Improving the Design of Roadside Ditches

Project summary:

A field-monitoring program began in the spring of 2000 to test the ability of a grassy roadside swale to remove pollutants in stormwater. A check dam was designed and installed into the vegetative swale. The check dam system incorporated some unique design features, including a peat filter to trap nutrients and metals and a low rock pool to trap water for the settling of suspended solids and for biological processing. The check dam was cost effective and simple to install. The system was quantified and evaluated hydrologically and qualitatively before and after the check dam installation. Pollutants monitored included total suspended solids, total phosphorus, and ortho-phosphorus. The average pollutant removal rates for the three storms following the installation of the check dam were 54 percent total phosphorus, 47 percent ortho-phosphorus, and 52 percent total suspended solids. Metals were also analyzed for two storm events, one before and one after installation of the check dam. Peat soil samples were analyzed for nutrients, organic content, water capacity, metals, and pH both before and after check dam installation. The results suggest that properly designed short vegetative strips and swales can reduce pollutant levels from the stormwater that drains off roadways.