Infiltration of Highway Stormwater Runoff into Vegetated Filter Strips under Wet Climate in Western Oregon

Stormwater management is an important aspect of highway operation, as the quantity and quality of stormwater generated from road surface can have important legal, economical, and environmental safety concerns. Vegetated filter strips (VFS) are best management practices (BMPs) that are designed to infiltrate and treat sheet-flowed runoff along roads and highways. The effectiveness of the VFS depends on factors like width, slope, vegetation type, flow rate, and soil texture. Despite the broad use of VFS, there is no standard design criteria for roadside infiltration strips in wet climate, such as found in Western Oregon. The objective of this study is to monitor and collect highway runoff generated from road surface within Western Oregon. The data will be used to develop design equations and protocols for sizing and implementing roadside infiltration strips under wet climates.

A monitoring network was established in Oregon from the west slope of the Coast Range to the west slope of Cascades which encompassed Willamette valley. Five monitoring sites were built on highway 18, 20, 22, 34, and Interstate highway 5 to collect surface runoff, soil moisture, and precipitation data. Ten feet by ten feet and ten feet by twenty feet plots were used along the roadside to harvest runoff water. A novel "Groovy Bucket" design together with a digital data logger were employed to monitor runoff flow rate. Physical properties of the soils on highway embankment were also investigated. The results showed that soil texture varies from loamy sand to silty clay among different locations. After the first monitoring period from October 2013 to April 2014, the data showed that VFS worked effectively as the buffer zone to store runoff water in the soils, however, a minimum width of 10 feet can not capture runoff generated by intensive rainfalls. Relationship between soil saturation and runoff flow rate revealed that wet antecedent soil conditions generated more runoff compared to dry antecedent soil conditions. Infiltration tests indicated that soil infiltrability of the filter strips decreased dramatically after receiving the winter precipitation (rainfall and snowfall), which reduces the ability of the filter strips for retaining runoff for the forthcoming storm events.