A bioswale is a vegetation-filled, shallow, open channel that controls the flow of stormwater – a ditch with purpose. The flood- and erosion-resistant native vegetation improves water quality through infiltration, sedimentation and filtration. Thus, they are often placed at the beginning of a BMP system. We also recommend that you talk to an engineer or surveyor to assess the drainage and water flow where you plan to build the bioswale.
- Watershed drainage area should be less than four hectares.
- The bioswale must be downstream from impervious surfaces to accept the polluted runoff.
- The bioswale must be 3 feet above groundwater to maintain dryness.
- Runoff should travel through the bioswale for a minimum of 5-10 minutes to ensure the infiltration, sedimentation and filtration processes are successful.
- Decide on a date of construction that falls approximately 3 months before the rainy season. The vegetation must become established before the first storm.
- Gather the following materials:
- Compost and natural soil – since soils with a high organic content have a high exchange capacity, they will remove a greater amount of metals. Mix the two components together to increase vegetation nutrients and infiltration capacity. You will need enough to cover the area of the bottom at 0.15 meters in depth.
- Gravel for filtration, enough to cover the area of bottom and 0.15 meters in depth.
- Seeds for vegetation – enough to cover the area of the bottom. The vegetation must be able to provide a dense cover with a strong root structure, stand upright in strong water flows, tolerate periodic flooding and drought, cannot be dormant during rainy season and should be in sun. You can find a list of native plants here.
- Dig the swale approximately 1-4 meters in width, 1-4 meters in depth and up to 30 meters in length. The optimal slope is 3%.
- Lay gravel to a thickness of 0.15 meters.
- Lay compost and natural soil mixture to a thickness of 0.15 meters, on top of the gravel.
- Plant and water seed normally. Ensure that seed is not flooded for extended periods of time during the germination phase.
- To prevent the seed from washing away, construct a flow diversion for protection until vegetation is established.
- Dead vegetation must be removed and replanted, which removes metals and pollutants already filtered by the plants.
- Invasive species and accumulated sediment must also be removed periodically.
- Irrigate as needed to maintain a healthy system.