Control and clean water with bioswales

Photos courtesy of the City of Salem, Ore.
Soil amendments were applied to improve growing conditions at the bioswale site.

The construction of bioswales is increasing as businesses and consumers look for more environmentally sound water treatment methods. These shallow depressions are designed to slow water runoff and filter out contaminants with natural materials, such as grasses, ground covers, perennials or riprap. They are frequently located next to paved areas, such as parking lots and roads, to control stormwater runoff.

Patricia Farrell, a natural resources specialist for the city of Salem, Ore., says a bioswale’s effectiveness is reliant upon proper design and planning. “Begin the engineering and design early in the process so there is adequate room for the desired water treatment,” she says.

Farrell says the swale’s length and gradient are determined by the amount of water to be treated. Its length should allow water to remain within the swale for about 10 minutes. The slope needs to be steep enough to prevent standing water, yet shallow enough to slow the water long enough for treatment. Generally, slopes of 1 to 4 percent are adequate.

The width of the channel bottom is also important for maximizing filtering area and for slower water; thus, the greatest width possible is recommended to prevent ponding and increase filtration efficiency. There should be roughness adequate for slowing water and maximizing its contact with vegetation. A maximum depth of 4 inches is recommended, not to exceed the vegetation’s height. The speed of the flow determines effectiveness; for water treatment, 1 foot per second should be the highest velocity.

Kroger Park bioswale

In the late 1990s, the city of Salem planned its first bioswale as a demonstration project to document the use of “green infrastructure” to reduce the pollution found in stormwater. The project was funded, in part, by an Oregon Watershed Enhancement Board (OWEB) grant. The remainder of the funding was provided by the city of Salem and other project partners. The city of Salem provided staff, educational materials, printing and plants.

The site chosen was Kroger Park, a 7-acre neighborhood park bordered by Pringle Creek and a public storage facility. Stormwater from the storage facility’s roofs and asphalt parking lot drains into the creek.

The bioswale was designed by Otak, Inc. and constructed by Henderson Lands Services LLC. In 2001, the 200-foot-long grass swale was built with a bottom channel width of 6.5 feet and a trapezoidal cross-section with 4-to-1 (horizontal-to-vertical) side slopes. The total width is 18.5 feet.

The area was hydroseeded to stabilize the soil and control erosion. A fiber matting was installed along the bank where grading had occurred.

A manhole flow splitter was installed to direct stormwater from the site of the storage facility to the swale, and a spreader bar helps maintain an even flow. Temporary irrigation was used to help establish the vegetation, the area was hydroseeded, and erosion control matting was put in place.

The channel bottom was planted with grasses, rushes and sedges. The surrounding area has a variety of trees, shrubs and plants, including 420 tall Oregon grapes, 70 common snowberries and 34 clustered roses.

As a bioswale must endure alternating periods of wet and dry conditions, plant selection is of particular importance. The National Wildlife Foundation recommends swamp milkweed, brown fox sedge, cordgrass, ironweed, spike and dark green rush, wild quinine, spotted Joe-Pye weed, sneezeweed and prairie blazing star. To add aesthetic appeal amidst the grasses, choices such as blue flag iris, dense blazing star, Riddell’s goldenrod and blue vervain may be considered.

Stormwater moves from the facility to the flow splitter, and then travels into the flow spreader area, which is filled with large rocks. Water slowly moves through small openings in the flow spreader bar and into the swale for filtration.

In the channel of the swale, water flow is slowed so that it moves through the 200-foot-long bed in about nine minutes. Vegetation and soil strip sediment and pollutants from the water. Ultimately, the treated water trickles into Pringle Creek.

Restoring the stream bank

In conjunction with the bioswale construction, the creek’s stream bank was restored. Invasive vegetation, erosion and other factors led the city to take on the makeover.

The creek’s steep banks were modified to create a graduated rise, and the vegetation was replaced with native plants. The new flora helped to filter water and stabilize the stream bank, and it also discourages human interference while making the area more wildlife friendly. Previously, lack of shade led to water temperatures that were too high for native fish.

Monitoring and maintenance

The Kroger Park bioswale became operational in 2002. The city monitors the stream bank restoration and water quality and maintains the site.

“The bioswale has been shown to reduce sediment loads and thereby reduce pollution to the stream,” Farrell says. “The plantings have also been very successful and are thriving, providing shade to the creek and wildlife habitat.”

She adds that due to the overall success of the project, OWEB reduced the bioswale’s required monitoring period from eight years to three, but the city continues its evaluations. Its goals are to reduce pollutants by the following percentages: total suspended solids, 75 percent; total phosphorus, 25 percent; metals, 20 to 60 percent; turbidity, 60 percent; and oil and grease, 60 percent.

Upland areas were planted with shrubs and trees.

Total suspended and dissolved solids have been reduced by half in the treated stormwater. Lead and zinc were also cut, but E. coli levels have not improved, which Farrell attributes to contamination by domestic and wild animals. She says the natural appearance of the bioswale area leads pet owners to believe it is an acceptable place for animal waste. The treated water is also tested for pH, chemical oxygen demand and nitrogen following three storms each year.

Farrell says that obtaining water samples has been difficult because much of the water infiltrates before it reaches the end of the bioswale. She recommends that others constructing bioswales establish an automated sampler at the outfall to increase accuracy and efficiency.

The project’s third year report to OWEB was submitted in 2006. It noted an overall 105 percent plant survival rate, although some vegetation was replaced in earlier years. The stream bank was reported to be stable, with no erosion. Progress had been made in increasing canopy cover around the creek, but a 60 percent cover rate will be difficult to achieve. This moderate progress reduced immediate outlook for higher wildlife populations, as well.

City parks personnel maintain the site with assistance from volunteers from the local watershed council. The primary tasks are mowing and clipping grass, with careful management to reduce the risk of summer fires. Weeding and overseeding are performed as needed. Maintenance costs range from $3,000 to $5,000 annually.

According to Farrell, the success of the project was demonstrated when the city constructed another bioswale to manage runoff along a city street. She reports more real estate developers in the area are using the technique to treatment stormwater from parking lots. She believes that bioswales can, and should, be an integral element of a landscape construction business.

“Try to incorporate them into your parking lots and landscaping as an amenity, rather than as an afterthought,” Farrell says. “Get your local ordinances to allow bioswales to count as part of the landscape requirement for development.”

Learn more about the Kroger Park bioswale at www.cityofsalem.net/export/departments/spubwork/admin/water_res/bioswale.

Based in Greensboro, N.C., the author writes articles about horticulture, landscaping, agriculture and travel.