ODOT Research: Soil Amendment For Stormwater Volume Reduction

The Project
The Ohio Department of Transportation (ODOT) Office of Hydraulic Engineering Research, in collaboration with ms consultants, inc. and the United States Geological Survey (USGS) has recently completed a research project to determine the effectiveness of soil amendment as a stormwater runoff reducing Best Management Practice (BMP). The 'Assessment of Existing and Potential Volume Reduction for Post Construction Stormwater Management' research project was the largest data collection effort of its kind in the state of Ohio, and resulted in the development of a new stormwater BMP for ODOT transportation projects.
The Problem
Ohio Environmental Protection Agency (EPA) regulations require post-construction stormwater management for all construction projects with earth disturbance over a certain threshold. The ODOT Location and Design Manual, which provides design guidance for all ODOT projects, outlines several stormwater BMPs that have been approved by the Ohio EPA for use on publicly-funded transportation projects. Currently, ODOT can only utilize BMPs that have been approved for use by the Ohio EPA. Traditional stormwater quantity BMPs, such as detention basins and bioretention cells, tend to have large footprints which require significant right‐of way acquisition. When right-of-way is not available, ODOT must turn to underground storage tanks or manufactured systems to meet Ohio EPA requirements. These practices are expensive to construct and maintain, which can jeopardize overall project feasibility. However, many ODOT roadway projects have common features (e.g. grassed medians and grassed shoulders) that likely contribute to overall volume reduction-through infiltration and evapotranspiration. ODOT wants to know whether amending the native soils within these grassed medians and shoulders will increase the infiltration capacity and evapotranspiration ability enough to serve as a stormwater management practice.
The Goal
The 'Assessment of Existing and Potential Volume Reduction for Post Construction Stormwater Management' research project focused on evaluating the effectiveness of soil amendment as a stormwater runoff reducing practice. The ultimate goal was to develop a new BMP for managing post-construction stormwater on ODOT projects, that could be sited in the ample available green space along uncurbed roadways. The prospective new BMP will provide ODOT a low-impact, cost-effective alternative to detention basins, bioretention cells, underground storage, and manufactured systems.
Research Approach
The research team installed real-time flow monitoring equipment along ODOT roadways to measure stormwater runoff across the state of Ohio. This equipment included an h-flume with wingwalls, crest stage gage, stilling well, tipping bucket rain gage, and a camera. Monitoring equipment was powered by solar panels. Flow data and photographs were collected at 1-minute intervals, and transmitted to the data analysis team via LTE cellular data.
Existing conditions flow monitoring was performed from March 2018 to December 2018. This allowed the research team to establish a baseline, and determine how much runoff is currently occurring along typical ODOT roadways. Following the existing conditions monitoring period, soil amendments were installed at the monitoring sites in spring 2019. Soil amendments consisted shallow incorporation of amendment material into the top layer of native soil. Natural and manufactured soil amendment materials were evaluated, with the intended goal of improving the soil porosity, texture, and capacity to hold moisture, in order to improve the soils long-term capacity for infiltration and exfiltration. Four unique soil amendment designs (comprised of sand, expanded shale, and compost; at varying depths) were installed. Following the soil amendment installation, post-amendment monitoring was conducted from May 2019 to September 2020. The post-amendment flow data was compared to pre-amendment data to quantify the stormwater runoff benefit of the soil amendments and determine which soil amendment design was the most cost effective.
Findings
The research team recorded over 4,200 rain events over the course of project, which resulted in nearly 45 million gallons of measured rainfall and nearly 13 million gallons of measured stormwater runoff. The amount of stormwater runoff recorded at the monitoring sites was generally lower than the research team expected. This was true for the existing conditions monitoring period and the post-amendment monitoring period. During the existing conditions monitoring period, the average runoff percent (total runoff volume divided by total rainfall volume) was approximately 27%. During the post-amendment monitoring period, the average runoff percent was approximately 24%. While the change in runoff volume from the existing condition to the amended condition was not as significant as the research team anticipated, the overall low runoff percentages are an encouraging finding and a positive outcome of the research. Soil amendments appeared to improve soil quality and increase the likelihood that the resultant runoff percent will be low, by providing consistent, homogenous, media within the top layer of soil. Cost-benefit analysis showed the soil amendment to be cheaper to construct and maintain than traditional stormwater quantity BMPs. The amendments can also be implemented in the grassed medians and side slopes within existing ODOT right-of-way, which further increases the cost efficiency of soil amendment compared to traditional stormwater quantity BMPs, which often require additional right-of-way acquisition.
Recommendations
Based on stormwater runoff performance, soil quality benefits, and overall cost-benefit analysis, the research team recommended the Amended Vegetated Filter Strip (AVFS) BMP for incorporation into the ODOT Location and Design Manual, as a post-construction stormwater quantity BMP. This new BMP will benefit ODOT, regulatory agencies, design engineers, and contractors/developers by providing a space-efficient, cost-effective, and easy to implement alternative to traditional stormwater quantity BMPs. The Amended Vegetated Filter Strip BMP is grassed and should be implemented in a strip, parallel with the roadway, so that it can receive sheet flow directly from the paved roadway areas. The AVFS BMP soil mix, comprised of sand and compost, will be incorporated into the existing soil to a depth of 6-inches. Detailed material specifications, construction procedures, stormwater treatment credit criteria, and maintenance requirements will be published in Volume 2 of the ODOT Location and Design Manual.
Discussion
This presentation will summarize the technical objectives, research methodologies, results, recommendations, and lessons learned from the 'Assessment of Existing and Potential Volume Reduction for Post Construction Stormwater Management' research project. It will also discuss the benefits of stormwater research (quantifying hydraulic performance, evaluating operation and maintenance needs, improving environmental stewardship, etc.), and who stormwater research can benefit: (State DOTs, regulatory authorities, stormwater designers, municipalities, maintenance crews, etc.).