NYC’s subways have a built-in tool for preventing floods. Where was it last week?

After Hurricane Sandy devastated New York and flooded its subway system in 2012, the Metropolitan Transportation Authority (MTA) knew it needed to act. The storm caused an estimated $5 billion in damage, mostly from water getting in where it shouldn’t have.

“Track, rails, signal cables, escalators—every piece of equipment that operates in the New York subway system was affected in some way,” says Robert Laga, deputy business lead for the MTA’s Construction and Development department.

Floodgates have been installed in 68 low-lying subway and PATH (Port Authority Trans-Hudson) stations in Lower Manhattan. Housed at the street level above the stairwells that lead down into the stations, the floodgates are made of a Kevlar-based fabric that can be rolled out horizontally ahead of flood events to create a kind of lid over the openings.

The flooding from that event came mostly from the sheer amount of rainwater that overwhelmed the stormwater system, with more than 3 inches falling in a single hour. Though Hurricane Sandy wasn’t as strong as Ida, the flooding was more intense because it overlapped with a high tide, causing a storm surge to flow from the Atlantic into the subway tunnels and underground stations. Both events are rare, but the overlap of a storm and a lunar-influenced high tide is more easily and reliably forecast. So when it comes to preventing floods from taking down the subway system, the MTA is planning around future Sandys rather than future Idas.

[Image: courtesy ILC Dover]

“We essentially came up with a system that I’ll loosely call a very high-tech window shade,” he adds. The Flex-Gate system involves a combination of tarp-like sheets of coated Kevlar and a restraining web of Kevlar that can withstand thousands of pounds of tension. With a simple crank, a worker can roll the gate out of its storage compartment along tracks on each side of the stairwell and latch it at the surface level, turning the gaping hole of a subway entrance into a waterproof seal. In tests, the system has been able to withstand 16 feet of water.

“It’s a significant investment in the subway system,” says Laga, adding that the cost to install the systems at each station ranged from $600,000 to $750,000, adding up to more than $40 million.  Though the Flex-Gate product can be used during flash flood events, Laga stresses that the MTA’s installation is not being used to prevent damage from flash flooding, which is harder to predict and faster to inundate the system compared to a tidal surge. By monitoring forecasts and tides, Laga says, MTA officials can decide within hours whether the Flex-Gate system should be activated. Though there have been a few close calls, Laga says the system has yet to be deployed.

Though the MTA is ready for the next storm surge, the flash flooding of last week’s Hurricane Ida underscores the challenge facing transit agencies like those in New York and Zhengzhou, China, where 12 people died recently when heavy rainfall flooded subway tunnels. Some extreme weather events can be predicted and planned for, but others can catch people and infrastructure systems by deadly surprise.

The new Flex-Gate systems are a recognition that cities need to do more to prevent such devastation, but the MTA’s Laga concedes that protecting the subway system is a moving target. “When they built the subway system years ago, there was no such thing as global warming or sea level rise,” he says. “Now there is.”