New York is reviewing its plans for a massive rainstorm after the flooding that has left parts of Houston underwater for days.
"We're looking at what a Harvey-type event would mean in New York City," said Daniel Zarrilli, senior director of climate policy and chief resilience officer to Mayor Bill de Blasio.
The amount of rain that has hit Houston since Hurricane Harvey made landfall late last week would be unprecedented for the northeast U.S., according to Allan Frei, a climatologist and chairman of the geography department at Hunter College in Manhattan. The most serious flooding in the region was Hurricane Irene in 2011, when 15 or so inches of rain left much of Vermont underwater.
Still, Frei said, climate change increases the odds that severe rainstorms like the one in Houston could strike New York City. And if New York ever got as much rain as Houston has over the past week, "it would be absolutely devastating."
"If a storm causes a big storm surge at the same time as it's raining, and if it hits during high tide, that would be -- I can't even imagine," Frei said. Not only would the sewer system get overwhelmed by the volume of water, he said, but it would probably be blocked up with debris as well, diminishing their capacity to drain the city.
The effects would be worse if the storm hit New York City and then kept moving north, which Frei called a common path for storm systems. "Then the Hudson River is going to have water from the upstate precipitation as well."
Superstorm Sandy demonstrated New York City's vulnerability to water: Floodwaters exceeded six feet in the southern tip of Manhattan, the East Village and Chelsea, as well as Brighton Beach and the eastern side of Staten Island.
Since then, the city has updated its protections, including $4 billion in upgrades for the subway system. But some of the same things that define New York -- its underground network of tunnels, and the limited number of options for getting onto and off of Manhattan -- leave it especially exposed to flooding, while complicating the task of evacuating the island if it came to that.
Most of New York City's sewage system is designed to handle no more than one and a half inches of rain per hour, according to Ted Timbers, spokesman for the New York City Department of Environmental Protection; infrastructure built since the late 1960s is better, but can still handle no more than 1.75 inches of rainfall per hour.
Rainfall above that amount will flood, Timbers said in a phone interview. "The system would just back up until it could release itself, like a bathtub."
On Sunday, Houston was getting as much as four inches of rain every hour.
Sea-level rise makes things worse. The water level around New York is 1.1 feet higher today than in 1900, and could rise as much as two feet more by 2050.
"With global warming and sea-level rise, what we are seeing is the effects of these storms amplified," Ernest Moniz, energy secretary for President Barack Obama, told Bloomberg TV Friday. "We're going to have to spend a lot of money, I think, in hardening our facilities, our infrastructure, against this kind of damage."
Frei, who worked with New York's Department of Environmental Protection on a project to look at the city's water supply, said the city's staff do a good job planning for severe weather. But he said that something like what happened in Houston is an entirely different scale of disaster.
"Whether they're prepared for 50 inches in five days, that I do not know," Frei said. "And I wouldn't be surprised if the answer is no."
Megan Pribram, assistant commissioner for planning and preparedness at the city's Office of Emergency Management, said the city has planned for 11 inches to 14 inches of rain, which is the historic high in the area. But she said her office would now look for lessons from Harvey.
"We'll be talking about it for a long time, and re-evaluating our planning," Pribram said in a phone interview. Still, she said, "I think New York is ready."