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.