This project examined the role that road shoulders play in the stormwater runoff process. The goal of the research was to determine the type of shoulder treatment that yields the least quantity of runoff of the highest quality. Three types of shoulder materials were tested: conventional asphalt, gravel, and porous asphalt. Porous asphalt allows water to penetrate and flow through the pavement to a sublayer, and it can be used in place of conventional asphalt on low-traffic roadways. Each of the three shoulder materials were tested in duplicate on a heavily traveled, two-lane road north of Redmond, Washington. Stormwater runoff from the road flowed onto the shoulder test sections and was collected in a stormwater collection system at the base of the test sections. Flow-weighted composite samples were collected, and both runoff quantity and quality were evaluated.
On the basis of results from 11 storms monitored between November 1995 and August 1996, several trends were identified. The porous asphalt shoulders demonstrated a greater potential to reduce runoff volumes and peak discharge rates than gravel and conventional asphalt shoulders. During typical wet season storms (0.76 cm [0.3 in.]), the porous asphalt and gravel shoulder test sections reduced runoff volumes by approximately 85 percent and 30 percent, respectively, in comparison to the conventional asphalt test sections. The ability of the porous asphalt shoulders to reduce pollutant loads far exceeded that of the gravel and conventional asphalt shoulders. During typical wet season storms the solids and pollutant loads from the porous asphalt shoulders were more than 90 percent lower than the loads from the conventional asphalt shoulders. The gravel shoulders yielded load reductions ranging from 10 percent to 70 percent lower than the conventional asphalt, although ortho-phosphorus loads exceeded those of the conventional asphalt shoulder by nearly 30 percent.
Removal rates were highest for those pollutants that were correlated with total suspended solids (0.70<r2<0.95), indicating that physical mechanisms of settling and filtration were critical in removing pollutants from the runoff over both porous asphalt and gravel shoulders. The porous asphalt shoulders were more efficient at removing soluble pollutants, particularly ortho-phosphorus, than the conventional asphalt and gravel shoulders. After one year of use the porous asphalt shoulders showed no signs of clogging, maintaining infiltration rates of 4445 cm/hr (1750 in./hr).