Nearly a year's worth of rain fell on the Chinese city of Zhengzhou over four days this month. Flooding killed 71 people, including 14 trapped in a subway station as waters rose.
Zhengzhou is part of a Chinese government initiative to build "sponge cities" in response to increasing urban flooding across the country. These cities aim to use natural processes to soak up rainwater rather than sending it coursing through concrete drainage pipes and channels that can get overwhelmed or cause other problems.
These systems are catching on around the world as city planners embrace the benefits they provide, such as stormwater management, urban cooling and aesthetic appeal.
But experts say the storm that drowned Zhengzhou was more than any city's drainage system could handle. And because of climate change, these kinds of storms are becoming increasingly common around the world.
"The intensity, the frequency of the storm events is much greater than we've been used to," said Neil Weinstein, executive director of the Low Impact Development Center, a nonprofit group focused on sustainable stormwater management techniques.
"The sponge city concept ... is really good at slowing down modest-sized storms," said David Sedlak, civil and environmental engineering professor at the University of California, Berkeley. "But it's not a replacement for the flood control infrastructure that many of our cities need to protect us from these super big floods and storms that arrive in a short period."
China launched its sponge city project in 2015 with 16 pilot cities. Another 14 were added the next year.
The move was a response to increased flooding in Chinese cities following the country's rapid urbanization.
In 2000, just over one-third of the Chinese population lived in cities. In 2019, that figure reached 60%, according to official statistics.
Building those cities meant paving over forests, fields and farmland. Unlike the green areas they replaced, pavement and the roofs of buildings do not absorb rainwater.
As the area covered by these hard, impervious surfaces increases, so does the amount of water running off of them during rainstorms.
"Runoff all of a sudden has nowhere to go," said Colleen Chiu-Shee, a graduate student at MIT’s Department of Urban Studies and Planning. Big storms can overwhelm drainage systems and lead to flooding.
At the same time, urbanization is leading to water shortages in some cities. Drainage systems designed to quickly move water off city streets do not give rainwater a chance to seep into the ground and recharge underground water sources.
Other cities worldwide have dealt with the same issues. For example, in Washington, D.C., and other U.S. cities, increasing development has led to untreated sewage spilling into streams when heavy rains overburden the sewer system.
Beginning in the 1990s, urban planners in the Washington suburb of Prince George's County, Maryland, began looking to natural systems to help control stormwater problems. The concepts, originally known as low-impact development, have traveled around the world under various names.
The basic idea is to get away from concrete pipes and channels that quickly move water out of the city. Instead, the goal is to slow down the water and give it a place to soak into the ground.
At its simplest, this includes grass-lined ditches called bioswales on roadsides or parking lots. At the other end of the scale are large constructed wetlands that absorb water and remove some forms of pollution.
Plants are key components of these systems. They absorb water through their roots and release it into the air from their leaves. The roots also help aerate the soil to allow more water to infiltrate the ground.
Green vs. gray
The presence of plants is why these systems are also known as "green" infrastructure, and it differentiates them from traditional "gray" infrastructure that relies on concrete and pipes.
Green infrastructure also has added benefits, Sedlak said. "One is that by growing more trees and vegetation in the city, you cool it and make it more livable. The other is, you infiltrate more of that water into the ground and you have a chance of getting that water back out later as a water supply."
That's why China calls them "sponge cities," said Shaw Yu, a retired University of Virginia civil and environmental engineering professor. They absorb water when it rains, "and then when water is needed, it could be pumped out, like when you squeeze the sponge."
Researchers are reviewing the sponge city program, and so far, "the results are very mixed," MIT's Chiu-Shee said. A lot depends on who is running the program and the priorities of the investors and developers, she added.
But no city is designed to deal with the kind of rain that soaked Zhengzhou, or the flooding that struck Germany and Belgium earlier this month.
"The whole world is learning about how badly climate change can really affect cities nowadays," she said.