Green Infrastructure Really Works
Urban rainstorms frequently result in flooded streets and yards, puddled parking lots, and wet, disgusting basements. Stormwater may eventually wash debris from roadways, clear up that annoying oil slick in the parking lot, and give the city a fresh-washed feeling, but this renewed cleanliness comes with a steep environmental cost: the oil, debris, and dirt washed off roads by rain is subsequently returned, often bypassing sewage treatment systems, to local water sources. Communities can reduce the incidence of this stormwater pollution by installing small-scale green infrastructure features—such as bioswales, permeable pavers, and rain gardens—that are commonly referred to as stormwater best management practices (BMPs).
Recognizing that one of the biggest impediments to the installation of green infrastructure is the lack of supporting performance data, CNT partnered with Hey and Associates (Hey) between 2009 and 2010 to document the capabilities of existing BMPs within the Chicago region. The team worked with support from the Illinois Sustainable Technology Center (ISTC) to develop quantitative performance data for one bioswale, two patches of permeable concrete, and 15 rain gardens. CNT and Hey hypothesized that, in places where simple and inexpensive BMPs are utilized, the runoff, pollution and flooding caused by stormwater can often be effectively mitigated.
The results of the research were published by ISTC in April, 2012 in a report entitled, “Monitoring and Documenting the Performance of Stormwater Best Management Practices.” The data included in the report can be used by communities to support their plans to implement green infrastructure systems.
The bioswale studied by CNT and Hey is located at Our Lady Gate of Heaven Parish, and had been constructed and analyzed by CNT as part of a monitoring project during the summer months of 2007 and 2008. The recent ISTC report expanded on this earlier research to include data on infiltration during winter months (December 15, 2009, through February 9, 2010). The initial testing on the bioswale showed infiltration rates of between 0.2 and0.8 inches per hour, the capacity for infiltration being limited only by that of the underlying soil. The recent winter study yielded similar results, indicating that freezing winter conditions do not negatively affect the effectiveness of bioswales. These green landscape features can and should be installed amidst large impervious surfaces (parking lots), even in regions with persistent freezing winter temperatures.
The two patches of permeable concrete, strategically located around drainage outlets in a parking lot at St. Margaret Mary Parish, were also constructed during a 2007 CNT project. The patches were positioned to efficiently capture parking lot runoff and minimize discharge of water to the small sewer that carries water from both the parking lot and the church gymnasium to the city sewers. This strategy has been so successful that, following the September 2008 record rainfall, Father Jim Barrett told his congregation that,
“At St. Margaret Mary, we were very fortunate in that there was little rain damage…Since we put the water permeable concrete in the Activity Center parking lot and the bioswale in the church parking lot, there has been no water in either building…Thank you Ozinga Concrete Company and the Center for Neighborhood Technology. You saved us hours in clean up costs.”
When the site was revisited in 2009, however, the infiltration rate for both permeable patches was significantly less than anticipated (1.08 in/hr and 1.31 in/hr). After the patches were cleaned with a power washer, the rates returned to an acceptable average rate of 19.3 in/hr, indicating that permeable concrete can be counted on to function satisfactorily only if properly designed, installed and regularly maintained.
The exhaustive data that CNT and Hey had gathered while monitoring the performance of four rain gardens between 2007 and 2009 inspired the researchers to expand the scope of the ISTC project to include an inventory of the green infrastructure elements located throughout the Chicago region. As part of this new inventory, CNT and Hey evaluated fifteen rain gardens to document their capacity to reduce stormwater runoff. The evaluations, which included a visual survey, infiltration rate testing, and synthetic drawdown testing, concluded that even the most economically planned rain gardens are effective methods of stormwater management. Fourteen of the fifteen rain gardens studied were capable of infiltrating runoff from a roof or pavement area six times that of the garden following a 100-year storm event. Because rain gardens are an easily implemented and highly effective element of green infrastructure, they provide an ideal solution for home and business owners looking to reduce flooding and runoff.
Many organizations and environmental programs have created informational resources to facilitate the construction of small-scale rain gardens. Rain Gardens: A how-to manual for homeowners, published by University of Wisconsin-Extension and raingardens.org, maintained by the West Michigan Environmental Action Council, both provide detailed instructions for building rain gardens, helpful fact sheets, and easy-to-follow design prototypes.
After evaluating the performance of the bioswale, patches of pervious concrete, and rain gardens, CNT and Hey concluded that green infrastructure is an effective way to manage stormwater runoff and volume. Installing these green landscape features throughout the Chicago region will reduce the volume of harmful runoff returned to waterways, curtail flooding, and create a more livable urban environment.