - Reduce pollutant sources Low
BP13 - Create riparian habitat Low
BP16 - Reduce peak temperature Low
BP17 - Absorb and/or retain CO2 Low
ES4 - Biodiversity preservation Medium
ES5 - Climate change adaptation and mitigation Medium
ES6 - Groundwater/aquifer recharge Medium
ES7 - Flood risk reduction Medium
ES9 - Filtration of pollutants Medium
ES11 - Aesthetic/cultural value Medium
PO7 - Prevent surface water status deterioration Medium
PO11 - Better protection for ecosystems and more use of Green Infrastructure Medium
PO14 - Prevention of biodiversity loss Medium
BP1 - Store runoff Medium
BP5 - Increase evapotranspiration Medium
BP6 - Increase infiltration and/or groundwater recharge Medium
BP9 - Intercept pollution pathways Medium
BP10 - Reduce erosion and/or sediment delivery Medium
Last updated: 09 Jun 2015 | Top