Stopping the Next Sandy: Tunnel plugs


A tunnel plug could prevent flood damage in subway tunnels. (Graphic via NY Times)

Over the years, the MTA has struggled to keep its subway system water-free. When the Sandy storm surge inundated the system’s East River tunnels, the problem was laid bare for all to see. As most experts agree that the next storm is simply a matter of when and not if, the MTA will have to do something to address its vulnerable infrastructure, and that something might just be a giant plug.

The immediate history of the MTA’s water problems started a few years ago when a torrential summer storm led to massive flooding. This wasn’t the first time vulnerable areas suffered water damage, and the MTA decided to do something about it. Street-level grates were raised a few inches, and staircases were elevated as well. For normal storms, these measures alleviated the water problems, but for hurricane storm surges, the MTA’s temporary and permanent preventative initiatives were quickly overwhelmed.

We lose sight of what happened to the subways because the MTA was able to respond quickly to the problem. Some service was up and running within days, and nearly everything is back to normal. But the system suffered extensive exposure to salt water, and even outside of the fiscal costs of the clean-up, key equipment — signals, switches, track beds, wiring — will now have a short shelf lives. Protection remains key.

To that end, scientists with the Department of Homeland Security are working on developing a tunnel plug, and The Times went in depth into the project today. The key parts:

The idea is a simple one: rather than retrofitting tunnels with metal floodgates or other expensive structures, the project aims to use a relatively cheap inflatable plug to hold back floodwaters. In theory, it would be like blowing up a balloon inside a tube. But in practice, developing a plug that is strong, durable, quick to install and foolproof to deploy is a difficult engineering task, one made even more challenging because of the pliable, relatively lightweight materials required…

A subway tunnel is hardly a pristine environment; it is full of grease and grime — and, often, rats. “That’s something we’ve talked about,” Dr. Fortune said. “We’ve actually put Vectran samples in tunnels, to see if rats ate it. They didn’t.”

There are also obstructions like tracks, as well an electrified third rail, pipes and safety walkways, all of which could cause gaps between the plug and the tunnel walls. Most of the obstructions can be dealt with by modifying a short section of the tunnel to accommodate the plug, which is 32 feet long when inflated. Sharp corners can be curved, flush tracks of the type used at grade crossings can be installed, the third rail can be discontinued for a stretch, and pipes can be made to swing against the ceiling.

Those modifications will reduce potential gaps but not eliminate them. In the most recent test, when Dr. Barbero and a colleague, Eduardo M. Sosa, inspected the front of the plug, they discovered a two-inch gap in one corner. The procedure called for filling the plug with water to pressurize it further, and then introducing water behind it to simulate a flood. But a plumbing failure, unrelated to the plug, ended the test prematurely. It was repeated successfully several days later, Dr. Fortune said, and the plug held back all but a small amount of water.

Henry Fountain’s article explores the construction of the plug. It consists of three layers of durable yet flexible materials. It also explores placement: The plug is designed to fold into the sides of the tunnel and can be deployed remotely. The key questions though concern cost and effectiveness. One plug costs around $400,000, and the MTA, for instance, would need a considerable number of these plugs to adequately protect the porous tunnels.

Effectiveness though remains the biggest concern. The MTA’s tunnels are vulnerable at key access points, but plugging tunnels would simply displace water flow to other vulnerable areas. What success is accomplished if floodwaters destroy a train station but spare the tunnels? The MTA also, as The Times notes, must deal with water that enters through ventilation grates and various other entry points. The plugs can only do so much.

Short of sealing up the system, though, these plugs may be one of the more promising areas of progress. The clock is ticking, and the MTA doesn’t have time on its side. Can something like this be in place before the next flood arrives? If the money is there, a solution will be too.

Categories : MTA Technology

21 Responses to “Stopping the Next Sandy: Tunnel plugs”

  1. Brian says:

    Wouldnt this not prevent flooding, rather it would just change the location of the flood?

    • Anon256 says:

      Indeed. It might have some limited use for protecting particular bits of especially valuable or difficult-to-repair infrastructure.

      • pea-jay says:

        All the more reason to build a comprehensive flood wall along our southern margin and flood gate at the Narrows and at the western end of the LI Sound. Better to keep the water out in a few points than retrofit every conceivable opening.

        • Nathanael says:

          As I say, that won’t work if the next flood is coming from rain dumped upstream in the Hudson, Hackensack, and Passaic basins. For that reason I advocate a floodwall strictly for Lower and Midtown Manhattan. That and a floodwall in the area of Long Island City.

          • Nathanael says:

            (The remaining flood-prone areas should be renaturalized and returned to their role as barrier islands and marshes.)

          • BBnet3000 says:

            In London they shut the Thames barrier during the low tide before the swell of a large rain storm comes down the river, this leaves a hole for all that extra water to fill so that it doesnt back up and flood during the high tide.

    • John-2 says:

      In the case of Whitehall Street, unless the plug to the tunnel was paired with plugs on the station entrances, all the plug would do in the wake of another Sandy would be to turn Whitehall into a mirror of the new South Ferry station, where as the low point on the 1 train’s route, the water simply had nowhere to go and filled up like a plugged sink.

      I would think that identifying all the points of entry for water in the various tunnels and then seeing how the openings can either be raised (if they’re vents) or sealed (if they’re simply openings where water snuck in) would be Step 1 in underground flood control, while looking at sealing any Zone A stations like Whitehall-South Ferry at their surface entrances would be the other initial step. Keep the water from getting into the fare control area before installing plugs at train level, that might then require major new pumps to keep the damage from simply shifting from the river tunnels to the stations nearest the water.

    • Bolwerk says:

      It could certainly reduce damage inside tunnels.

      Also, it might have applications in places where tunnels flood but stations don’t. The L Train 14th Street Tunnel flooded, but I don’t think either Bedford Avenue or First Avenue did.

  2. Larry Littlefield says:

    Electric air pump?

    I was told by someone who knows that another major infrastructure provider used air pressure to keep water out of critical infrastructure in the street. When Lower Manhattan was blacked out, the air pumps shut down, and the infrastructure was destroyed.

    The barriers should not rely on ongoing electricity.

    Moreover, better to let the tunnels flood but protect the interlockings. You really need to place these things to guard the portions of the system that are not tunnels and are not in the flood zone. Would a 5 foot higher flood get to Chambers Street? That would be a real disaster.

  3. Scott E says:

    An interesting idea, and I certainly think it has potential.

    My one concern is that I hope there are enough air-leaks in the tunnels to maintain pressure. If a plug is put at both ends of an under-river tunnel, and a surge pushes these plugs in, say, 10 feet on each end, you’ve now compressed the air in the tunnel to a high-pressure environment. This may cause more fragile parts of tunnel infrastructure (i.e. light bulbs) to shatter, but under enough pressure, it could threaten the structural integrity of the tunnel itself. Add to that the water that inevitably will seep through tunnel walls and through gaps at the plug, and the generally low-atmospheric pressure during a storm, and you’ve got a delicate situation. I’m sure this is all stuff that the design engineers have thought of.

    • Gamma says:

      10 feet on a 2,000 feet tunnel is nothing. The pressure will never raise high enough. To just double the pressure you will need to cut the length in half. Also during normal operating the tunnels are designed to resist the pressure (e.g. the weight of the earth and water on top of them) trying to crush them in while having inside atmospheric pressure only. If the use of the plugs somehow raised the inside pressure that would only make the task of resisting the crushing pressure easier. This applies to deep underwater tunnels. In shallow tunnels just below street level the water level will never be high enough to move these plugs any substantial distance causing the inside pressure to rise.

      • SEAN says:

        This is an interesting concept, but you need to consider water displacement as water will seek it’s own level. Having said that, I do think the idea should be explored & tested to insure it’s viability. I don’t see an issue with air pressure since the tunnels under the East River are lengthy & could remain dry if seeled propperly.

  4. TP says:

    How about building infrastructure and equipment that’s waterproof? Is that possible? The third rail could include some kind of waterproof cover that could be activated in emergencies when service is shut off? I’d think there has to be a way to build equipment that can withstand being submerged in salt water without being destroyed.

    • Larry Littlefield says:

      My solution would be to place the barriers to protect the interlockings, around Chambers Street for example. And let the tunnels act as “dry wells” to draw the water away from the rest of the system. That seemed to work during Sandy!

      The infrastructure in the tunnels is less complicated, and easier to replace. And perhaps it could be made more robust in an underwater situation, as TP proposes.

  5. Someone says:

    If the plugs work, the Homeland Security will seem like geniuses. If they fail, we are all doomed.


  1. [...] to begin discussing ways to avoid these problems in the future. Whether the answer lies in tunnel plugs, flood doors or an as-yet-undiscussed solution, the region needs to begin planning for the next [...]

  2. [...] the Montague St. Tunnel or the Brooklyn-Battery Tunnel from future flooding isn’t as easy as installing a plug or dropping doors across the tunnel entrances. As Tom Abudllah, Transit’s Chief Environmental [...]

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