Hydrologic Performance of Vegetated Compost Blankets for Highway Stormwater Management

Abstract

In an attempt to reduce the impacts of stormwater, stakeholders are exploring ways to improve the performance of existing stormwater control measures adjacent to highways, including vegetated filter strips (VFSs). This study assesses the hydrologic performance of a VFS amended with a vegetated compost blanket (VCB) through evaluation of dynamic flow modification, event volume storage, and cumulative performance. Over 2.25 years, 278 rainfall events were observed at a highway median in Maryland for two VCBs of 7.6-cm depth and 30-m width along the highway, one at 3-m length and one at 6-m length. Supplemental greenhouse mesocosm experiments provided supporting information on stormwater storage and slope impacts. VCBs were found to significantly reduce both stormwater flow and volume. Peak flows were significantly reduced by 39% for the 3-m and 72% for the 6-m VCB in comparison to highway runoff. At the highest flowrates, both VCBs were unable to significantly reduce flow, and instead acted as conveyance. Total influent stormwater volume capture over the entire study period was found to be 44% and 55% for the 3-m and 6-m VCBs, respectively. A useful design estimate for representative storage capacity, using saturated moisture content and wilting point of the media, was determined through use of a hockey-stick fit for inflow-outflow curves. Improvements in VFS soil hydraulic properties and vegetative cover over the course of the experiment were observed, indicating additional mechanisms for hydrologic improvement. To reduce and manage excess stormwater, engineers have designed green systems called stormwater control measures to absorb and filter stormwater. One of these systems is a vegetated filter strip (VFS), a thin layer of soil on the sides of highways often planted with grasses. To test whether a layer of compost placed on top would significantly absorb more stormwater, two large vegetated compost blankets (VCBs) were studied on a highway median in Maryland, and small test setups that simulate a highway median topped with a VCB were studied in a greenhouse. It was found that the VCBs lowered the highest peak flows during storms by 39%–72%, with longer flow path VCBs having a better performance. The systems were also able to store 44%–55% of the total volume of stormwater over a 2.25-year study period. An equation was created to estimate the volume of stormwater a VFS with VCB should be able to store; the equation uses the compost and soil volumes and standard compost and soil properties. The VCBs laid over a VFS in this study were effective in reducing the amount of highway stormwater more than a VFS alone.