How do you prevent New York City subway flooding in extreme rain?
How do you prevent New York City subway flooding in extreme rain?
At one point during the morning of Sep. 29, every subway line in New York City was either fully or partially suspended, rerouted or running with delays.
It was pouring, up to 3 inches an hour in certain parts of the city. The sewers were overwhelmed. Streets and highways flooded — so did 45 subway stations.
It was a mess. It was also familiar.
Two years earlier, also in September, the remnants of Hurricane Ida dumped a record amount of rainfall on the city in a single hour and nearly brought the subway system to a halt.
“We designed our sewers to absorb a rainfall of 1.75 inches per hour,” said Rohit Aggarwala, New York City’s chief climate officer and commissioner of the Department of Environmental Protection, which manages the sewers. “And until 2021, that was a pretty conservative number.”
It’s not anymore. This kind of sudden, torrential rain is becoming more common in the Northeast and other parts of the country as the climate changes.
Since 2021 in New York, several storms have dumped more than 1.75 inches of rain an hour and overwhelmed the sewer system. When that happens, the water flows along the streets and then down, into any other openings it can find — including the subway (which, of course, is mostly underground).
A lot of work needs to be done both underground and above ground by many different departments and agencies to make the subway, so critical to the life and economy of the city, more reliably functional in extreme rain. Some of it is going to take a long time.
“Moving concrete and steel and pipes and electrical cables and reengineering the city is the work of years and decades,” Aggarwala said. “Unfortunately, the climate is catching up to us really quickly.”
Flooding from extreme rain is particularly challenging to predict and prepare for, according to Eric Wilson, vice president of climate resilience and sustainability planning at the Metropolitan Transportation Authority, which runs the subway.
“There’s just a lot of uncertainties associated with rainfall,” he said. “There are patterns. Like, we’re understanding that where rainfall does accumulate, it can overtop sidewalks and go into our subway stations. That’s one of the biggest causes of inland flooding in subway stations. And so we take steps to mitigate that as we learn more.”
Even seemingly small changes to infrastructure around stations can make a difference.
You can see a few examples of how the MTA and the city are trying to mitigate stormwater flooding at the Chambers Street station in downtown Manhattan on the 1/2/3 trains — if you know where to look.
Chambers Street flooded in another torrential rainstorm in 2007, but not in Ida in 2021, and not in September’s storm either, according to the MTA. That is, at least in part, because of infrastructure installed around the station in 2009.
Unlike most of the metal grates you see all over the city that allow airflow into the subway, the vents above the Chambers Street station are not flush with the sidewalk. They’re raised a few inches “to prevent stormwater that overtops this curb from penetrating down into the vents,” Wilson said.
Looking at them, it doesn’t seem like a few inches would make much of a difference, but it does, he said.
So does raising station entrances, so that instead of walking off the sidewalk straight down the stairs, you step up a few inches from the sidewalk first before walking down into the subway.
Much like the elevated vents, Wilson said, the elevated step at Chambers Street is designed “to prevent stormwater from just flowing directly down into the stairs.”
And down the block from the subway entrance, the curb is much deeper by the storm drain, or catch basin, than it is anywhere else, and the road itself is angled to funnel rainwater toward the drain.
“All these kinds of very nuanced things that you might not notice are actually really critical to keep stormwater from entering the subway station,” Wilson said. So is keeping catch basins clear of trash and leaves, something the city’s been pushing in a big way.
At this point, 28 subway stations out of 472 have some of this infrastructure in place to prevent flooding from extreme rain, and work is underway on others.
These kinds of interventions are “likely to be good enough for the places that will be affected by flood accumulations of, let’s say, 4 inches or 5 inches,” said Marcel Negret, a senior planner at the nonprofit Regional Plan Association. But for other stations and other parts of the subway system, “where you would see deeper levels of flooding, you probably need something else.”
The MTA spent $64 million after the 2007 rain storm to mitigate flash flooding in the subway. The current capital plan includes $75 million more for stormwater mitigation and additional funding for related projects like wells and pumping infrastructure.
But it’s not just up to the MTA. The city also needs a lot more permeable surfaces above ground — more rain gardens, green infrastructure, buildings and roads that can absorb water before it gets to the sewer or the subway.
Rohit Aggarwala, the city’s chief climate officer, said that work is happening, but it’s going to take time. There’s no way around it.
“We also have to start as a city, as a community, recognizing that rain is now a thing that we have to take seriously,” he said. “I think New Yorkers have this tendency to assume that, ‘Yeah, when there’s a blizzard, I might have to stay home for a day, but when it rains, no matter how hard it rains, everything should work normally. There shouldn’t be any flooding in the street; all the trains should run.’ That’s not the world we live in.”
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