New Bioswale Treats Stormwater in South Salem
As part of the City of Salem's increasing efforts to improve stormwater quality, the 12th Street Bioswale has recently been brought on-line. Begun in 2007 as a retrofit of an unused right-of-way strip along 12th Street, the new 270-foot bioswale is now treating a portion of the stormwater runoff from 16 acres of nearby residential development.
What is a bioswale?
A bioswale is a vegetated channel that treats polluted stormwater run-off from nearby surfaces such as roads and rooftops. Although a bioswale may look similar to a ditch, it is specially designed to provide natural water treatment by utilizing the filtering action of native plants and the soil. While a ditch is designed to convey water quickly, a bioswale has a wider bottom and gentle slope that allows excess flow to infiltrate and sediment to settle out. Native water-loving plants cover the bottom of the swale, trapping pollutants and encouraging infiltration through the soil.
Transforming a Right-of-way into a Bioswale
The photos below show the steps taken to transform an unused strip of right-of-way along 12th Street Cutoff into a working bioswale.
Although the 12th Street Bioswale is one of the first large bioswales constructed in Salem, it certainly won't be the last. The City of Salem is currently revising its Design Standards so that both new development and redevelopment of older properties will be required to treat stormwater before it enters a stream. Bioswales are just one of the many different ways stormwater will be treated in Salem.
Stormwater Filtration Systems
A bioswale is a type of natural stormwater filtration system. However, there are many types of stormwater filtration systems which can be used to remove stormwater pollutants from urban areas. The filtering or cleaning method employed may be mechanical, natural, or a combination of the two. Mechanical methods typically use underground structures that either settle out pollutants or direct water through a filtering system. Natural methods are generally on the surface and employ vegetation and soils to capture pollutants and infiltrate a portion of the flow. Whichever method is used, the practice of reducing or removing stormwater pollutants is required by permits held by the City of Salem from the Oregon Department of Environmental Quality, and are part of Salem’s "Best Management Practices" or BMPs.
Bioswale Design Considerations
Like any other treatment option, a bioswale will only be effective if it is designed correctly. Below are several important design elements considered when creating a bioswale:
- Slope: The bottom of the swale has a gradual slope of only 1-4 percent; enough to slow down flow without creating ponding.
- Channel Width: A flat channel bottom provides room for the flow to spread out, increasing filtering and infiltration capacity. A wider width also prevents flows from concentrating into gullies, which could cause erosion damage to the swale.
- Speed of Flow: Bioswales are designed to keep the water flowing slowly. Slower speeds create longer 'residence time,' giving natural processes a chance to work. A residence time of ten minutes or more is a typical goal.
- Surface Roughness: Swales are planted with native vegetation of moderate height that provides surface roughness to slow down water flows.
- Flow Depth: The depth of flow through the bioswale must be lower than the vegetation in order for plants to have a slowing effect. Typically, the goal is a water depth of four inches.
- Channel Length: The length of the channel should be long enough, given the flow speed, to provide approximately ten minutes of residence time.