A Green Approach to Reducing Flood Impacts

Ed Brown

As rains get heavier and more frequent, flooding, especially in cities, is becoming a serious problem. The traditional way of managing stormwater has been to quickly get it off the road and into the storm sewer system to be sent downstream, said Lauren McPhillips, Assistant Professor of civil and environmental engineering and of agricultural and biological engineering at Penn State. “With the stormwater out of sight, the problem was out of mind.” However, whisking the water away increases risks of extreme flooding downstream.

Another problem, equally serious, has to do with combined sewer systems. There are more than 850 municipalities across the country with a combined sewer system, which transports both stormwater and sewage together to a treatment facility before it is discharged. When these systems reach capacity during a rain event, they are designed to release both stormwater and untreated sewage at a designated “combined sewer overflow point,” which then enters a local body of water and introduces significant amounts of harmful bacteria.

Bishwodeep Adhikari is one of the researchers working with McPhillips to study ways to manage stormwater in urban environments. When asked what drew him to this project, he said, “One reason I was interested in Lauren’s green stormwater infrastructure (GSI) project, was that I'm originally from Nepal, where there is a lot of urban flooding.”

“Even in the U.S.,” added Adhikari, “it's very intense because of the large expanse of highly impervious areas: paved streets, parking lots, and the like. Whenever there are large rainfall events, we get street flooding, which leads to large quantities of stormwater runoff. It affects the quality as well as the water levels of the downstream water bodies receiving this water.”

Lancaster, PA, has adopted the GSI approach to mitigating these problems, experimenting with many small installations of vegetated areas — bioretention basins, tree trenches, rain gardens, and more.

A GSI Basin is basically a system with soil and various types of vegetation, for example, switchgrass, resilient to high-ponding, high-stormwater runoff. The soil composition is also engineered to be most effective. “We were able to take advantage of the fact that these basins have been in place for a while so we could install instrumentation to analyze how well they are performing. We collect samples of both inflow and outflow water,” said Adhikari.

McPhillips and her group are studying ways to optimize this infrastructure, both for Lancaster and other cities that might be interested. The team is sampling water from two Lancaster stormwater basins — one beside a major street that needs more salt in the winter vs. one beside a parking lot that has less salt applied — while examining how well plants and soil in the GSI retain pollutants in each. They are also performing experiments on lab-scale GSI systems, known as mesocosms, at Penn State University Park.

“The first task for the project was to look at how well these green infrastructures are performing in retaining metals, like copper and zinc, and other nutrients,” said Adhikari. “The metals bind with soil minerals and are retained by the process of adsorption, especially if the soil has a good amount of organic material — we are studying the soil textures to determine which have the best adsorption.” The team recommends regular maintenance to measure when the amount of metals in the soil has reached the point where the soil should be replaced.

According to Adhikari, these green areas can also improve the urban environment by reducing the temperature effects of urban “heat-islanding.” In urban areas, there are many dark surfaces like pavement that absorb a lot of heat. Vegetation shades or transpires (gives off water vapor) to generate air cooling. The team is measuring those temperature effects.

“We are making recommendations to the city of Lancaster, based on our research, on how to best use the GSI infrastructure by engineering the soil and vegetation,” said Adhikari.

This article was written by Ed Brown, Associate Editor, SAE Media Group. For more information, visit here  .

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