California Urban Area Bioswales

Although requiring more space than conventional concrete-lined open channels, habitat-friendly bioswales are now a common substitute for impervious channels in California urban and suburban areas.

Bioswales are an ecological method to meet stormwater treatment regulations for pollutant removal while maintaining stormwater management efficiency.

Designed with drought-tolerant, native habitat vegetation, bioswales remove stormwater pollutants by filtration, sedimentation, and sorption. The bioswale slows runoff to allow for more effective infiltration into the underlying soil.

The following are key areas to evaluate when considering developing a bioswale:

What do you need to treat? Bioswales are particularly effective for treating two types of run-off: “first flush” storm flows after initial rains that carry high-pollutant loads in small amounts of water, and “nuisance run-off,” such as excessive irrigation water or car wash flow. Pollutants most effectively removed include suspend solids (sediments), heavy metals, litter, and dissolved nutrients/fertilizers.

Do you have available land? Bioswales of all shapes and forms can be effective. Surface area is an essential consideration so water contact with vegetation can be maximized for pollutant uptake. A good rule-of-thumb for highest efficiency pollutant removal is 1,000 square-feet per tributary acre of run-off. Side slopes and depth should be shallow; longitudinal slopes should be kept at a half to one percent to reduce velocity and maximize surface contact time. Suitable land can be found in landscape buffers – road median strips, landscape strips, and HOA landscape areas.

Can the proposed site manage flow properly? As a “flow-based” treatment alternative, bioswales function best under low runoff conditions. The influent stormwater flow rate may need to be moderated somewhat upstream with a diversion structure to allow low flow events to enter the bioswale and higher flow events to bypass the bioswale. Bioswale effluent should then be directed to a regional water body or stormwater retention facility. It is essential to determine early in the design phase if the proposed site can achieve the correct drainage patterns either through natural or graded means.

What gets planted in the bioswale? Native plants, primarily a palette composed of grasses, is ideal for a California bioswale. Grass’ compact form immediately slows storm and nuisance flows and filters sediments. The dense fibrous root systems prevent surface erosion and maximize toxic uptake. Seasonal moisture is a major consideration when selecting the plant palette. Rushes, sedges and other grass-like wetland species can be used if the site’s hydrology can support them. Post-installation maintenance requirements are key to consider when designing native vegetation as part of a bioswale.

What will it look like when it’s done? Bioswales provide a natural look to a functional drainage and treatment system. Potenial sites, such as road median strips, can benefit from a bioswale’s aesthetic. Visual appeal can be enhanced by accenting the low grassy swale with ornamental border plantings and rockscaping, to mimic a natural wash.

“Bioswales have been underutilized because of the misconception that they require significant amounts of open space” said Chuck Greely, PE, LEED AP, Dudek’s project manager who specializes in bioswale and low impact design. “Effective bioswales can be implemented fairly easily as part of new site design projects, but there are many opportunities to retrofit existing drainage facilities. A thorough engineering evaluation is necessary to determine a proposed site’s suitability for bioswale storm water treatment.”