Legacy LID: Stormwater Treatment in Unimproved Embankments along Highway Shoulders in Western Washington

There are over 7,000 miles of highway in Washington managed by the Washington State Department of Transportation. In non-urban areas, the majority of these roadways drain to unimproved roadside embankments. In order to quantify the stormwater quantity and quality treatment provided by these embankments, monitoring equipment was installed at 4 monitoring sites with edge-of-pavement, 2-meter downslope, and 4-meter downslope runoff collectors. The runoff collectors were installed parallel to the roadway and designed to intercepted overland flow. The data indicate that on average 79% of the runoff from the highway was infiltrated within the first 2 meters of the edge of pavement, while 83% was infiltrated within 4 meters. On average, there was a 72% reduction in peak flow rate between the edge-of-pavement and the 2-meter stations and a 90% reduction in peak flow rate between the edge-of-pavement and the 4-meter stations. Water level observations in the ditch at the bottom of the embankment at two of the sites indicated that runoff seldom emerged at the base of the embankment.