When Hurricane Harvey unleashed five feet of rainfall on the city of Houston in 2017, the floodwaters displaced thousands, killed 16, and cost the city billions. This rainfall came from a ferocious storm off the coast, but experts say the city's growth worsened its effects.
Already, projections have shown that greenhouse gas emissions, warming the climate and oceans, will result in more intense hurricanes. Now, a new study suggests that cities themselves can play a role: In Houston, researchers found that urbanization increased the risk of flooding that August 21 times over.
Scientists who study these storms know that large stretches of paved surfaces lead to increased storm runoff, inundating cities with catastrophic floods. On this front, Houston, which experienced the largest urban growth in the country over the previous decade, was especially vulnerable. As Pacific Standard reported in August, city officials allowed developers to pave over prairie land that might have absorbed some of the floodwaters, adding to the devastation—one reason why researchers are now proposing to restore wetlands and other green spaces as a way to manage rainfall.
But urbanization's effects go beyond runoff. In the study published Wednesday in Nature, researchers looked specifically at the "role of the city" to learn how Houston's geography responded—and contributed—to heavy rainfall, says hydrologist Gabriele Villarini, an associate professor at the University of Iowa.
Villarini and his team found that, not only did the city experience increased flooding because of "extensive concrete and asphalt coverage," but Houston's urban development also directly magnified the rainfall itself, due to an effect that experts call atmospheric drag. "On the very fundamental level, you can think of the [city's] area as a rough surface," which causes friction that slows down storms like Harvey, Villarini says. "One of the byproducts of this effect is that all the ingredients we need for heavy precipitation get enhanced because of the condition of the air as it moves over the city."
Simply put: the greater the drag over the city surface, the heavier the rainfall. These models show Houston was not just ill-equipped to manage rainfall; its city surfaces actually compounded the storm's impact.
Of course, this effect exists alongside the broader threat of climate change, which continues to loom large: In another study released Wednesday, researchers found that climate change increased rainfall from Hurricanes Katrina, Irma, and Maria by a range of 4 to 9 percent. While most scientists have focused on future warming effects, this study looked at evidence from historical storms, and what they found suggested that "climate change to date has already begun to increase tropical cyclone rainfall," the authors wrote.
Looking ahead, they predicted a similar trend: People can expect more rainfall—and faster wind speeds—based on projections from most of the storms sampled.
As more areas of the country are hit with the kind of flooding seen in Harvey, the role of the city will become increasingly important for survival. Villarini notes, however, that more research is necessary to tell whether this effect is unique to Houston and Harvey or applies to all cities and hurricanes.
"Most likely we will not be able to reduce the impacts to zero," he says. "But hopefully some of [the takeaway] is, 'How do we prepare or mitigate the flood impacts from such extreme events?'"