Source: Flickr
Image: Aaron Volkening
Bioswales are shallow depressions in the earth designed to gather and redistribute stormwater runoff. They use organic mediums such as soil, sand, compost, vegetation and stones to filter out contaminants. Some are mechanical and some are natural. Others use a combination of natural and mechanical techniques. Removing stormwater pollutants is known as "best management practice", or BMP, and is often a requirement of the Environmental Protection Agency.
Source: Flickr
Image: ShiftBristol
Wet swales, like the one pictured above on Fernhill Farm in the UK, intersect the groundwater, similar to a linear wetland cell. The design consists of a shallow permanent pool and wetland vegetation to provide stormwater treatment. This one employs several pools. A farm is a more suitable setting for a wet swale as homeowners in residential areas normally do not care for standing water. However, on a farm the water is not only purified, but the swale produces resources such as willow and creates a diverse wildlife habitat. The water is so clear by the time it reaches the last pond in the series, folks have been known to go for a swim.
Pepsi-Cola Bottling Co. in Eugene, Oregon
Source: Google Images
Image: Derek Godwin
Above, grasses, sedges and rocks collect and slow the rainwater, allowing it to seep into the ground below. A dry bioswale like this uses a deep fabricated soil bed in the bottom of the channel. A sand/soil mixture which meets permeable soil requirements is brought in to replace the existing soil. An underdrain system is also incorporated, which is typically a layer of gravel encasing a perforated pipe.
Source: Flickr
Image: Darien Library
When designing a bioswale the most important components to consider are: flow depth, flow velocity, channel bottom width, slope, roughness coefficient and length of the channel. The flow depth should not be taller than the vegetation with a maximum of 4" recommended. The flow velocity should be 1' per second. You need to keep the velocity low enough to provide adequate time within the channel. A 4-5' per second velocity is ideal for 2-year storm events and should not erode the structure. A wider channel bottom provides maximum filtering surface, which prevents flows from concentrating and gullying. In regards to slope you want it steep enough to prevent ponding and shallow enough to regulate velocity. Slopes of 1.0% - 4.0% are recommended. Roughness will vary with the type of vegetation used, but you want to maintain sufficient contact with the plant life to slow the water velocity and provide adequate time for filtering. Ten minutes of residency time is ideal in regards to the length of the channel.
Source: Flickr
Image: Plan For Opportunity
As stormwater runoff flows through bioswales, pollutants are filtered out by soil and vegetation. The parts of the plants above ground such as stems, leaves and stolons inhibit flow, causing pollutants and particles to settle. The pollutants are incorporated into the soil for immobilization and decomposition. The bacteria present in healthy soils can actually break down carbon-based pollutants such as motor oil.
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