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Bioswale, County Court Boulevard, Brampton

This article is about installations designed to capture and convey surface runoff along a vegetated channel, whilst also promoting infiltration.
For underground conveyance systems which promote infiltration, see Exfiltration trenches.


Swales are linear landscape features consisting of a drainage channel with gently sloping sides. Underground they may be filled with engineered soil and/or contain a water storage layer of coarse gravel material. Two variations on a basic swale are recommended as low impact development strategies, although using a combination of both designs may increase the benefits:
Bioswales are sometimes referred to as 'dry swales', 'vegetated swales', or 'water quality swales'. This type of BMP is form of bioretention with a long, linear shape (surface area typically >2:1 length:width) and a slope which conveys water and generally contains various water tolerant vegetation,
Enhanced grass swales are a lower maintenance alternative, but generally have lower stormwater management potential. The enhancement over a basic grass swale is in the addition of check dams to slow surface water flow and create small temporary pools of water which can infiltrate the underlying soil.
Grass swales are a relatively common landscape feature already and a great opportunity for retrofit, to reduce flow and improve water quality by encouraging settling and infiltration behind a series of check dams.
Retention swales can be imagined as linear, sloped dry ponds. They typically make relatively small contributions to water volume and quality control than many other BMPs, but they may feature as part of a site-wide treatment train approach.

Take a look at the downloadable Enhanced Grass Swales Factsheet below for a .pdf overview of this LID Best Management Practice:

Enhanced swale.png

Swales are an ideal technology for:

  • Sites with long, linear landscaped areas, such as parking lots
  • Connecting with one or more other types of LID
Types of Swale
Property Bioswale Enhanced grass swale
Surface water Minimal
Any surface flow can be slowed with check dams
Ponding is encouraged with check dams
Soil Filter media required Amendment preferable when possible
Underdrain Common Uncommon
Maintenance Medium to high Low
Stormwater benefit High Moderate
Biodiversity benefit Increased with native planting Typically lower

Planning Considerations[edit]

Bioswale with check dams
(vertical scale exaggerated)
Stepped bioretention cells alternative for slopes >6 %
(vertical scale exaggerated)

Enhanced swales and in some cases bioswales include check dams that can be designed to pond substantial volumes of water on the swale surface between flow events, so provide both stormwater conveyance and infiltration functions. For information about constraints to infiltration practices, and approaches and tools for identifying and designing within them see Infiltration. For guidance on infiltration testing and selecting a design infiltration rate see Design infiltration rate.

A linear design (surface area typically >2:1 length:width) is a common feature of swales:

  • An absolute minimum width of 0.6 m is required for bioswales to promote healthy plant growth, and to facilitate construction,
  • Grassed swales are usually mown as part of routine maintenance, so the cross section will be triangular or trapezoidal in shape with maximum side slopes of 1:3. The minimum width for this type would be 2 m. See Best cross sections

Swales may be graded along longitudinal slopes between 0.5 - 6 %:

  • Between 1 - 6 %, check dams are recommended to bring the compensation gradient <1 %.
  • Slopes > 6% can accommodate a series of stepped bioretention cells, each overflowing into the next with a spillway.

For a table summarizing information on planning considerations and site constraints see Site considerations.


Distance between dams is determined by equalling the elevation of the crest of each dam, with the elevation of the toe of the upstream dam.
(vertical scale exaggerated)

Pretreatment and inlets

To minimize erosion and maximize the functionality of the swale, sheet flow of surface water should be directed into the side of the BMP. Gravel diaphragms, vegetated filter strips and shallow side slopes are ideal. Alternatively, a series of curb inlets can be employed, where each has some form of flow spreader incorporated. Single point inflow can cause increased erosion and sedimentation, which will damage vegetation and contribute to BMP failure. Again, flow spreading devices can mitigate these processes, where concentrated point inflow is required.


A review of swale-like practices was published by STEP in 1999. The project page and additional tools are available here.


While few field studies of the pollutant removal capacity of bioswales are available from cold climate regions like Ontario, it can be assumed that they would perform similar to bioretention cells. Bioretention provides effective removal for many pollutants as a result of sedimentation, filtering, plant uptake, soil adsorption, and microbial processes. It is important to note that there is a relationship between the water balance and water quality functions. If a bioswale infiltrates and evaporates 100% of the flow from a site, then there is essentially no pollution leaving the site in surface runoff. Furthermore, treatment of infiltrated runoff will continue to occur as it moves through the native soils.

Volumetric runoff reduction from bioswales
LID Practice Location Runoff Reduction* Reference
Bioswale without underdrain Washington 98% Horner et al. (2003)[1]
Scotland 94% Jefferies (2005)[2]
Bioswale with Underdrain Maryland 46 to 54% Stagge (2006)[3]
Bioretention without underdrain China 85 to 100%* Gao, et al. (2018)[4]
Connecticut 99% Dietz and Clausen (2005) [5]
Pennsylvania 80% Ermilio (2005)[6]
Pennsylvania 70% Emerson and Traver (2004)[7]
Bioretention with underdrain
Ontario 64% CVC (2020)[8]
Maryland and North Carolina 20 to 50% Li et al. (2009) [9]
North Carolina 40 to 60% Smith and Hunt (2007)[10]
North Carolina 33 to 50% Hunt and Lord (2006) [11]
Runoff Reduction Estimate* 85% without underdrain;

45% with underdrain

Enhanced grass swales[edit]




Simple grass swales[edit]


Check dams[edit]

Also see Jen's Pinterest board of check dams

See Also[edit]

  1. Horner RR, Lim H, Burges SJ. HYDROLOGIC MONITORING OF THE SEATTLE ULTRA-URBAN STORMWATER MANAGEMENT PROJECTS: SUMMARY OF THE 2000-2003 WATER YEARS. Seattle; 2004. http://citeseerx.ist.psu.edu/viewdoc/download?doi= Accessed August 11, 2017.
  2. Jefferies, C. 2004. Sustainable drainage systems in Scotland: the monitoring programme. Scottish Universities SUDS Monitoring Project. Dundee, Scotland. https://www.climatescan.nl/uploads/projects/8126/files/1277/SNIFFERSR_02_51MainReport.pdf
  3. Stagge, J. 2006. Field evaluation of hydrologic and water quality benefits of grass swales for managing highway runoff. Master of Science Thesis, Department of Civil and Environmental Engineering, University of Maryland. https://drum.lib.umd.edu/items/42be6ce6-e4ef-4162-a991-c273607d422d
  4. Gao, J., Pan, J., Hu, N. and Xie, C., 2018. Hydrologic performance of bioretention in an expressway service area. Water Science and Technology, 77(7), pp.1829-1837.
  5. Dietz, M.E. and J.C. Clausen. 2005. A field evaluation of rain garden flow and pollutant treatment. Water Air and Soil Pollution. Vol. 167. No. 2. pp. 201-208. http://citeseerx.ist.psu.edu/viewdoc/download?doi=
  6. Ermilio, J.F., 2005. Characterization study of a bio-infiltration stormwater BMP (Doctoral dissertation, Villanova University). https://www1.villanova.edu/content/dam/villanova/engineering/vcase/vusp/Ermilio-Thesis06.pdf
  7. Emerson, C., Traver, R. 2004. The Villanova Bio-infiltration Traffic Island: Project Overview. Proceedings of 2004 World Water and Environmental Resources Congress (EWRI/ASCE). Salt Lake City, Utah, June 22 – July 1, 2004. https://ascelibrary.org/doi/book/10.1061/9780784407370
  8. Credit Valley Conservation. 2020. IMAX Low Impact Development Feature Performance Assessment. https://sustainabletechnologies.ca/app/uploads/2022/03/rpt_IMAXreport_f_20220222.pdf
  9. Li, H., Sharkey, L.J., Hunt, W.F., and Davis, A.P. 2009. Mitigation of Impervious Surface Hydrology Using Bioretention in North Carolina and Maryland. Journal of Hydrologic Engineering. Vol. 14. No. 4. pp. 407-415.
  10. Smith, R and W. Hunt. 2007. Pollutant removals in bioretention cells with grass cover. Proceedings 2nd National Low Impact Development Conference. Wilmington, NC. March 13-15, 2007.
  11. Hunt, W.F. and Lord, W.G. 2006. Bioretention Performance, Design, Construction, and Maintenance. North Carolina Cooperative Extension Service Bulletin. Urban Waterways Series. AG-588-5. North Carolina State University. Raleigh, NC.