Hydrologic Performance of Vegetated Compost Blankets for Highway Stormwater Management

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.