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Zhengzhou subway flooding a warning for other major cities

Rescue workers enter a station of Zhengzhou, China’s Metro Line 5 to inspect floodwaters on 26 July.

China Daily via REUTERS

The scenes were apocalyptic. On 20 July, a flash flood in Zhengzhou, a city of 10 million on the Yellow River in China, caused a low-lying, kilometer-long section of the city’s Metro Line 5 tunnel to fill with water, trapping more than 500 riders in a subway train. In real time, passengers posted terrifying videos and photos on social media sites, showing people standing in chest-deep water that was still rising. Rescuers, hampered by extensive street-level flooding, arrived 4 hours later, but 14 people did not make it out alive.

Scientists and engineers are still piecing together the chain of events that led to the tragedy, but already they are warning that the lessons go far beyond China. “The intensity and frequency of extreme weather is increasing with climate change, [and] major metropolitan areas around the world are at increased risk,” says Liu Junyan, climate risk project leader in Greenpeace East Asia’s Beijing office. Municipal drainage systems in Hong Kong or New York City “couldn’t handle so much water” either, says Chen Ji, who studies the effects of climate change on water resources at the University of Hong Kong. Just 3 days ago, several stations in the London Underground were inundated.

Many cities may not be aware of the flood hazards their decades-old subway systems face. “To date relatively little research has been carried out on the study of [flash] flooding events affecting metro systems,” Edwar Forero-Ortiz and colleagues at the Cetaqua Water Technology Centre, a private research institute in Barcelona, Spain, wrote in a July 2020 Hydrological Sciences Journal paper. Even less is known about how climate change is adding to the risks. Against that background, “I think that this flood is very important in terms of providing a warning to subway system managers” that they need to take measures to mitigate flooding, says Taisuke Ishigaki, a flood disaster specialist at Kansai University.

Floods occur in China nearly every summer, but they typically get little notice. Flooding in June and July 2020 along the Yangtze River and its tributaries claimed more than 200 lives and displaced more than 700,000. Such flooding typically occurs after prolonged rainfall within a river basin, providing authorities and residents with days, if not weeks, of warning.

Zhengzhou subway flooding a warning for other major cities

Floodwater trapped passengers in a crowded metro train in Zhengzhou, China, on 20 July. Fourteen people died in the flooded subway.

EPN/Newscom

The flash flooding—experts call it pluvial flooding—that hit Zhengzhou, in contrast, showed how quickly torrential downpours can turn deadly in a densely packed modern city. The capital of central China’s Henan province, Zhengzhou, is in a semiarid region far from China’s coasts where annual rainfall averages 600 millimeters (mm). But on 20 July, 670 mm of rain fell, with 201.9 mm in a single hour. Water saturated the city, pooling in low-lying areas and flooding many streets. Hydrologist Hu Caihong of Zhengzhou University says the city’s drainage system could cope with a 100-year flood—one that historical data suggest has a 1% chance of occurring in any given year. But Hu says what hit last week was a 1000-year interval storm.

The city’s metro system, which Hu says is designed to withstand a 200-year storm, continued to operate. To prevent flooding, the system’s entrances are raised above ground level. But water had pooled in a metro rail yard in a low-lying area of the city. Around 6 p.m. on 20 July, the water pressure toppled part of a retaining wall protecting an access tunnel that leads from the rail yard to the subway. Water funneled through the access tunnel into Metro Line 5. System operators suspended operations on the entire network at about 6:10 p.m., but by then, the disaster was unfolding.

Ishigaki says it is the first subway flooding accident he knows of that has claimed lives. It is not clear whether the Zhengzhou subway had flood gates. Japan’s subway systems typically do have massive gates that can close level entrances and passageways from office building basements and underground malls, tunnel entrances where surface train lines go beneath the streets, and ventilation shafts. “In principle, the idea is to not allow water to flow into the subway,” Ishigaki says.

But Japan also faces greater challenges, Ishigaki says. Most of its major cities are located where rivers meet the sea, and the subways face inundation not just from rain, but from flooding rivers, storm surges, and earthquake-triggered tsunamis, which could deliver volumes of water orders of magnitude greater than local heavy rainfalls. Such events can be forecast—even tsunamis now come with 1 hour or more of advanced warning—allowing subway system operators to halt trains and clear passengers before they shut the gates.

Flash floods are much harder to predict, as the Zhengzhou tragedy and deadly floods 1 week earlier in Germany and Belgium showed. Yet cities have to anticipate all possibilities, Ishigaki says. “Unfortunately,” Forero-Ortiz wrote in an email to Science, “we have found that neither the authorities responsible for transport systems nor urban planners around the world take flood risk into account in their systems.”

With reporting by Bian Huihui.

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