Field Evaluation of Hydrologic and Water Quality Benefits of Grass Swales with Check Dams for Managing Highway Runoff

A grass swale is a Low Impact Development (LID) technology useful in managing complex stormwater management challenges, especially in linear applications such as highways. In this study, the addition of vegetative check dams on swale performance is evaluated. Check dams will reduce runoff velocity, increase the retention time and therefore allow more time for water quality improvement processes to occur. Two full-scale grass swales in the median of a four-lane highway on Maryland Route 32 have been monitored for 24 storm events. Each swale includes two vegetated check dams and one swale has a pretreatment grass filter strip adjacent to the swale. The project was designed as an input/output comparison between the hydrologic and water quality characteristics, specifically Total Suspended Solids (TSS). Results suggest that grass swales improve the water quality of the runoff and help to reduce the runoff peak. The swales manage to reduce the average peak runoff from 45 to 18 (MDE-CD) and 20 L/s/ha (SHA-CD). MDE-CD performs slightly better due to the ability of the pre-treatment area to increase the infiltration time. Furthermore, the swales managed to reach the TSS research goal of 30 mg/L for 85% (SHA-CD) and 72% of the storm events (MDE-CD). Without those swales, only 28% of the storm events met the research goal. Compared to a previous study, however, addition of check dams shows no improvement for TSS removal.