Performance of Vegetative Controls for Treating Highway Runoff

Vegetative storm water controls include grassed swales and vegetated filter strips. The effectiveness of these controls for removing pollutants found in runoff from highways and other urban areas has not been demonstrated to the satisfaction of regulatory agencies; therefore, these technologies have been limited to applications as pretreatment devices for other structural runoff controls. Many highways in Texas and other areas discharge storm water to grassy medians and shoulder areas, which act as both filter strips and grassed swales. The runoff flows overland down the sides of the median as in a filter strip and then parallel to the highway as in a grassed swale. This study investigated the capability of vegetated highway medians for treating storm water runoff in the Austin, Tex. area. Two medians on major highways were monitored to document pollutant removal efficiencies. The medians were designed solely for storm water conveyance and differed in slope and vegetation type. In addition, the highways adjacent to the medians had different average daily traffic counts and contributing drainage areas. Removal efficiencies at the two sites were remarkably similar despite these differences and were comparable with those observed in structural controls such as sedimentation/filtration systems (i.e., greater than 85% removal of suspended solids). The majority of pollutant removal occurred on the sides of the median, indicating the effectiveness of vegetated filter strips. Consequently, the effect of the length of the median (measured parallel to the highway) on the amount of removal was minimal. These data indicate that medians with side slopes of less than 12% and a length of at least 8 m (measured from the pavement edge to the center of the median) are effective in reducing storm water loads from highways.