Use of Compost and Compost Amended Vegetated Filter Strips for Hydrologic Mitigation: Effectiveness and Design Guidance

Focus Area

Water Quality/Wetlands


Natural Resources







Research Idea Scope

Need for the Research

Highways can contribute to hydrologic changes with subsequent degradation of streams. Permits and regulations are beginning to require management of stormwater to minimize adverse hydromodification. Traditional methods of stormwater control emphasis detention, which requires considerable land. Many urban LID practices, such as bioretention cells, are not applicable in the highway environment. There is a need for hydrologic mitigation techniques that are effective in
linear right-of-ways. Filter strips have been used for treatment of highway runoff, but their effectiveness in hydromodification has received less attention. Washington State DOT (WSDOT) has conducted research on quantifying infiltration in road shoulders and adjacent rights of way in semi-arid eastern Washington, but this has not been extended to other regions. Other studies by WSDOT show that vegetated filter strips (VFS), vegetated filter strips enhanced by compost amended soil (CAVFS) and compost blankets, used for water quality purposes have hydrologic benefits. However, design criteria and methods for quantifying the hydrologic benefit are not well established. 

Scope of the Research

This project will examine how the volume and flow rate of highway runoff is modified by flowing through vegetated filter strips and compost amended filter strips.

Research would consist of a review of relevant existing research and literature, followed by controlled field experimentation. Variables included in the testing include vegetation type and density, soil characteristics, compost characteristics, rainfall intensity, duration and frequency, and roadside slope. The study would look at the modification of the runoff hydrograph, specifically the size and timing of the peak flow rate, duration of runoff, and the total volume of runoff. The results would be analyzed to develop quantitative guidance that can be used by engineers to design and size drainage and secondary stormwater flow control measures (SCMs). 


The proposed U.S. EPA rulemaking will most likely require hydromodification mitigation for highways. In addition, runoff volume reduction is being considered as a surrogate measure for quantitative pollution reduction requirements in NPDES permits. Tools are needed by DOTs to comply with the likely rules. The research should be started now to coincide with the release of the final EPA rule in 2012. 

Urgency and Payoff

The proposed U.S. EPA stormwater rulemaking will likely require state DOTs to mitigate for hydromodification impacts. Roadside filter strips used by DOTs may provide partial or complete hydromodification mitigation. The proposed research would quantify this benefit and give DOTs a tool for hydromodification mitigation compliance. This research could lead to flow control “credits” for SCMs primarily intended for water quality treatment. On the project level the result would be reduced size of detention facilities (with savings in construction and right-of-way costs), or elimination of the need for additional flow control SCMs altogether. 

Suggested By

Center for Environmental Excellence, Stormwater Community of Practice