A bioswale is a shallow depression created in the earth to accept and convey stormwater runoff. A bioswale uses natural means, including vegetation and soil, to treat stormwater by filtering out contaminants being conveyed in the water.
Stormwater Filtration Systems
A bioswale is a type of stormwater filtration system. However, there are many types of stormwater filtration and management systems that can be applied to urban environments to remove stormwater pollutants. Some are mechanical, natural, or a combination of the two. Some mechanical options use manhole type structures to allow sediments or pollutants to settle. Other mechanical methods direct water into a filtering media, such as sand, that filters out pollutants and allows cleaner water to continue on its course. Whatever method is used, the practice of removing stormwater pollutants is generally known as a "best management practice," or BMP, which may be a requirement of the U.S. Environmental Protection Agency or the Oregon Department of Environmental Quality.
Bioswale Design Consideration and Performance Objectives
A Bioswale, like any other treatment option, will only be effective when it is designed correctly. Here are several important elements to consider when designing a bioswale:
- Slope. The slope should be steep enough to prevent ponding and shallow enough to slow water velocity. Soils must not readily drain water; the goal is to get cleaner water to flow downstream. Recommended slopes of 1.0% to 4.0% should be used.
- Channel Bottom Width. A wider channel allows for maximum filtering surface and for slower water spreading throughout the channel. A maximum width prevents shallow flows from concentrating and gullying, thereby maximizing the filtering by vegetation structure.
- Roughness coefficient. This varies with the type of vegetative cover and flow depth. There should be sufficient roughness to slow water velocity and to allow water to contact vegetation within its journey through the bioswale.
- Flow depth. This should not be taller than the vegetation, mainly grass. Maximum depth of 4" is recommended.
- Flow Velocity. Flow should be sufficiently low enough to provide adequate residence time within the channel. A maximum flow velocity of 1.0 feet per second for water quality treatment is required. 2-year storm events should be non-erosive, usually having not greater than a 4-5 feet per second velocity.
- Length of Channel. The length of the channel should be long enough to provide approximately 10 minutes of residency time.