Whether a heavy rainstorm leads to destructive flooding, however, depends on a combination of factors: the amount of rainfall, the way that water flows and collects on the landscape, and how all that water is managed. Over time, studies have found, the United States and other countries have managed to reduce their vulnerability to many types of dangerous flooding by building dams, levees and other protective measures.
Still, plenty of risks remain. Cities such as New York are often more vulnerable to sudden downpours because so much of their land area is paved over with impervious surfaces like asphalt, which means that runoff is channeled into streets and sewers rather than being absorbed into the landscape. In Houston, researchers have found that the transformation of open land into paved parking lots and housing developments helped worsen flooding after Hurricane Harvey in 2017.
New York’s subway system, built a century ago, was also not designed to handle more extreme rainfall fueled by climate change. The Metropolitan Transportation Authority has invested $2.6 billion in resiliency projects since Hurricane Sandy inundated the city’s subways in 2012, including fortifying 3,500 subway vents, staircases and elevator shafts against flooding. Still, this week’s flash floods showed that the system remains vulnerable.
And as heavy rainfall increases, experts say, more will need to be done. That could include adding more green space in cities to absorb excess runoff, as well as redesigning sewer systems, roads and public transit networks to cope with heavier precipitation. It also includes updating flood-risk maps to account for climate change, so that people have a clearer sense of where it’s risky to build and where they should buy insurance against flooding.
“Pretty much all the infrastructure we’ve built today was designed to deal with historical weather conditions, and that’s no longer enough,” said Jennifer Jacobs, a professor of civil and environmental engineering at the University of New Hampshire. “It’s tough in places like New York City, because there’s just not much room for the water to go, but we need to think more creatively about drainage and how we design our systems for higher levels of precipitation.”