Bioswales

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This article is about installations designed to capture and convey surface runoff along a vegetated channel, whilst also promoting infiltration.
For underground conveyance which promotes infiltration, see Exfiltration trenches.
For design recommendations on grassed channels, see Enhanced grass swales.

Overview[edit]

The fundamental components of a bioswale are:

  • A graded channel
  • Planting
  • Filter media, to permit infiltration into the facility (not necessarily to soils below)

Additional components may include:

Planning considerations[edit]

Design[edit]

Bioswales are sized as narrow linear bioretention cells. Drainage time of bioswales are typically lower than other geometric configurations of similarly sized bioretention facilities, owing to the higher hydraulic radius of the sides.

Materials

Bioretention: Planting design Bioretention: Plant list

Performance[edit]

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

  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=10.1.1.365.8665&rep=rep1&type=pdf. 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=10.1.1.365.9417&rep=rep1&type=pdf
  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.