Exploring green stormwater infrastructure to manage severe urban flooding and pollution
Concrete is as ubiquitous in cities as it is impervious. Amid worsening storms due to climate change, it only deflects runoff toward the drain, threatening sewer capacities. Lauren McPhillips is partnering with researchers across the commonwealth to engineer stormwater solutions using nature to replace hard surfaces and help control the flow.
Labyrinths of concrete and metal creep below and along our city streets, exposed only by pockmarks of manhole covers and the dark of drains. They lay in wait for the next passing storm, ready to quickly sweep away the water rushing from driveways, roofs and roads. The water is gone for now — but what is sacrificed?
“The most traditional way of managing stormwater was to quickly get it off the road, into the storm sewer system and send it 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.”
Modern urban drainage systems began by whisking the water away at the risk of extreme flooding downstream, she explained. Methods to control stormwater flow like detention basins were then added to hold the water in place until it could be released safely.
But as cities grew, each new development project installed more concrete and other impervious surfaces in place of the softer, natural areas that slow and absorb water. This not only diverted more and more water to the drain, severely tightening stormwater system capacities, but also removed the natural filtration ability of soil and plants to treat the cocktail of contaminants stormwater often carries before it heads downstream, McPhillips added.
“Fertilizers wash off lawns and end up in the drain,” she said. “Brakes shed different types of metal. Even atmospheric deposition — exhaust from cars, buildings, factories — can be sources of contaminants.”
McPhillips, who on Twitter goes by “@PSUstormwater,” studies ways to sustainably manage stormwater in urban environments, a deepening issue as the frequency and severity of extreme flooding and other severe weather events increase due to climate change. She and her team are currently engaged in two projects in Lancaster, Pennsylvania, researching how nature can be re-integrated into cities to “green the grey” and mitigate these water quantity and quality issues.
Lancaster, McPhillips explained, is one of 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. These systems are common in older cities in the Midwest and Northeast, including Pittsburgh, Chicago, Philadelphia and New York.
When these systems reach capacity during a rain event, they are designed to release both stormwater and sewage untreated at a designated “combined sewer overflow point,” which then enters a local body of water. Preventing these overflow instances — which are happening more often — has become a national enforcement priority for the U.S. Environmental Protection Agency (EPA).
“Cities are motivated for different reasons, but a big motivator has been the EPA saying you have to find ways to reduce these combined sewer overflows,” McPhillips said. “It’s forcing them to spend money.”
In 2018, Washington D.C. expanded its existing grey infrastructure to increase stormwater capacity, which worked — to an extent. Most overflows were prevented, but the water that did spill over still introduced significant bacteria to local waterways.
Some cities, like Lancaster, are experimenting with many smaller installations of “green stormwater infrastructure” (GSI), which integrates nature into engineered solutions for stormwater management. The city is awash with small, vegetated areas — bioretention basins, tree trenches, rain gardens and more — that fit seamlessly into the community while also managing the city’s stormwater.
“Lancaster has extensive GSI implementation, making it an ideal site for the work we’re interested in,” McPhillips said. “They use rain gardens a lot, which are located between the street and the sidewalk. The water comes off the street into these contoured bowls with diverse vegetation where the water accumulates and percolates through the soil, allowing plants to take it up and treat it.”
Plants in GSI, like switchgrass, are chosen for their tolerance to a wide range of moisture conditions, and their deep roots retain water and nutrients. GSI soils — usually sandy with a bit of organic matter — are specifically engineered to help retain contaminants like copper and zinc, both heavy metals commonly found in urban stormwater runoff.
But according to McPhillips, a barrage of salt, used to melt snow and ice in the winter and spring, can severely affect the vegetation and soil’s health, including how well they retain contaminants.
“Salt kicks the metals out of the soil particles,” McPhillips said. “All northern areas with freezing precipitation deal with it. What are ways we can better design GSI so the salt doesn’t impact its ability to treat the stormwater and filter the contaminants?”
To understand this effect on GSI performance, McPhillips is collaborating with researchers across Penn State — Shirley Clark from Penn State Harrisburg, Hong Wu from the College of Arts and Architecture and Margaret Hoffman from the College of Agricultural Sciences — as well as Sybil Gotsch, formerly a researcher from Franklin and Marshall College in Lancaster, now at the University of Kentucky.
The team is sampling the water from two Lancaster stormwater basins — one near a street that needs more salt in the winter versus one 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.
Twice a week, Penn State environmental engineering graduate student Alex Brown waters the mesocosms to simulate rain events.
“I often spend my time maintaining the mesocosms in the greenhouse,” Brown said. “Normally, this means preparing synthetic stormwater, which is rainwater that we add pollutants to in known concentrations.”
According to McPhillips, the results of these experiments will hopefully give the team a more nuanced understanding of how well a variety of GSI plants and designs hold up to the salt barrage.
If the researchers find salt has an effect on performance no matter the GSI design or location, then maybe the results could motivate cities to use less salt in the winter, McPhillips said.
“The salt itself is a pollutant,” she said. “If we’re going to see the impacts of salt no matter what, then we have figure out how to get less salt in the system.”
Cities are motivated to reduce pollution in their waterways, McPhillips explained, but she’s found that individual community members are often more excited about GSI’s tertiary benefits.
In addition to controlling the flow of stormwater and retaining contaminants, GSI may help mitigate urban heat islands — pockets of high temperatures caused by dense grey infrastructure like buildings and pavement retaining heat. The new green spaces also help sequester carbon, increase ecological biodiversity and liven community aesthetic.
“There seems to be a lot of interest from community members about these other benefits of these green stormwater strategies,” McPhillips said. “Cities that don’t have combined sewers are also seeing these successes and are starting to implement it, too.”
To this point, the majority of GSI-related research has focused on its water quantity and quality services, she explained, but much is still to learn about how cities can properly take advantage of these other benefits. Thanks to a grant from the Pennsylvania SeaGrant College Program, McPhillips is leading another project to quantify these services, which she hopes will give cities actionable data to help inform their community members and increase the usage of GSI.
Co-investigators on the project include Clark, Wu, Hoffman and Gotsch, as well as Daniel Brent from the College of Agricultural Sciences, and Jennifer Fetter and Tyler Groh, both from Penn State Extension. Students from Franklin and Marshall measured heat and infiltration rates over the summer, while Penn State students collected soil samples and analyzed accumulated carbon. The researchers also started surveying vegetation and collecting insects to investigate how GSI can influence ecosystem habitats.
“Community members don’t have to wait for the city to start integrating GSI to reap the benefits, though,” McPhillips said. "I built my own rain garden last summer leveraging some native plants recommended by Penn State Extension.”
For those interested, the EPA offers a stormwater calculator to help anyone from homeowners to urban planners integrate GSI into their development projects.
“My home is on a slope, and I’m sure my neighbors are happy to have less water flowing down into their yards now during a big rain storm,” McPhillips said. “But on the sunny days, we get the bonus of having nice flowers to look at, and sometimes butterflies stopping by. It’s a win-win.”