For many New Yorkers, the waterlogged subway stations around the city on Monday night were a familiar disruption to their commutes.
Though service returned to normal by Tuesday morning, the storms that struck the region served to highlight — once again — just how overmatched New York City’s subway system is by the increasingly extreme weather caused by climate change.
The subway system, which is more than a century old, is relied on by millions of passengers every day and weaves the five boroughs into a singular city. But it has a longstanding infrastructure problem that is only getting worse as rainfall gets heavier and more frequent.
Even as some improvements have been made, the M.T.A. is in a race against time; in 25 years, the likelihood of torrential rainfall events in the region is projected to almost double.
Twenty of the 472 stations in the sprawling subway network had to be temporarily closed on Monday night, including bustling stops along the 1, 2, 3 and 6 lines in Manhattan, which run the length of the island and link to the Bronx. There were widespread service delays as water sloshed onto busy tracks and platforms. Amateur video footage showed streams of water flowing through the 23rd and 28th Street stations, on the 1 line.
Bored through layers of dense bedrock, the subway system is surrounded by the groundwater that runs beneath the city. Even on a dry day, transit workers are routinely dispatched to plug leaks and pump out typically between 10 million and 13 millions of gallons of water from the system, according to the Metropolitan Transportation Authority, the state agency that operates the city’s mass transit system. On Monday night, transit workers pumped out more than 15 million gallons of water from the system, in part because the city’s sewage system backed up, the M.T.A. said.
The city’s sewer system, like the subway, is more than 100 years old. Both crucial systems were designed for a smaller city that had yet to deal with the level of storm surge and extreme rainfall that it sees currently.
Most of the city’s drainage infrastructure consists of a combined sewer system, which means that a single pipe carries both storm-water runoff and sewage from buildings. These pipes were built for about 1.75 inch capacity. So during an extreme rainfall event like Monday’s, when parts of the city saw more than 2.5 inches of rain, those pipes will overflow.
The impact of rainfall and flooding is also magnified in the subway system because of its porous nature. With rain, any opening that ends in a subway tunnel is a cause for concern, everything from a station entrance to an elevator shaft, said Klaus Jacob, a geophysicist and professor emeritus with Columbia University. The subway system is ventilated, with open grates spread throughout the city.
Janno Lieber, the head of the M.T.A., said that “the system is not a hermetically sealed submarine,” though the transit agency says it has taken a number of steps to prevent flooding at stations.
The M.T.A. is developing automatic closures for the subway system’s 39,000 air vents, according to a recent report by the authority. But those kinds of upgrades are expensive; the authority has requested an additional $6 billion for storm-water and coastal flood resilience improvements.
Dr. Jacob said the blame for recurring subway flooding did not rest solely with the transit agency. “It’s not just an M.T.A. issue — the flash flooding has to do with insufficient drainage, and that’s a city issue,” he said, referring to the Department of Environmental Protection, which oversees the city’s water supply and sewer system.
“What happened last night is really quite simple,” said Rohit Aggarwala, who heads the department. “The pipes were designed for a certain amount of water. A lot more water fell from the sky.”
The department has dedicated itself to major sewer upgrades, green infrastructure projects and to cleaning the city’s 150,000 catch basins, Mr. Aggarwala said. But with a sewer budget of only $1 billion a year, and an estimated $30 billion worth of needed work, it will take decades to complete, he explained.
Winnie Hu is a Times reporter covering the people and neighborhoods of New York City.
Hilary Howard is a Times reporter covering how the New York City region is adapting to climate change and other environmental challenges.
Stefanos Chen is a Times reporter covering New York City’s transit system.
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