In a “rain garden” off of Chestnut Street, PhD candidate Leena Shevade goes to work clearing a small square of soil and placing a water-filled canister in the hole. She is measuring how long it will take 90 millimeters of water to drain into the soil under a young tree. Then, she’ll measure a spot over in the sun, one on the other side of the garden, and one among the grasses. She’ll probably hit seven spots this morning, each one three times.
These are painstaking, seemingly-incidental measurements, and a labor-intensive act of science. But the success of an entire city’s green infrastructure can depend on such details.
Shevade’s research aims to maximize the management of urban stormwater, a pressing concern in cities where impervious surfaces can lead to massive flooding. In this vest-pocket park and others like it, Shevade studies root zone configuration, ponding, inflow depths on the infiltration performance, and soil conductivity – small-scale factors that impact the effectiveness of urban green infrastructure systems. Her work is part of a vast, collaborative effort among scientists to mitigate extreme weather-related events brought on by climate change, particularly in the Boston-to-Philadelphia corridor. In order to draw reliable conclusions on that effort, every hunch about how to manage runoff has to be tested, modeled, and quantified.
There is no benefit if rain gardens like this one don’t work the way they’re supposed to, Shevade says.
“If it rains and you’re in the forest where there is no pavement and no streets or roofs, only 10% of the rain will turn to runoff,” explains Shevade, who began her PhD work at Drexel in 2014 in the Department of Civil, Architectural and Environmental Engineering (CAEE). “But in cities, it’s more like 90% to 95% turns to runoff. So the natural processes like infiltration, transpiration, etc., we have just eliminated those almost completely from the urban areas.
“What happens in cities like Philadelphia and New York where it is a combined stormwater and wastewater system is that we just built up around those. Now, we don’t have much space. If we want to upgrade that system, it is difficult to do because we have built around it so much that there’s no space to add new pipes. There is already so much stress on these systems. So you ask, what can I do to integrate improvements? That’s where green infrastructure comes in.”
Solutions for Urban Runoff
Shevade defines urban green infrastructure (GI) as a decentralized approach to stormwater management that uses engineered design to mimic an area’s natural hydrology. One such example is the rain garden off of Chestnut Street at Woodland Walk where she does part of her research. While they are welcome pockets of greenery in a concrete world, rain gardens are primarily designed to manage runoff by concentrating water in shallow depressions of soil, grass, trees and shrubbery.
But the growth of GI is outpacing the data on how effective its mitigations are. So, Shevade uses her measurements to determine the dominant variables affecting its performance. She is writing her doctoral thesis, “Key ecohydraulic drivers of spatial performance of urban streetscapes” on those variables.
As part of her work, Shevade uses $10,000 of instrumentation awarded to her in a recent research grant from the METER Group, Inc., a Pullman, WA-based firm. Her Grant A. Harris Fellowship award recognizes graduate students who make significant contributions to agricultural, environmental, or geotechnical science. The grant provided Shevade with a SATURO infiltrometer and 10 Mini-Disk Infiltrometers, which she received this spring.
Shevade was first drawn to GI research when she attended a meeting of the Consortium for Climate Risk in the Urban Northeast, a program overseen in part by Drexel’s Dr. Franco Montalto, an associate professor in the College of Engineering’s Department of Civil, Architectural and Environmental Engineering. Montalto is a co-investigator in the six-state, NOAA-supported consortium.
“Cities like New York and Philadelphia started building decentralized green infrastructure because, on paper, it appeared to be a more cost-effective approach to reducing runoff than one that involved replacing or expanding the sewer system,” says Shevade. “But Dr. Montalto started asking how well the built systems actually were performing. He believes we need precise data to quantify exactly how much water these systems can hold and how they work under different climatic conditions. This is a big and evolving research area.”
Motivated by uncertainty of water
Shevade knows a thing or two about stormwater. As a young girl growing up in Mumbai, India, she was struck every year by the fact that drought and flooding could occur nearly simultaneously in one region. For example, the water pressure in her family’s apartment building was so weak that they had to walk downstairs to retrieve water from a common area. Much later, during the biggest flood she ever experienced, it rained 1,000 millimeters—or about four inches—in one day. Shevade walked six miles to her home knee-deep in runoff. And while she came through it just fine, her sister had to be rescued from a second-floor apartment on a raft made out of office furniture.
“I grew up in India so I know how scarce the water is, and why these proper catchments are needed,” she says. “That’s why I thought, this is where I want to work. This is the area where I should do my research. Because it’s what’s needed right now.”
Shevade got her undergraduate degree in civil engineering and her master’s in environmental engineering, both at Mumbai University. After six years of consulting work with a private firm in India, she moved to the United States with her husband and began her PhD at Drexel. She teaches several freshman design courses.
“Leena’s research will help us to better understand how to adapt cities, in the context of climate change, to extreme weather,” says Montalto, Shevade’s thesis advisor. “She is doing very detailed research that seeks to understand how to get the stormwater in contact with new urban vegetated surfaces. But she also wants to understand what impact that water has on the vegetation – vis-à-vis the soil’s moisture content.”
Shevade expects to complete her PhD work within the year.