PERFORMANCE OF VEGETATIVE CONTROLS FOR TREATING HIGHWAY RUNOFF

Vegetative storm water controls include grassed swales and vegetated f ilter strips. The effectiveness of these controls for removing polluta nts found in runoff from highways and other urban areas has not been d emonstrated to the satisfaction of regulatory agencies; therefore, the se technologies have been limited to applications as pretreatment devi ces for other structural runoff controls. Many highways in Texas and o ther 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 pa rallel to the highway as in a grassed swale. This study investigated t he capability of vegetated highway medians for treating storm water ru noff in the Austin, Tex. area. Two medians on major highways were moni tored to document pollutant removal efficiencies. The medians were des igned solely for storm water conveyance and differed in slope and vege tation type. In addition, the highways adjacent to the medians had dif ferent average daily traffic counts and contributing drainage areas. R emoval efficiencies at the two sites were remarkably similar despite t hese 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 o ccurred on the sides of the median, indicating the effectiveness of ve getated filter strips. Consequently, the effect of the length of the m edian (measured parallel to the highway) on the amount of removal was minimal. These data indicate that medians with side slopes of less tha n 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.