Bioswales: An Alternative to Storm Sewers

Chicago Wilderness Corporate Council member Gewalt Hamilton Associates, Inc. (GHA) is an 80-member professional civil engineering firm with three Chicago area locations. Learn about one of GHA’s green infrastructure projects that helps manage stormwater runoff and prevent flooding at a high school. 

By Marie Hansen and Heather Miller, GHA, Inc.

Niles Township High School District 219 serves more than 4,800 students within two campuses in northern Cook County. With one of the first five schools in the US to earn LEED certification for an existing building, the District has been a leader in sustainability and environmental stewardship for more than a decade.

Challenge
At the south end of the Niles West High School campus, the District Office backs up to the athletic fields. During a storm, rainwater flows south off the fields and towards the Office parking lot, pooling in the garage. The result is frequent flooding – even during minor storm events.

Traditionally, this type of drainage problem would be resolved simply by installing a series of pipes and inlets through which the flow stormwater would be directed off the site. However, in this particular situation, the traditional approach presented two major problems.

First, the athletic fields are situated within ten feet of the problem area and would be significantly disturbed by the construction activities involved with installing this type of underground improvement.

Second, the cost of the pipe and inlet infrastructure materials exceeded the District’s budget for this project.

Cross-section of a typical bioswale

In search of a solution to this persistent problem, the District opted for a more environmentally friendly application which aligns with their sustainability goals. As an alternative stormwater management approach, GHA designed a series of bioswales through which stormwater is collected and redirected away from the District Office.

Bioswale at the Niles West High School and District 219 Office campus

Solution
Similar to rain gardens, these shallow depressions consist of a soil mixture overlaying a gravel base, with a perforated drain pipe. This construction promotes infiltration of water, rather than the runoff typical of compacted soil. The swales are enhanced with native vegetation which also helps increase infiltration and slow the rate and flow of stormwater as it leaves the site.

Not only was the cost of constructing the bioswales considerably less than the traditional storm sewer alternative, this approach provides several other advantages.

The use of green infrastructure adds beauty to the site. As the plantings mature and become established, they will provide a habitat for birds and butterflies.

Native vegetation helps filter out pollutants, thus improving stormwater quality. Small weirs were also added to the bioswales to help decrease the flow rate, which allows more time for filtration as the runoff is conveyed through the series of swales.

The slowed rate of discharge promotes some stormwater to seep back into the ground, replenishing groundwater supplies. This infiltration process decreases the amount of stormwater leaving the site, which reduces demand on the municipal storm sewer system.

An established bioswale with a colorful display of native flowers and grasses

Results
Two years after planting, the bioswales are well on their way to becoming fully established, as roots deepen and the plants mature. Even during this year’s heavy April rains, the District Office avoided flooding in all areas, with only a minor pooling in the lower level garage dock.

Not only did this application of sustainable design solve the District’s drainage problem, the bioswales provide additional benefits. By reducing runoff, improving stormwater quality, and recharging groundwater supplies, these beautiful swales enhance the campus and offer a tangible representation of the District’s commitment to reducing their impact on the environment.

 

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