Landscape Architecture, Ecological Restoration and Planning

The Greening Series

Rain Gardens


Tools for greening your site

By S. Edgar David, RLA


Trail Systems (coming soon)


Bio-swales or vegetated swales, as they are often called, are used in the landscape to convey stormwater and depending on how they are designed perform many beneficial functions.  Bio-swales are modeled after natural streams, but designed and engineered to manage storm water runoff from adjacent parking areas, buildings and uplands.  The bio-swale is designed to collect rain fall from smaller more regular rainfalls in shallow pools behind riffles in the stream channel, but allow water volumes from larger rainfalls to flow through the stabilized channel.  Bio-swales are often described as linear rain gardens. The collective system of riffles and pools benefits wildlife habitat by capturing and providing a source of water, promotes groundwater recharge by allowing infiltration and performs many other ecosystem services all while reducing the impacts of erosion and flooding. Created bio-swales are attractive urban amenities that can be a beautiful feature in the landscape.

This diagram illustrates the bio-swale’s natural attenuation used to address surface run-off, in this case surface run-off from a parking lot.  By providing islands of green space planted with appropriate native species including, rushes, grasses, flowering plants and certain woody plants selected for their particular abilities to metabolize common run-off components, a bio-swale can be created.  The bio-swale works by employing several mechanisms: filtration, adsorption and metabolism. The filtered water can then percolate into the ground or be discharged into the local storm water system.


How Bio-swales Function
Bio-swales are low-gradient, vegetated channels through which surface water runoff is directed.  As needed the centerline of the channel is stabilized with rock vortex weirs which serve as check dams to slow and detain water in pools, the classic riffle and pool sequence of a natural stream.  The centerline is further stabilized with cobbles of mixed sizes to protect the channel from erosion during significant storm events when peak flows need to be conveyed through the site.  Often boulders of native stone are used in creative ways to express the swale as a feature in the landscape


Immediately adjacent to the stabilized centerline the bio-swale is thickly planted native herbaceous plants needed to provide a dense network of roots to provide soil stabilization, slow runoff, absorb rain water and filter the water as it infiltrates the soil. These important ephemeral wetland plants must be tolerant of flooding and intermittent soil saturation, but also adapted to drier soil conditions between rainfalls.

Benefits of Bioswales

Filtering Pollutants

Bio-swales by design include a sequence of riffles and pools like a natural stream.  By nature the pools capture rainfall from the smaller more regular storm events in the depressed areas behind the riffles. These pools can often be expanded to capture more runoff and include a broader shallow area filled with more diverse perennial species.  In this way Bio-swales are thoroughly integrated with Rain Gardens in a sequence often described as a linear rain garden.

Click here to see SED Greening Series 'Rain Gardens'

Studies have found that properly designed and constructed bio-swales treat runoff for pollutants and are able to achieve excellent removal of heavy metals, total suspended solids, oil and grease, bacteria, nutrients, fine sediment and some pesticides and herbicides. Vegetation in a bio-swale helps to slow water velocity and encourage deposition of fine sediment and heavy metals. These pollutants are then immobilized and no longer pose a threat to downstream water quality. When water carrying pollutants infiltrates into the soil, some of the pollutants are decomposed by bacteria. Pollutants that can be broken down in this way are oil from roads and parking lots, and some pesticides and herbicides. Excess nutrients from fertilizers are taken up by the plants growing in the soil. Urban runoff often contains potentially harmful bacteria from septic or sewer leaks or domestic animal feces. The concentration of these bacteria is reduced through break down by solar radiation when runoff is retained in a bio-swale.  Bio-swales remove suspended solids through settling and filtration. Dissolved pollutants are removed and/or transformed as runoff infiltrates into the ground.


Retention and Infiltration

Bio-swales can be used to temporarily store runoff water and increase infiltration resulting in reduced runoff volumes and rates.  Increased infiltration recharges groundwater and sustains stream base flows.


Thermal Pollution

Bio-swales also reduce thermal pollution. Stormwater can rise in temperature as it washes across impervious surfaces such as hot parking lots in summer. Heated stormwater flowing into streams can impact fish and other wildlife that depend on cold water streams to live and breed.

Heated runoff from impervious surfaces can be cooled as it filters through a bio-swale. One study observed a temperature drop of 12°C between stormwater entering a bio-swale and stormwater filtering out of the bio-swale.


Wildlife Habitat

Bio-swales planted with native flowering perennials can be attractive landscaping features that provide food, water and shelter for birds, butterflies and many beneficial insects.

Contact SED Design to today to see how we can help you design and engineer your bio-swale and solve your site drainage issues in the most creative and beneficial ways.