Vegetated Swales for Managing Stormwater Runoff from Secondary Roads

Secondary roads play an important role for connecting local communities to principal county and state highways and, in some cases, may account for a large proportion of the state-maintained road systems. Existing swales alongside secondary roads can help mitigate the impact of roadway runoff. Limited information is available to characterize their runoff constituent concentrations and pollutant reduction potentials. Consequently, three secondary roadway segments typical of low-to-medium traffic volumes were monitored within the Piedmont region of North Carolina. Event mean concentrations (EMCs) of monitored constituents were found to be substantially lower than those reported for North Carolina primary roads and the national highway runoff data derived from the Federal Highway Administration. In general, secondary road runoff exhibited site-average EMCs of 32 mg/L total suspended solids (TSS), 0.65 mg/L total nitrogen (TN), and 0.13 mg/L total phosphorus (TP). Roadside swales are effective to attenuate the inflow EMCs to 15 mg/L TSS and 0.48 mg/L TN, achieving EMC removal efficiencies of 53% TSS and 25% TP. There was essentially no change in TP EMCs as runoff passing through the swale. Pollutant-load reductions for runoff moving through roadside swales were 77% for TSS, 67% for TN, and 33% for TP. Runoff attenuation through infiltration in vegetated swales accounts for all or most of the load reductions and points to the importance of maximizing infiltration rates in roadside swales. As a result, state agencies can incorporate the water quality benefits of swale treatment with secondary roads when developing their watershed strategies for nutrient load reductions.