Performance and Design of Vegetated BMPs in the Highway Environment

Abstract

The objectives of this paper are to document the pollutant removal of existing vegetated areas adjacent to highways and to identify the minimum design requirements needed to obtain substantial pollutant reduction. The pollutant removal performance of highway shoulders (buffer strips) is based primarily on eight test sites located on highway rights-of-way in California that were not designed for stormwater treatment. At each location, multiple stormwater collection systems (over 30 systems total) were installed and e quipped with automated samplers to collect highway runoff after passing through vegetated buffer strips of varying widths. The vegetated buffers at each of the eight sites differed in characteristics such as slope (up to 50 percent), width, vegetation t ype and coverage, soil, and hydraulic loading. The data collected between 2001 and 2003 indicate that concentration reductions consistently occur for TSS and total metals and frequently for dissolved metals. Conversely, concentration increases were observed for dissolved solids and occasionally for organic carbon. Nutrient concentrations were generally unchanged by the buffer strips. The pollutant removal and design of roadside swales is based primarily on the performance of two sites located in Austin, Te xas, and which were constructed solely for stormwater conveyance. The data collected between 1994 and 1997 indicate that the majority of the pollutant removal occurs on the side slopes of the swales rather than along the length. Minimum discharge concentrations observed at these sites were similar to those observed from buffer strips in California. The greatest pollutant removal occurs when the geometry of the swale maximizes the length of the side slope; consequently, the optimum cross-section geometry for highway medians is Vshaped rather than the trapezoidal geometry normally illustrated in guidance manuals. In addition, the relatively constant side slopes facilitates mowing and is safer for the traveling public than channels with abrupt changes in side slopes. Swales along highways are normally sized to convey large, infrequent events, which when combined with the low side slopes implemented for safety reasons results in a broad bottom that reduces the concentration of runoff constituents. Consequently, a flat bottom is not required for effective operation.