A bioswale is an urban landform used to convey surface water in order to enhance infiltration and reduce surface runoff. Bioswales are typically moderate gradient devices (approximately one to five percent in channel slope) and may be covered by grasses, landscape fabric, mulch or other vegetation or leaf litter. These landforms are typically integrated into an urban landscape design to enhance the visual appearance, but also may be used in agricultural settings as drainways to intercept runoff containing silt, pesticides or nutrients. In urban settings the bioswale may serve to reduce sediment load and other water pollutants from reaching natural watercourses. A typical bioswale has gently sloping or curved sides to emulate the appearance of a natural watercourse; in fact, if established with appropriate native vegetation, the bioswale may become a riparian corridor or wetland restoration element of the natural landscape.
The course of a bioswale may be straight, but where available land permits, may take on a curved or meandering channel; these non-linear shapes allow more time for water to infiltrate and thus are generally more effective in infiltration enhancement and pollutant trapping. In addtion, the curvature typically enhances the aesthetic appearance and produces a more natural looking landscape element. Bioswales are most typically viewed as intermittent streams, unless local precipitation is very high, or other continuous sources of surface runoff are present, such as persistent over-irrigation or natural springs. typical bioswale design calls for water retention on the order of 60 to 120 hours following a storm event. (Russ) In any case the flow depth is normally quite shallow in order to maximize the ratio of land surface to water volume, and hence produce the highest possible capture of infiltration and pollutant trapping.
Channel gradients are typically in the range of one to six percent to create slow to moderate stream velocity. Cross gradient slopes are typically steeper than the channel gradient; a typical design standard for the cross gradient slope is a gradient of five to 33 percent. The overall channel flow capacity is often sized to a given flood capacity standard; ability to convey a ten-year flood is a common standard. Often the mid-channel bottom has an exfiltration trench that abets the flood capacity, and may also contribute to total infiltration by lining the bottom of the trench with rock and sand; such a design feature is particularly useful in cases where the characteristic bioswale soil is rather impermeable.
Common urban applications of bioswales are to intercept large quantities of surface runoff from low permeability man-made surfaces, such as parking lots, roadways and roofs. Thus bioswales may be useful in industrial parks, office complexes, retail centers and high density apartment projects. Particularly important applications occur around parking lots and roadways, where substantial water pollutants are generated from vehicular fluids (e.g.motor oil drippage, wiper fluid), lead, sediment, etc., and conveyed by rainfall runoff to natural receiving waters, in the absence of a bioswale or retention pond.
In addtion to water pollution control discussed above, the bioswale may have substantial benefits in flood control, where receiving waters are subject to periodic flooding; this outcome is produced since greater infiltration and reduced surface runoff are products of the bioswale landform. Furthermore, beyond the mere interception of water pollutants travelling to receiving waters, residence in the bioswale trap may allow actual decomposition or destruction of certain of these pollutants. For example, nitrogen and phosphorus pollutant loads may be comsumed in vegetation growth within the bioswale. In the case of certain waterborne pathogens, there may be both organism retention and destruction occurring, since some pathogens may be retained for a sufficient time span to accomplish organism death. The potential exists for organic material interception to result in the bioswale becoming a carbon sink, thus contributing favorably to the reduction of atmospheric carbon. An example of this outcome is where the sediment load contains a large fraction of organic material or considerable aquatic biomass is present; in these cases the organic material may become entrained in the soil interstices and increase the soil retained carbon. As a wider consequence of bioswale interception of pollutants, the benefits may include protection of receivng waters biota, by reducing pollution impacts to fish and other aquatic organisms and by reducing nutrient loading that may cause excess algae blooms.
Even though the bioswale concept has been known for at least four decades, not until the 1990s were there many examples of implementation by landscape architects in development projects of significant urban scale. Some of the earliest scientifically designed bioswales for large scale applications are found in the western United States. In 1996 in Willamette River Park in Portland, Oregon a total of 2330 lineal feet of bioswale was designed and created to capture water pollutant runoff prior to entering the Williamette River. Intermittent check dams were installed to further enhance silt capture, with the outcome reduction totalling 50 percent of all suspended solids entering the river system from the park. (France).
A large scale bioswale was designed at the Carneros Business Park, Sonoma County, California in 1997, (Francis) the project design team coordinating with the California Department of Fish and Game and County of Sonoma to produce a detailed design to channel surface runoff at the perimeter of a large parking area. Surface runoff consists of building roof runoff, parking lot runoff and overland flow from properties to the north of the project site. In this case the bioswale fed into a portion of a natural watercourse which was being restored, subsequent to a century of degradation from overgrazing and other agricultural uses. (Hogan) A total of two lineal miles of bioswale was designed into the project. The purpose of the bioswale was to minimize stormwater runoff contaminants entering Sonoma Creek. The designed bioswale channel is a nearly linear grass-lined shape. The downslope gradient is approximately four percent and cross-slope gradient is approximately six percent.
- U.S. Army Corps of Engineers. Design Schematics for a Sustainable Parking Lot, TR-03-12, U.S. Army Corps Research and Development Center. (3.4MB .pdf)
- Thomas H.Russ. 2000. Redeveloping Brownfields: landscape architects, planners, developers. McGraw Hill. New York. 289 pages
- Robert Lawrence France. Handbook of Water Sensitive Planning and Design, CRC Press LTD (2002) ISBN 1-56670-562-2
- C. Michael Hogan. 1998. Hydrology and biology studies for Carneros Business Park, Lumina Technologies. Prepared for the William A. Saks Company and the County of Sonoma, California
- John Francis. 2008. Philosophy of Mathematics. 160 pages