LAST WEEK in New York City stormwater gushed into subway stations and turned streets into bubbling rivers. Across America’s north-east, flash floods caused by extreme rain from the remnants of Hurricane Ida killed more than 50 people. In July similar scenes played out in Zhengzhou, a city in China, where nearly 300 people were killed in a deluge. Flooding caused by storm surges and tropical storms has long been a risk for cities close to sea level such as Miami or Amsterdam. But, by increasing the amount of water that clouds hold and changing precipitation patterns, a warming atmosphere also means that storms are dumping more water onto cities. How can they learn to cope?

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Urban environments are not well suited to extreme rainfall because concrete stops water draining away. Therefore one of the best things cities can do is to become more porous. By 2030 China wants to cover 30 cities with sponge-like features, which promise to soak up or re-use 80% of rainfall, at an estimated cost of $1trn. These include “permeable pavements”, which allow water to move through them, and “green roofs”, which use plants and soil to collect and filter rainwater. Lower-tech solutions help too. Planting trees and shrubs along streets can reduce run-off. One study in British Columbia found that urban trees can intercept roughly half of rainfall. Cities can also stop water flowing where it shouldn’t. In Rotterdam, the second-largest city in the Netherlands, playgrounds, sports facilities and parks are built to make space for water. On a sunny day in Benthemplein square, a game of basketball can get under way. But when the skies darken the sunken court doubles as a rainwater basin. And simply tidying up makes a difference. Rubbish left out on the streets for collection, as it is in many American cities, blocks water from flowing into drains.

Many of the worst problems occur below street level. Sewers often struggle to cope, regurgitating their contents back up into homes, streets and waterways. London is building a new “super sewer” that will be complete by 2025, partly to avoid this. Greater Chicago, which is in essence built on a swamp, has since the mid-1970s been building the “Deep Tunnel”, designed to divert sewage and storm water into holding reservoirs. But despite much of the system being operational, it continues to be overwhelmed during extreme storms, forcing the city to dump effluent into Lake Michigan. Flash-floods can also play havoc with underground transport systems. In July at least 14 people died in Zhengzhou after rising water trapped commuters in tunnels. Such tragedies can be avoided. In Taipei subway entrances have been elevated to stop water coming in during typhoons. Tokyo’s metro system is equipped with sliding doors capable of withstanding floodwater. And at least 11 of the 13 people who died in New York as a result of the flooding last week were in basement apartments, many of which are illegal and lack safety measures such as multiple exits. The city’s new flood plan—released in May—outlined a scheme to notify people living in basements of flood risks but did not intend to start doing so until 2023.

In many cases, officials and residents fail to treat flood risks with the gravity they deserve. For example, the European Flood Awareness System, an early warning system developed by the European Commission, sent out specific warnings to German officials four days before a violent downpour began in July. Despite that, many residents in the worst-affected regions were caught off guard, and more than 180 people died in Germany. And in Zhengzhou, a forecast of heavy rains from meteorologists on July 19th was largely ignored by the city’s authorities, and its 10m inhabitants were warned too late. Days later more than 300,000 residents of Ningbo, another Chinese city, were relocated after the first warnings of a typhoon. Despite widespread flooding, no deaths were reported. Like many consequences of climate change, heavier and more sudden rainfall is something that cities must adapt to. But sometimes it is so severe that the only thing to do is get people out of the way.

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