Effectiveness of Grass Swales at Reducing Stormwater Runoff from Urban Highways in Wisconsin (BQY49)


Grass swales offer some degree of water-quality benefit by promoting infiltration, decreasing runoff velocities by increasing time of concentration, filtration of pollutants through the grass media, and uptake of pollutants by plant roots (U.S. Environmental Protection Agency, 2004). Wisconsin Department of Transportation (WisDOT) has to control at least 40 percent of total suspended solids (TSS) of existing highways; newly constructed highways will require control of at least 80 percent TSS as well as 60 percent control of the pre-constructed infiltration volume. These conditions are to be implemented by 2013 to meet federal and state requirements, including U.S. Environmental Protection Agency’s National Pollution Discharge Elimination Standard, Wisconsin Department of Natural Resource’s (WDNR) NR216 and NR151, and WisDOT’s TRANS 401. Since the WisDOT already incorporates grass swales along most highways, they could potentially meet their permit requirement by maintaining grass swales on highways. This study will be focused on measuring the effectiveness of grass swales at reducing stormwater runoff flowing from urban highways. Urban highways have higher flow volumes and concentrations due to additional paved lanes and higher traffic counts. A summary of studies by Nara and Pitt (2005) reported grass swales were able to reduce TSS concentrations by 60 to 90 percent. However, many of these studies only examined concentrations in the grass swales and did not consider volume reductions. This study will evaluate the infiltrative capacity of grass swales and their potential to reduce pollutants. This will be done by monitoring a section of grass swale separated into two contributing components: 1) vegetated side slopes and, 2) grassed channel. An additional section will be instrumented to monitor the grass swale as a whole. This study is a cooperative agreement between WisDOT, U.S. Geological Survey (USGS), and WDNR.


The objective of the proposed grass swale study is to determine the volume and pollutant reduction by monitoring the two individual components and by monitoring the whole treatment practice. The specific objectives are the following:  To determine filter strips pretreatment capabilities at dissipating sheet flow and concentration from an adjacent highway edge.  To determine channel flow treatment capabilities in a grass swale by excluding flow from the vegetated filter strip.  To determine treatment capabilities of a grass swale under the typical WisDOT design.  To evaluate the change in particle-size distributions from vegetated-filter strip, and grass swales.  To verify the infiltration rates from vegetated-filter strips, and grass swales.  To calibrate and verify vegetated-filter strips and grass swales calculations in WinSLAMM.  Document any relationship between vegetated-filter strip and grass swales.

Study Design

The WisDOT incorporates infiltration practices in urban highway to reduce the stormwater runoff flowing from an impervious highway. The two infiltration practices that work in tandem to control highway runoff are vegetated filter strips and grass swales. Vegetated filter strips are located down slope of the highway and are designed as pretreatment to grass swales. Filter strips reduce flow velocities, infiltrate runoff, and remove particulate pollutants such as sediment. 12 Grass swales are vegetated open channels that collect and transport stormwater runoff. Grass swales offer some degree of water-quality benefit by decreasing the amount of impervious surface, promoting infiltration by decreasing runoff velocities and thus increasing time of concentration, filtering of pollutants by grass media, and uptake of pollutants by plant roots (U.S. Environmental Protection Agency, 2004). The purpose of this study is to understand how grass swales work and apply the results to the WinSLAMM model. The study design looks at three aspects of the grass swale. One part of the study is to monitor the swale under the typical WisDOT swale design. This will measure upstream and downstream changes in flow and concentration when a grass filter strip is flowing into the swale. Flows will be measured by pre-rated flow structures. The second part of this study is to isolate the swale channel by berming off the filter strip to prevent flow from the entering into the swale. The third part of this study is to isolate the swale filter strip area which are designed to filter, infiltrate and settle particles be slowing water velocity.

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