Sustainable Urban Design: Educator’s Toolkit for Project-Based Learning, page 18 of 20

Image of a swale alongside a neighborhood street in Seattle
A bioswale alongside a neighborhood street in Seattle, WA. © The Nature Conservancy

Green Stormwater Infrastructure: Bioswales

Bioswales are landscaping features that slow and collect polluted stormwater runoff where it will infiltrate soils and be treated by natural elements. Bioswales are similar to rain gardens but whereas rain gardens are typically smaller and used for residential purposes, bioswales are designed to handle larger quantities of water generated from impervious surfaces like parking lots and city streets. They tend to be long and narrow, often require engineered soils for adequate drainage, and are much deeper than a typical rain garden. In order to facilitate stormwater infiltration, bioswale soils may need to be amended with compost and sand. In some cases, rock trenches or perforated underdrains can be installed down the center of the swale.

Like rain gardens, bioswales can be filled with low-maintenance, native plants that can withstand extreme conditions. Deep-rooted native plants can help build soil quality and prevent erosion. When stormwater infiltrates a bioswale, the purified water slowly recharges groundwater and prevents contamination of our waterways with polluted, unfiltered stormwater runoff.

Get Started

  1. Bioswales are often parabolic or trapezoidal in shape and should be large enough to handle the amount of rainfall received in 24-hours during a 10-year storm event at your location. We recommend consulting with an expert who can help engineer a bioswale that will work for your location.
  2. Find other community partners who can support this work and help explore grants and other opportunities for funding and fundraising.
  3. Get the necessary permissions and permits from all stakeholders.
  4. Evaluate soil conditions at the site. See pages 15-17 in the Rain Garden Handbook for Western Washington for information on how to do this.
  5. Create a design that takes into consideration the inlets where water will flow into the swale. Will you use rocks or grass filter strips to capture sediments at the inlet? Which substrate seems best considering the maintenance involved in clearing out sediments?
  6. Determine the equipment you will need to build the bioswale, identify adult volunteers who can help with the installation, and consider any safety precautions during installation.
  7. Schedule several work days to bring the project to completion.
  8. Be sure to document the project along the way and have students submit a press release with photos of the build days and be sure to celebrate your success!

Common Challenges

  • Bioswales are larger projects that typically require the assistance of an engineer and heavy equipment. This would be a great project to find a community partner to work with.
  • Sediment must be removed periodically to ensure proper flow; clogged soil media can lead to a bioswale fail!

Maintenance Schedule and Other Notes

  • Inspect the bioswale after storms to ensure proper flow and drainage.
  • Dead vegetation must be replaced to help the swale function and for aesthetics.
  • Deep-rooted native plants are best for infiltration and help reduce maintenance.
  • Don't use herbicides or pesticides in swales.

Related Lessons and Classroom Integration

Additional Project Resources