Bioretention Engineered Media: Creating Specifications for Performance
Bioretention devices, including rain gardens and planters, are increasingly popular stormwater control measures (SCMs) that provide the technical benefits of green infrastructure or low-impact development (LID).
Hydrologic mitigation objectives are accomplished primarily by infiltration through the system to surrounding soils, and evapotranspiration of runoff stored in the growing media (often an engineered mixture).
Runoff water quality improvement is thought to be dependent on residence time, filtration, and/or sorption capability.
The aesthetic co-benefits of bioretention depend on matching plants to media conditions, including low nutrient content to ensure water quality benefits.
Maintaining the infiltration and storage capacity of the media is critical to the ongoing success of all of these functions. However, many design guidelines lack adequate engineered media specifications to achieve these goals.
This presentation compiles recent research exploring typical media specifications and implications for a bioretention system’s ability to satisfy the functional objectives.
Various engineered media have been tested in situ, sampled from field installations, or mixed in the laboratory, and subsequently characterized according to soil texture and particle size distribution, as per the minimum criteria required by many design guides.
These same media have also undergone more detailed assessment, including: infiltration rate, by laboratory falling head methods or field double-ring infiltrometer; water retention characteristics, including porosity, field capacity and wilting point; and nutrient leaching potential.
Results confirm that the minimum metrics of soil texture, particle size distribution and porosity (usually assumed based on soil texture) often bear no discernible relationship to infiltration, runoff storage capacity, or water quality.
This presentation offers suggestions for establishing meaningful criteria for specifying engineered media contributing to resilient bioretention, based on scientific evidence linking characteristics to actual performance.
1. Hydrologic mitigation and water quality improvement in bioretention systems rely primarily on the characteristics of engineered media. System performance can be manipulated by careful configuration of the engineered media.
2. Soil texture, particle size distribution, and porosity of an engineered media often bear no discernible relationship to infiltration, runoff storage capacity, or water quality.
3. Field testing and laboratory investigations of bioretention systems provide evidence to create meaningful specifications for engineered media, with direct relationships to performance including hydrology, water quality and ability to support plant life.
ABOUT THE PRESENTER
Dr. Elizabeth Fassman-Beck is an associate professor of civil engineering at Stevens Institute of Technology (New Jersey). Her research in urban stormwater management quantifies the hydrologic and water quality performance of green infrastructure technologies.
Elizabeth has led award-winning research on green roofs, is the 2018 ASCE-NJ Educator of the Year, and recently gave a TEDx presentation on mitigating CSOs using green infrastructure.