China’s “Sponge Cities”

A reader sent me a link to a fascinating article by Ann Scott Tyson in the Christian Science Monitor about China’s “sponge cities.” It reminded me of a recent study by Dr. Matthew Berg about the need for conservation to be a component of all flood solutions.

Tyson’s Sponge City article focused on recent Chinese floods from July 17-21, 2021. They hit a large manufacturing center called Zhengzhou with a population of more than 10 million especially hard. Rains there were eerily reminiscent of Hurricane Harvey.

While the Sponge City investments didn’t save Zhengzhou from a 5000-year flood, they did provide many benefits. And many lessons.

Sponge Cities and Green vs. Gray Infrastructure

The title of Tyson’s article is, “To curb urban flooding, China is building ‘sponge cities.’ Do they work?” Answer: Not by themselves. At least not yet. And not for a 5000-year storm.

The Sponge Cities concept has a large green component as opposed to being all gray. Think of gray infrastructure as dams and other concrete based solutions. China has constructed 97,000 dams since the 1950s, says Tyson. That doesn’t even include the dikes and levees also built to prevent riverine flooding in cities built on floodplains. But those alone were not preventing flooding.

China has had a large urban migration in the last 40 years. Since 1978, the country’s urban population expanded fivefold. As concrete replaced green space, urban drainage systems in most Chinese cities proved insufficient to cope with rising flood risk, says Tyson. Hence, a push for more green solutions

Says Tyson, “China’s Sponge City program aims to use pervious pavements, rain gardens, green roofs, urban wetlands, and other innovations to absorb water during storms. The soil then purifies that water and gradually releases it – much like a sponge. The government has invested more than $12 billion in the program since 2014 to help cities create sponge features on 20% of their land, with the goal of retaining or reusing 80% of annual precipitation by the 2030s.”

But the Sponge City idea involves more than just green features. Guy Carpenter, who models climate risks for insurance companies, points out that the concept also includes construction of large capacity drainpipe networks, underground stormwater storage tanks, and other flood-control facilities.

One-Hundred-Year Level of Protection Inadequate for Record Rains

Says Carpenter, “The aim of the Sponge City project is to protect the city from floods with return periods up to about 1-in-100 years (1% annual chance). Both the peak rate of precipitation and the total rainfall amount of this event far exceeded the tolerance of the design scope of the Sponge City.”

Zhengzhou in Mainland China’s Henan Province was struck by tropical cyclones Cempaka and In-fa. They dumped more rain on Zhengzhou in a day – 28 inches – than it usually receives in a year. From start to finish, Zhengzhou received 32.5 inches. The one hour peak was 7.9 inches.

According to Guy Carpenter, who models climate risks for insurance companies, $7.7 billion of those $12 billion “sponge city” dollars were invested in Zhengzhou. Zhengzhou is one of China’s major manufacturing centers and where Apple manufacturers most iPhones.

The Sponge City investments were not wasted. Despite the severity of the rainfall, they eliminated 125 previous flood-prone locations (77% of total). They have also proven effective with light to medium precipitation, reducing flood peaks, promoting the sustainable circulation, and recovery of rainwater.


Yale Climate Connections reported that preliminary damage estimates exceed $10 billion.

The South China Morning Post reported that the flood caused at least 66 deaths, including 14 in the local subway system and six in a tunnel.

Extreme flooding in Zhengzhou, China, on July 20, 2021, after 28 inches of rain fell in 24 hours. (Image credit: UN Climate Change Twitter feed)

The heavy rainfall almost caused several reservoirs to breach. More than 230,000 people in surrounding areas had to be evacuated as a precaution. Workers are still shoring up the reservoirs.

The severity of this event simply exceeded the flood control and storm-water drainage facilities in the city including the ‘sponge city’ additions. The event had knocked out transportation, communications, water supply, power and other industries.

Zhengzhou’s Hurricane Harvey

Sounds a lot like Houston during Harvey. And it was, relatively speaking.

The following tables from Harris County Flood Control District’s final report on Tropical Storm Imelda compare rainfall totals for different durations during several recent storms including Harvey and Allison. These are the max totals recorded inside Harris County. In some cases, the storms produced higher totals in neighboring counties.

Max rainfall rates recorded in Harris County during different durations three major storms.

Remember, Zhengzhou got 28 inches in a day, 7.9 inches in an hour and 32.5 inches for the storm. So their totals are comparable to Harvey’s.

Yet Zhengzhou gets 40% less rain on average than Houston. Zhengzhou’s annual rainfall is 29.7 inches. In contrast, Houston averages 49.77 inches.

So you can see how the Zhengzhou flood totally overwhelmed the city’s defenses. The Wall Street Journal reported that the food had a recurrence interval of 5000 years. This was China’s Hurricane Harvey!

Lessons from Sponge Cities and Smart Cities

The South China Morning Post also describes the chaos that reigned in Zhengzhou, much as it did in Houston after Harvey. The Chinese government had invested heavily not just in Sponge Cities, but in Smart Cities. The latter were supposed to provide people with extra evacuation time. However, the Post reported Zhengzhou was cast back into the “digital dark ages” when the disaster knocked out the Internet and electricity. Warnings did not get from officials to the people who needed them.

Millions of people in Zhengzhou struggled with basic communication, transport, buying food and even keeping people alive,” says the Post.

In my opinion, this story underscores the need to:

  • Take advantage of every flood-mitigation technique possible
  • Use aggressive (not conservative) estimates of rainfall probabilities
  • Rely on fail-safe warning systems
  • Build far from rivers; give them room to flood
  • Be prepared during hurricane season.

Posted by Bob Rehak on 7/31/2021

1432 Days after Hurricane Harvey