Cities Fight Flooding by Turning Rooftops Into Prairies
As storms intensify with a warming climate, sometimes the best way to prevent flooding is to find a way to stop storm water from reaching the ground in the first place.
That’s the theory behind green roofs—thin layers of soil and vegetation growing on the tops of buildings to help absorb rainwater and stop it from potentially flooding the city streets below.
Green roofs have been embraced in cities ranging from New York to Portland, Oregon. Recent installations include several new green roofs in Roanoke, Virginia—a city that has flooded three times since June. Others have recently been completed or are being built on an ice rink in Colorado, on top of luxury stores in Paris, and at Amazon.com’s new campus in Seattle.
“Green roofs have become very popular as a form of what I would call decentralized or distributed infrastructure,” said Patricia Culligan, a civil engineering professor at Columbia University whose research has shown that green roofs can absorb as much as 61 percent of annual rainfall on a building, preventing it from flooding streets and overwhelming sewer systems. “They’ve been a strategy in Germany to deal with environmental issues for decades, but now they’re really taking off in a big way in the U.S.”
Noah Garrison, director of the environmental science practicum at the Institute of the Environment and Sustainability at the University of California, Los Angeles, said green roofs play a major role in reducing flooding. That’s because both the soil and the vegetation take up water that would normally fall to street level, where roads, sidewalks, parking lots, and other structures are incapable of absorbing it. “The green roof acts like a sponge and will not release water until either it’s oversaturated or the intensity of the rain is so hard that it can’t soak up fast enough,” he said. “With small events, you can almost eliminate runoff.” That water eventually evaporates, leaving the roof ready to absorb the next rainfall.
When big storms hit, Garrison said, green roofs can at least slow the flow of water. “A lot of runoff gets held to the roof, so you can kind of space out the peak flood time,” he said.
He noted that the best solution to water retention is large green spaces—a 30- or 40-acre park, for example—but added that smaller, on-site controls such as green roofs “are absolutely necessary and beneficial, as well as a cheaper way of dealing with storm water runoff than conventional systems.”
Culligan said individual green roofs each play a role, but it’s even better to consider them as part of a broader system. “One unit does well, but how many can we put in an urban landscape?” she asked.
Not every building is a candidate, she pointed out. Some may have large heating and air-conditioning units that take up too much real estate, while older buildings may not have the structural capacity to support the weight of a green roof. Still, there’s plenty of potential. “We estimate that about 10 to 20 percent of rooftops in New York City, maybe more, could be retrofitted,” she said.
Besides reducing urban flooding, green roofs also help reduce pollution. Storm runoff typically picks up oil, trash, and other pollutants from roadways, which then get pushed into sewers and waterways. If green roofs reduce that runoff, they contribute to cleaner creeks, rivers, and oceans. What water does come down from a green roof is often cleaner. “Rooftops typically have less pollution, and different types, than ground surfaces,” Garrison said. Culligan’s research, meanwhile, has shown that more intense rooftop gardens can even help to reduce the acidity of rain that passes through the system.
Most green roofs use a type of desert succulent called sedum as vegetation. “Sedum establishes well, and it grows everywhere,” said Northwestern University doctoral candidate Kelly Ksiazek-Mikenas, who is studying whether other plants could fill the same role. Her work involves experiments with different types of native grasses and wildflowers, which she found could take up more water while providing a potential habitat for wildlife such as bees, butterflies, and birds.
Of course, Ksiazek-Mikenas acknowledges that biodiversity isn’t the main objective of most green roofs. “People aren’t planting green roofs with the motivation of supporting urban wildlife,” she said. “But animals are here. There are ways that people can share some of our spaces with other organisms. That’s going to be really beneficial for us and for the planet in general.”