April 13, 2024

Kongjian Yu has a plan for urban flooding: ‘Sponge Cities’

Cities around the world face a daunting challenge in the era of climate change: Intense storms are turning streets into rivers, flooding subway systems and inundating residential neighborhoods, often with deadly consequences.

Kongjian Yu, a landscape architect and professor at Peking University, is developing what may seem like a counterintuitive answer: let the water in.

“You can’t fight water,” he said. “You have to adapt to it.”

Instead of installing more drainage pipes, building flood walls and channeling rivers between concrete embankments, which is the usual approach to water management, Yu wants to dissipate the destructive force of floodwaters by slowing them and giving them space to spread out.

Yu calls the concept a “sponge city” and says it’s like “doing tai chi with water,” a reference to the Chinese martial art in which an opponent’s energy and movements are redirected rather than resisted.

“It’s a whole philosophy, a new way of dealing with water,” he said.

Through his Beijing-based company, Turenscape, one of the world’s largest landscape architecture firms, Mr. Yu has overseen the development of hundreds of landscaped urban water parks in China, where runoff from flash floods is diverted to penetrate the soil or be absorbed by the soil. constructed wetlands.

Yu said growing up in a village in Zhejiang province at the end of the Cultural Revolution showed him how previous generations in rural China “made friends with water.” Farmers in your region have built terraces, berms and ponds to direct and store excess water during the rainy season.

This was in stark contrast to the urban landscapes of modern China. Traditionally, cities in China set aside areas capable of absorbing flood waters. But this nature-friendly urban design largely ended with the Industrial Revolution, Yu said. More recently, millions of hectares were paved to build cities, some of them appearing practically overnight.

“We have been using conventional drainage infrastructure for 200 years and have not solved the problem of flooding,” he said, noting that much of China has a monsoon climate subject to extremely heavy rain squalls that pose an increasing danger as climate change advances. This is because warm air can hold more moisture, resulting in stronger storms.

Currently, 65% of urban areas in China experience some degree of flooding every year, according to Yu. The country is the world’s second largest producer of greenhouse gases, behind the United States.

“The concrete drainage systems that came from the West simply can’t handle it,” Yu said. “We need a new solution.”

The sponge city program was formally inaugurated by President Xi Jinping in 2015 with pilot projects in 16 Chinese cities and has since expanded to more than 640 locations in 250 municipalities across the country.

You can see the concept in Houtan Park, a mile-long green strip along Shanghai’s Huangpu River that Yu designed in a former industrial area.

Terraces planted with bamboo and native herbs and grasses are bisected by wooden walkways that zigzag between constructed ponds and wetlands. Wetlands filter water, slow river flow, and provide habitat for waterfowl and spawning fish.

The goal, at least on paper, is that by 2030, 70% of the rain that falls on China’s spongy cities during extreme weather events will be absorbed locally, rather than accumulating on the streets.

Whether it is possible to convert enough land is a key question.

Edmund Penning-Rowsell, a research associate at the University of Oxford who focuses on water security, said the scale of sponge city projects would have to be enormous to deal with flooding alone. “Take New York City,” he said. “How many Central Parks would it take to absorb this type of problem? You’d probably need half of Manhattan.”

Zhengzhou, in northeastern China on the banks of the Yellow River, was an early adopter of the sponge city concept, spending hundreds of millions of dollars building related projects from 2016 to 2021. But torrential rains flooded much of the city. city ​​in July 2021. creating scenes of destruction and killing hundreds of people, including at least 14 in a subway tunnel.

Why were the floods so disastrous in Zhengzhou? Yu said that some of the money earmarked for sponge projects was diverted to other programs and that the land set aside for them was insufficient. If permeable surfaces or green spaces make up 20 to 40 percent of a city’s area, he said, “we can virtually solve the problem of urban flooding.”

Niall Kirkwood, a landscape architecture professor at Harvard who has known Yu for years, acknowledged that it can be difficult, and sometimes impossible, to convert land into city centers that are already densely built. Still, he said, Yu’s impact as an innovator has been incalculable.

“He created a clear and elegant idea of ​​valuing nature, of partnership with nature that everyone, the man on the street, the mayor of a city, an engineer, even a child, can understand,” said Professor Kirkwood.

Where large tracts of land are unavailable, sponge city projects are replacing concrete and asphalt with permeable pavement, installing green roofs, and creating trenches called bioswales that channel stormwater runoff and use vegetation to filter debris and pollution.

The sponge city concept is not exclusive to China. One of Yu’s overseas projects is the Benjakitti Forest Park, a labyrinth of lakes, trees and miniature islands in Bangkok that opened to the public in 2022 and occupies more than a hundred hectares on the site of a former tobacco factory.

Separately, in 2007, the Dutch government started a program called Room for the River, which consists of more than 30 projects around four rivers, including the Rhine. The idea is to restore natural floodplains in key areas around sites that need protection. The Danish capital, Copenhagen, is using “flood parks” that turn into temporary ponds during heavy rain. Philadelphia and Malmo, Sweden, also have projects.

In addition to flood control, these projects have the advantage of being a cheap way to recharge local aquifers and a low-tech adaptation to help the city’s overheated neighborhoods, because water evaporation has a cooling effect.

John Beardsley, curator of the Oberlander International Landscape Architecture Prize awarded to Yu last year, echoed Professor Kirkwood, saying that Yu’s impact on politics in China, a country that has been more likely to arrest environmental activists than the Taking your messages to heart has been amazing.

Beardsley attributes this to Yu’s deft political skills and infectious enthusiasm, as well as the Chinese government’s powerful push to appear to be solving the problem of urban flooding, which has grown alarmingly in recent years.

“Kongjian has managed to be very critical of the government’s environmental policies while maintaining his practice and academic appointments,” he said. “He is brave and deft in that regard, threading a very narrow needle.”

“Sponge cities are not a total solution, but they make a significant impact,” Beardsley said. “I mean, we need to start doing something.”

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