The following is an email interview with Dr. Allen P. Davis, P.E. the Director of the Maryland Water Resource Research Center at the University of Maryland. Dr. Davis is a leading researcher on bioretention and has published numerous studies quantifying the benefits of its use in urbanized watersheds and low impact development (LID) concepts. In 1993, he received the National Science Foundation Young Investigator Award.
Green Infrastructure Digest (GrID): Over the last 20 years, you and your department have been instrumental in building the current body of knowledge regarding the design and effectiveness of bioretention systems to address stormwater run-off in urban areas. In regard to your current stormwater research, what issues are you and your department studying? Beyond your current projects, what are the issues that you think need to be studied in the coming years?
Dr. Allen P. Davis: While we find bioretention to be effective in the management of urban runoff, we still have many unanswered questions and opportunities for improvement. First, we need to be able to quantify performance results. Bioretention systems are all not the same, we should not expect each of them to perform identically, and our (and others) research show that they don’t. Bioretention performance will depend upon the characteristics of the contributing watershed and surrounding soils/hydrogeology, surface area, media depth, placement of underdrains, media characteristics, flow patterns, vegetation, and other factors. As we better understand the fundamentals of bioretention, we can better predict the effects of these parameters, leading to better designs and more effective watershed management.
Additionally, we are interested in improving the performance of bioretention in removing nitrogen and phosphorus compounds. These nutrients are the pollutants of primary concern for many water bodies, certainly for us in the Chesapeake Bay watershed. Bioretention performance for these nutrients is marginal and modifications to standard designs are being investigated to improve N and P removal.
The list for research topics is very long: what are the fates of captured pollutants? what is the role of biological processes (hydrocarbon degradation, plant uptake, nitrification/denitrification)? what are the best vegetation and vegetation management practices?
GriD: Over the past few years, green stormwater infrastructure has increasingly been employed within various Cities’ stormwater overflow control plans. As you know, bioretention is one of the dominant tools within the green infrastructure toolbox. Are bioretention facilities an effective tool for reducing stormwater run-off particularly in CSO events within our urbanized areas? If so, what impact has your research or others been able to demonstrate? If not, why?
Dr. Davis: Bioretention clearly can play a role in stormwater volume reduction. The larger the bioretention facility, the greater the reduction. CSOs present a greater challenge than suburban bioretention because of the lack of available land in cities. Some creative thinking can help to improve infiltration and storage in highly urbanized areas, but this is a major challenge. Some cities are looking to expand green space, even through opening up vacant lots for stormwater management. This can be helpful, but must be done on a large scale to show meaningful results.