It's a surprising sight: a large garden next to a sound-barrier wall along I-95 in Philadelphia's Fishtown neighborhood, with an undulating grade, marked by tall grasses, wildflowers, evergreens, and other trees.
Unexpected as it may seem, it's part of a meticulously planned, large-scale experiment.
By 2023, 70 "rain gardens" like this one are planned to border PennDot's massive, years-long I-95 reconstruction project. The main phase spans eight miles from I-676 to Cottman Avenue and is aimed at widening and modernizing the heavily used corridor. Smaller-scale work is then planned for Delaware and Bucks Counties.
The gardens are part of a novel collaboration among PennDot, Villanova University, and Temple University to prevent rainwater from flowing from the highway into the city's overtaxed storm-water system.
Philadelphia has a combined storm-water system, meaning all water going into streets and storm drains ends up at one of three treatment plants that can become overwhelmed. When that happens, dirty water has nowhere to go other than directly into the Schuylkill and Delaware River.
The gardens involve thousands of calculations, precisely calibrated contours, and an assessment of thousands of plants to see which ones are hearty enough to thrive in a roadside environment. Previous attempts to use Jersey Shore marsh-type grasses did not go well, for example.
One recent day after a heavy rain, Robert Traver, a professor of civil engineering at Villanova and chair of the university's Urban Stormwater Partnership, was out inspecting one of the rain gardens. This one, running along Richmond Street, between Shackamaxon and Marlborough Streets, covered about 1.7 acres.
"It's dry. It shows it's working," he said, referring to the fact that the gardens are designed to allow water to percolate into the ground or evaporate, rather than pooling. He then popped open a pole-mounted box, filled with wires and labels.
"This is our weather station. It measures wind, solar, radiation, humidity, and rainfall," Traver noted.
"We've really pioneered the monitoring so we can figure out everything we've done wrong. Because this is not easy."
He can check the data anytime on a mobile-phone app or in his campus office. He has six students — four candidates for master's degrees and two seeking doctoral degrees — helping with the project and analyzing data. They also test the soil.
Traver transferred some of the experimenting he has done on the Villanova campus, including several rain gardens there, as well as a former detention basin converted into a wetlands that drains and filters runoff from 40 acres of campus property.
"We have a ton of water that's being removed from the system," Traver said of the I-95 rain garden, noting that most of it is being absorbed into the soil, rather than flowing into the street and then into the sewers. "It also provides greening and cooling."
Residents have adopted the rain garden as a small park, walking their dogs along it. One neighbor even installed hummingbird feeders. At Halloween, they've been known to put up scarecrows since the garden was installed in 2015.
Previously PennDot would have simply installed concrete detention basins, which can overflow, or become stagnant pools where mosquitoes breed, said Elaine Elbich, a portfolio manager for PennDOT.
"This was our test," Elbich said. "When we get into the interchanges [the rain gardens] get much bigger."
She said it takes two years for the plants in a rain garden to take root and flourish. Once they do, they are mostly maintenance-free, except for a yearly weeding and trash removal.
A big test of the technology will be the rain garden being built at the interchange at Girard and Aramingo Avenues. Construction crews have already dug large sections there, unearthing soil that will eventually be covered with plant life.
Also on hand to inspect three of the rain gardens along a two-block section of Fishtown was Edwina Lam, a senior project engineer with AECOM, a subcontractor for PennDot, and a former student of Traver. Lam walked to a rain garden installed for the project that ran behind a home. It was guarded by a tall metal fence to keep out trespassers.
"We're trying to make it as maintenance-free as possible. And we also wanted to make it so it doesn't feel like you live next to a highway," Lam said. "Originally, people were skeptical."
She said the goal is to be able to handle a 1.5-inch rainfall in a 24-hour period, the goal set by the city's Water Department to comply with state and federal regulations under the Clean Water Act. In a normal year, only three or four rainfalls should exceed that.
Laura Toran, a professor in Temple's geology department, explains that the group used the sophisticated LIDAR (Light Detection and Ranging) surveying method to map the rain gardens' area. Toran said such precision was needed to engineer proper drainage.
"Water doesn't always go where we expect it to," she said.
Sasha Eisenman, an assistant professor in Temple's landscape architecture and horticulture department, considered 7,000 plants in the area and found 75 percent of them could survive in a rain garden, which catches road salt, oil, and other contaminants, she said.
Narcissus and black-eyed susan have done well, for example, as have sumac, pussy willow, elder berry, black willow, beach grasses, and switchgrasses.
Early on, some vegetation planted without Temple's help died, especially some varieties placed in gullies.
The rain gardens seem to be working. Traver said a recent test at the site showed the garden took in more than 1.67 inches of rain without overflowing, exceeding its requirements.
Alan Fody, a manager handling storm water for the Philadelphia Water Department, said it's critical to control road runoff.
"Roadways are typically the dirtiest storm-water runoff," he said. "There's a lot of hydrocarbons, bacteria, sediments, and other pollutants. The highway is a large area and a lot of it goes into the combined sewer system."
He said just the stretch of three rain gardens running along Richmond Street, between Shackamaxon and Marlborough Streets, helps drain about 45,000 square feet on a 500-foot stretch of road. Collectively, they can potentially prevent about 32,538 gallons of water from entering the city storm system during a heavy rain.
He said PennDot is also installing pipes in some areas of the construction that run parallel to the city's, but don't drain into a wastewater-treatment facility. That way, if too much rain falls for the garden to handle, the water can be returned to the Delaware River rather than further taxing the sewage-treatment system.
As Fody says, "Every little bit helps."