Source: Qingfu Xiao; E. McPherson; Qi Zhang; Xinlei Ge; Randy Dahlgren, “Performance of two bioswales on urban runoff management,” Infrastructures
Davis, CA (September 27, 2017) – A U.S. Forest Service study shows that after eight years, bioswales using engineered soil mixes are effective at reducing surface runoff, removing pollutants from surface runoff, and supporting tree growth in an urban landscape.
Bioswales are shallow drainage courses that are filled with vegetation, compost, and/or riprap. As part of the surface water runoff flow path, they are designed to maximize the time water spends in the swale, which aids in the trapping and breakdown of certain pollutants.
Bioswales have been widely recognized as an effective decentralized stormwater BMP to control urban runoff. Their effects are threefold; (1) vegetation intercepts rainfall reducing net precipitation (2) plant uptake of water via transpiration reduces soil moisture, thereby increasing subsurface water storage capacity, and (3) root channels improve infiltration.
Traditional bioswales are designed to remove silt and other pollutants from surface runoff waters. New bioswales are being developed for harvesting surface runoff and supporting urban tree growth. Bioswales that integrate engineered soil mixes (ESM) and vegetation are being used to enhance treatment and storage of surface runoff.
This study evaluated the effectiveness of two bioswales eight years after construction in Davis, California. The treatment bioswale was constructed with engineered soil mix that replaced the native loam soil. Four Red Tip Photinia (Photinia X fraseri Dress) trees and two Blueberry Muffin Hawthorn (Rhaphiolepis umbellate (Thunb.) Makino) shrubs were planted in the bioswale. Runoff flowed into the bioswale from an adjacent 171 m2 panel of turf grass.
An identically sized control bioswale consisting of non-disturbed native soil was located adjacent to the treatment bioswale. Surface runoff quantity and quality were measured during three experiments with different pollutant loads. When compared to the control, the treatment bioswale reduced surface runoff by 99.4%, and reduced nitrogen, phosphate, and total organic carbon loading by 99.1%, 99.5%, and 99.4%, respectively. After eight years, tree growth characteristics were similar across both sites.
As demonstrated by this study, bioswales are effective at reducing surface runoff, removing pollutants from surface runoff, and supporting tree growth in an urban landscape. After eight years, the bioswale using ESM was functioning at the designed capacity. The bioswale with ESM reduced much more runoff and pollutants when compared with a traditional control plot bioswale using native soil. The study confirms that bioswales with ESM can be a highly effective stormwater BMP in the suite of green infrastructure strategies.