Monitoring and Documenting the Performance of Stormwater Best Management Practices

The Center for Neighborhood Technology (CNT) and Hey and Associates (Hey) worked under the support of the Illinois Sustainable Technology Center (ISTC) to monitor and document the performance of stormwater best management practices during 2009 and 2010. There were three components of the project: (1) we conducted real-time monitoring on a bioswale and two patches of permeable concrete and documented the results; (2) we developed and implemented an inventory of green infrastructure features throughout the 6-county Chicago Region; and (3) we selected 15 rain gardens for infiltration testing and three of those for additional synthetic drawdown testing and documented the results. The most important results can be summarized as follows: A bioswale can be an effective method of infiltrating stormwater from a large impervious surface. The limits to its performance may be the permeability of underlying soils. However, if there are existing drainage structures to serve as a backup system, the bioswale can be utilized with confidence; Permeable pavement can also be an effective method of infiltrating stormwater from a parking lot. It can be utilized with confidence if it is placed so that it surrounds existing drainage structures and if there is a maintenance program that prevents clogging. Rain gardens can be an effective method of infiltrating stormwater from a roof or other impermeable surface. While soil conditions vary greatly throughout the region, and often vary substantially within a single rain garden, rain gardens can be used with confidence as long as caution is taken not to divert water toward a vulnerable situation. Despite the many reasons often given to doubt the capacity of our soils to infiltrate stormwater, rain gardens are nearly always successful. For example, one measure of success could be the capacity of a rain garden to infiltrate a 100-year storm event from a tributary area six times larger than the garden. Thus, a garden could be considered effective if it infiltrates seven inches of rainfall from an area six times the garden’s area plus the area of the rain garden, or the equivalent of 49 inches during a 24-hour storm. Our testing indicated that all but one of the 15 rain gardens tested would successfully infiltrate the water from a 100-year storm event.