Geotechnical Risks of Infiltration BMPs: Estimating Groundwater Mounding and Soil Moisture Effects of Stormwater Infiltration
Infiltration-based stormwater best management practices (BMPs) are some of the most effective methods for reducing stormwater runoff volume and associated pollutants. Stormwater infiltration is also emerging as a tool for groundwater supply augmentation. However, infiltration of stormwater can pose geotechnical risks, especially in urban areas, and can also reduce the effective lifespan of adjacent pavement or other structures that depend on soil strength. The geotechnical risks associated with elevated soil moisture and groundwater are fairly well documented, but tools to estimate the effect of infiltration BMPs on soil moisture and groundwater levels are not readily accessible to most project designers. This represents a significant knowledge gap for estimating the risk of infiltration as part of project development and review. While major projects may justify a site-specific evaluation, many smaller projects would benefit from a more efficient screening method for evaluating the risk posed by infiltration.
As part of National Cooperative Highway Research Program (NCRHP) project 25-51, Geosyntec conducted variably saturated vadose zone/groundwater modeling of four case study sites and more than 3,000 idealized scenarios to evaluate the effect of infiltration BMPs on groundwater mounding and soil moisture. The result were packaged into a decision support tool to help project designers evaluate the degree of risk posed by infiltration BMPs as a function of site-specific factors.
HYDRUS was used as the primary modeling platform and was validated via comparison of modeling data to monitoring data from four infiltration BMPs, consisting of two vegetated filter strips, a bioretention basin, and an infiltration basin. Models were developed using information that would typically be available to a project designer. Results were then compared to actual monitored data for BMP inflow and bypass and groundwater levels. Results indicated that HYDRUS can effectively model BMP surface ponding and total infiltration for volume-based BMPs and provides relatively strong agreement with groundwater level monitoring data. Results also suggest that appropriate ranges of uncertainty should be considered based on inherent uncertainty in soil parameters.
After model validation, HYDRUS was used to simulate more than 3,000 idealized infiltration scenarios to assess the maximum height and extent of groundwater mounding for different combinations of site geometry, BMP type and design configurations, BMP footprint to tributary area loading ratio, climate zone (including one in California), depth to groundwater, soil properties, and soil layering. Scenarios also considered the effectiveness of design features, such as vertical impermeable barriers and elevated underdrains, for reducing risk.
Model results from scenario simulations were compiled into a planning-level infiltration BMP geotechnical risk assessment user-tool. The tool presents summary statistics and visualizations of groundwater mounding and soil moisture based on user-selected parameter combinations that best match the user’s site. Results are intended to help a user categorize their site into one of three conceptual risk categories: clear high risk (i.e., unlikely to be feasible), moderate or uncertain risk (i.e., possible, but warrants further evaluation), or low risk (i.e., likely safe to proceed without further evaluation).
This tool is intended for use during the project planning stage and could be an effective “bridge” for communicating project feasibility between different entities (for example, project applicants and plan checkers) in support of the CASQA 2017 conference theme. Improving understanding of real risks of stormwater infiltration can also promote responsible and effective application for groundwater augmentation. A brief demonstration will be provided and input will be sought from the audience about key functionality that could improve usefulness of the final tool.
Mr. Gray has more than 10 years of experience in water resources in the academic and consulting sectors. Much of his work is related to stormwater quality and green infrastructure projects including treatment design, monitoring, modeling, geospatial analysis, and applied research. As a research faculty member at Oregon State University Mr. Gray developed innovative stormwater treatment devices and filter media using organic materials to remove heavy metal and other pollutants from stormwater runoff. Mr. Gray has authored peer-reviewed and trade journal publications and has presented his findings at trade conferences throughout his career.