Making cities 'spongy' could help fight flooding — by steering the water underground

JINHUA, China — In the shade of a willow tree, Li Tao and his buddy dabble lines in a slow-moving river channel and occasionally pull out a tiny fish.

"It's good to have a place like this for people to relax," says Li, his shirt off in the midday heat.

This place — called Baisha Creek — has come full circle.

The bank used to be concrete, but several years ago, work began to restore wetlands here. Rushes now grow in tall stands, and on islands in the river, weeds thrive on the shallow bank and cicadas buzz in the trees.

Yu Kongjian, the man behind the project, describes a similarly natural scene that existed here 50 years ago when, as a 10-year-old, he was nearly swept away by the water.

"It was a huge flood. But I survived. You know why? Because I grabbed the willows, weeds, the reeds, along the riverbank," he says.

Decades of turbocharged development meant that the riverbank, as well as many others in China, got paved over to channel the water away from growing cities. Textbooks say that's the thing to do, according to Yu. But, he adds, it hasn't worked.

"All these industrialized solution has some failure or has some bad side effects," he says.

Not only would concrete banks have made it harder for a drowning kid to clamber to safety, he explains, but flooding has gotten much worse because of "gray infrastructure."

Almost all of China's medium and large cities are now susceptible to floods. And Yu says 60% of them experience flooding every year. Extreme weather from climate change is exacerbating the problem.

So Yu has been evangelizing a solution he calls "sponge cities." That is, urban landscapes that are softer and purposely designed to absorb more water.

In Yu's words: "You are actually playing tai chi with nature, not boxing with nature."

Gareth Doherty, an associate professor of landscape architecture at Harvard University, says the concept is revolutionary.

"Because he's pushing boundaries on what it is that landscape architects do," he says. "He's pushing boundaries in terms of the environmental aspects of cleaning water and working with monsoon, working with rain, working with environmental processes rather than against them."

In fact, the concept is not unique to China; it goes by different names elsewhere — green infrastructure, low-impact development, sensitive urban design. But it's all about giving water space and creating conditions so that it can be absorbed back into the earth, instead of flowing into channels, pipes or streets.

So far, Yu has succeeded against long odds to popularize the concept in China, where the government has been fixated on development as a top priority and where the percentage of people living in urban areas has risen from less than 20% when economic reforms started in the late 1970s to around 65% now.

Removing the concrete

When Yu started to pitch the sponge city idea more than 20 years ago, he says, people thought he was trying to undermine China's development. He says he was even called a spy for the United States, where he studied design. But he says that he has delivered more than 600 presentations to Chinese officials and that the concept has slowly caught on.

One of Yu's early projects was Yanweizhou Park, located a few miles from where he slipped into the creek as a kid, in the city of Jinhua, in eastern Zhejiang province. A flood wall at the confluence of two rivers was failing to protect the area from annual inundation.

"So instead of building the flood wall higher and higher, I removed the concrete wall and [terraced] the riverbank," he says.

He planted natural grass, willows and reeds and installed overflow ponds and permeable footpaths. There's even a flashy elevated walkway painted in red and yellow that's meant to resemble a dragon.

Groundskeeper Sun Zheng'an says the park's touted sponginess seemed to work this summer when a typhoon hit. Heavy rain and winds toppled trees. The river rose, covering low-lying parts of the park, and then subsided.

"The water drained quickly, and there wasn't much pooling," he says.

About a decade ago, Chinese leader Xi Jinping endorsed the concept of sponge cities in the wake of floods in Beijing that killed 79 people. Now, dozens of cities around the country have sponge elements.

And it has catapulted Yu to fame among landscape architects. Some even call him the Frederick Law Olmsted of China — a reference to the man who designed New York's Central Park about a century and a half ago and put landscape architecture on the map.

A problem of scale

This summer, heavy rains hammered Beijing and the surrounding provinces, leading some to, once again, question the efficacy of the sponge city effort.

In a town called Wangping, in the Beijing suburb of Mentougou, a barren swath of mud, rocks and debris stretches alongside a river. It used to be a wetland park — the kind that's supposed to help mitigate flooding — but a torrent of rain caused the river to run high and wild, wiping it out. An overturned car on the opposite bank is a testament to the seriousness of the deluge.

Some experts say this kind of devastation shows the very limitations of the sponge city concept in a changing climate.

But Erica Gies, author of a book about water management called Water Always Wins, says that this may not be the right conclusion.

"People see a city in China flood from a heavy rain and then they say sponge cities doesn't work, but that's a fundamental misunderstanding of scale," she says.

And scale is the biggest challenge that sponge cities — and what she calls the "slow water" movement — face in China and beyond.

"It's a growing movement. But it needs to grow a lot faster and at much bigger scale for it to really help reverse the really extreme scale at which we have altered the natural water cycle," she says.

Getting sponge cities to work requires volume and properly targeted projects.

"If you have a sponge city project in 5 square miles of a city but your city covers hundreds of square miles, it's not surprising that that wouldn't be sufficient to prevent the flooding," Gies says.

Whether the sponge city concept can reach critical mass is an open question.

Yu, for his part, says he'll continue to lobby officials at home and abroad because many still don't get it.

"We still trust that concrete can solve the problem. We still trust that technology can solve the problem," he says.

And that mentality, he says — not the concrete — is the hardest thing to break.

With additional reporting by Aowen Cao. [Copyright 2023 NPR]