What Are Sponge Cities and Why is China Building so Many of Them?

They call them sponge cities (海绵城市). They are new urban design strategy that is rapidly being implemented in cities all over China. The idea is to create “cities that can breathe.” By breathe they mean permeate. Yes, sponge cities are cities that water can flow right into, be collected, and reused.

Water is one of the most wasted natural resources. Trillions of gallons of water precipitate down upon the cities of the world only to be diverted into drainage systems which carries it away to far off lakes, rivers, and oceans. All the while, cities pump water in from distant sources or up from aquifers that are quickly being depleted all over the world. At the same time, these drainage systems are often inadequate, leading to mass flooding events that have only intensified as cities continue to expand and changing weather patterns result in more frequent and more intense storms.

Porous bricks in the Yuelai sponge city in Chongqing.

This state of affairs is especially exemplified in China, a country that is paving over thousands of square kilometers of soil, building dozens of new cities, and expanding hundreds more each year as part of an urbanization boom that’s on a scale never before seen in history. This has lead to a situation were half of China’s 657 cities don’t meet national flood prevention safety standards, and 230 of these were hit by severe flooding in 2013 alone. According to EE News:

Experts say that climate change has played a role in the increase of urban flooding, producing more rainfall during shorter periods. Besides that, rapid urbanization has outpaced stormwater removal. Although China’s drainage networks extended to reach 288,838 miles as of 2013 — 20 times longer than that of 1981 — it still can’t catch up with the fast expansion of Chinese cities.

Beyond this, as China continues its urbanization push, myriad natural waterways are being decimated and aquifers are being drained, which is leading to water shortages in a full half of the country’s cities, according to the Ministry of Housing and Urban-Rural Development.

Conveniently, China’s duel problem of flooding and water scarcity is being combated with the same solution: sponge cities.

Yuelai Exhibition Center, the site of a sponge city in Chongqing.

Although China has been experimenting with mass urban water reuse systems for over a decade, it wasn’t until Xi Jinping declared that cities “should be like sponges” that the sponge city movement was born. As of now, 16 pilot cities have been selected to be part of a national program to create sponge cities, and plenty more local municipalities have jumped into open seats on the bandwagon with similar programs of their own.

The idea behind a sponge city is to re-rig the urban landscape to be a place where each drop of water can be collected and reused locally. So rather than the usual course of deflecting rain water away from urban areas with storm sewers and drainage systems, a sponge city will absorb this vital resource like a sponge. This is done with porous concrete that rain water can permeate through into the shallow aquifer or submerged cisterns below and with gardens (sometimes on rooftops) that can hold the water in the soil or divert it to holding tanks. This captured water can then be re-purposed as drinking, cleaning, or irrigation water.

I flagged down a taxi and rode out to a sponge city that’s being built on the outskirts of Chongqing. The place was called Yuelai New City, and is the site of Chongqing’s new International Expo Center. The place was out there — at the newly opened last stop of metro line 6, almost parallel with the city’s airport. The ride took nearly an hour from the city center through highway-landia and conglomerations of urban sprawl.

Building in a sponge city.

There isn’t really much going on yet in Yulai New City. There is basically an exhibition center, a hill where what appears to be a luxury condo complex is being built, some construction lots, and a valley that leads down to the Jia River.

I stepped out of the car at the exhibition center and quickly realized that there isn’t really much to see in a sponge city. Parking lots and gardens are to be found everywhere in China, and the visible elements of this place appeared very much like a conventional city on the surface. But the action in a sponge city happens underground.

What was noticeably different here were the streets themselves. Sponge cities are, well, spongy. The porous concrete isn’t hard and unforgiving, but is soft and springy. Walking around on the roadways, sidewalks, and plazas of the exhibition center felt like walking on a modern athletic track in a new pair of sneakers — I could feel the concrete compressing slightly beneath each step. Although this wasn’t such a drastic difference that the urban dwellers of the future couldn’t readily get used to it.

As I waked around the grounds of the exhibition center I noticed how integrated green space was into the design. Each parking lot was decked out with grid lines of gardens; trees and bushes where springing up at the edges of the lots and evenly between the parking spaces themselves, setting up a gauntlet for rainwater collection.

A grid of green space.

Like Yuelai New City, many sponge city projects tend to be happening in newly developed areas of China’s largest cities. Shenzhen, Xiamen, Beijing, Wuhan, Chongqing, and Zhenjiang all have major sponge city projects in the works, and this trend seems as if it may become a fundamental part of China’s urban design strategy from here on out.

By inhibiting flooding while retaining water, sponge cities turn precipitation back into the blessing it is meant to be rather than the burden it has become.