The approach derives from the standard flood-control method used in other cities, where water is pumped to above-ground reservoirs during storms. Chicago, for example, stores water and sewage in a nearby quarry during periods of heavy rain.
“What makes this system particularly noteworthy is just its scale, because it’s underneath one of the largest cities in the world,” said Patrick Lynett, a civil engineering professor who studies cities’ water-management tactics at the University of Southern California in Los Angeles. “It allows them to have this flood control out of sight.”
Tokyo’s size and density leave it no space for a typical above-ground solution. The Tokyo-Yokohama metropolis is the world’s second-biggest by land area after the New York City area, with more than double New York’s density, according to data compiled by the London-based City Mayors Foundation.
It’s also facing more occurrences of flooding as the frequency of storms increases. The city’s first underground reservoir, which is even larger than the Furukawa project, was called into action five times last year, compared with a usual rate of once or twice per year, according to Yoshiaki Takahashi, who oversees the facility in western Tokyo. It was used most recently on Aug. 10, when Typhoon Halong, the 11th of the season, poured 17 mm of rain in an hour on the capital.