Resilience

11.04.12

Hurricane Sandy’s Lesson for Flood-Proofing a Subway

How raised entrances, flood gates, and huge balloons could save New York’s subway from disaster.

If there was any doubt about just how serious a threat rising oceans and extreme weather pose to New York’s subway system, Hurricane Sandy put an end to them. Despite covering entrances and vents, erecting flood walls across tracks, and readying pumps, each of the seven subway tunnels leading into lower Manhattan took on water. The South Ferry station flooded to the roof. The 86th Street tracks looked like a river. Five days after the storm, there’s still no date set for when full service will return.

It’s a disaster scenario New York has envisioned many times before, most recently in a 2011 report by the New York State Energy Research and Development Authority, which estimated that a storm like Sandy could cause $10 billion in damage to transportation infrastructure, plus another $40 billion in economic losses. Now that it’s happened, long-talked-about upgrades to the subway have taken on new urgency. The havoc wrought by Sandy should prompt a “fundamental rethinking of our built environment,” Gov. Andrew Cuomo said at a press conference this week. “The challenge is not just to build back, but to build back better than before.”

What would a better, more resilient subway look like? For starters, it would have higher entrances and ventilation grates, says MTA spokesman Adam Lisberg. The MTA has been working to elevate subway entrances and air vents ever since a sudden summer deluge in 2007 dropped 3.5 inches of rain and overwhelmed the subway’s pumps, shutting down 19 lines. So far the MTA has raised entrances at 30 stations and ventilation grates on low-lying lines, often improving the street in the process by turning them into benches or bike racks.

It’s a good start, and an easy way to prevent flooding in an unexpected downpour, but it won’t do much against several feet of storm surge, as we saw with Sandy. Flooding like that calls for something along the lines of Bangkok’s subway entrances, which are elevated over a meter above street level and have built-in flood gates. The Bangkok metro continued to operate even as a foot of water lapped at the entrances during the 2011 monsoon.

The subway also needs more pumps, and better pumps, and they need to be on backup power. “One of the stories that’s emerging is that all infrastructure is interconnected,” says Radley Horton, a climate scientist at Columbia University who served on New York’s climate-change adaptation panel. “When the power is out it’s hard to drain the tunnels—that’s had a lot of impact on the subway system.” The power outage is also partly responsible for the gas shortage, he adds, which has made it impossible for many stations to keep pumping. Not that the current system could have held off Sandy anyway. The subway’s 700 pumps of varying vintage must drain 13 million gallons of water on a normal day; when there’s a storm, even a brief one like 2007’s, it can quickly be overwhelmed.

Robert Puentes, a senior fellow with the Brookings Institution, recommends “mundane but obvious” fixes like higher entrances, storm hatches, and a more resilient electrical grid, as well as more speculative projects, like a giant balloon that can be inflated to plug tunnels. The giant plugs are being developed by the Department of Homeland Security to protect subway tunnels from terrorist gas attacks, but the manufacturer claims they can work for floods as well. Though they wouldn’t be able to hold back all the water, they could limit flooding to a level that the pumps could handle.

“We can cork a wine bottle, so why not a subway tunnel?” says Lisberg, half-joking. He saw the plugs on Twitter but says he has no idea if they work. The MTA did try something similar on the west side yard of Penn Station: a giant tube full of water known as an aqua dam. It burst, but Lisberg says it helped reduce flooding.

Then there are the massive projects designed to reduce flooding in the city itself, like the oft-proposed sea gate, essentially a giant wall in the ocean that would blunt the force of storm surge before it reached the city, with gaps and doors to allow ships to pass. St. Petersburg, London, and the Netherlands have similar barriers. The Dutch engineering firm Arcadis estimated the cost of its proposed New York barrier at $6.5 billion, which might not seem prohibitive after we see the final bill for Sandy. Other, greener options, like wetland restoration or oyster reefs, could also help slow waves before they reach the city.

“We can cork a wine bottle, so why not a subway tunnel?”

There is debate over how such a sea gate would impact the coastal environment and whether it could potentially increase storm surge for communities left outside it. It’s also possible that such a flashy project could provide a false sense of security, Horton says, leading to careless behavior behind the wall and discouraging the city from making the many smaller improvements it needs to become more resilient.

And there are many, many improvements to make. New York’s subway system is 108 years old and not built to withstand the sort of superstorms that will become increasingly common. Cuomo noted this week that “100-year storms” seem to hit New York every two years now. “For us to sit here today and say this is a once-in-a-generation and it’s not going to happen again, I think would be short-sighted,” he said. “I think we need to anticipate more of these extreme weather type of situations in the future and we have to take that into consideration in reforming, modifying, our infrastructure.”