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Saving subways with a lot of hot air

High-tech balloons that keep floodwaters at bay

Author Bret Stetka Illustration James Provost

At the U.S. Department of Homeland Security, balloons aren’t just for celebrating birthdays and anniversaries—they’re also for halting encroaching floodwaters. Since 2007, in collaboration with West Virginia University and the Pacific Northwest National Laboratory, the DHS has been testing a set of giant tubular balloons that might be used to plug subway and other transportation tunnels in the event of a natural disaster like Hurricane Sandy. The idea is that these inflatable mega-corks would be rapidly deployed to hold back water, and thus prevent or minimize damage to transportation systems. So far the DHS has tested the balloons in a mock subway tunnel in West Virginia, as well as in a field study in the Washington, D.C., subway system. Researchers believe the technology will be ready for the real world in about two years. Here’s how they did it.

1. To build a balloon that could conform to the contours of a subway tunnel, withstand potential puncture hazards and fold away neatly like a parachute, the researchers needed a material  that was both strong and pliable. It also had to be waterproof, obviously. The engineers decided on Vectran™, a fiber spun from liquid crystal polymer.

2. After trying several variations, the team settled on a three-layer system: a watertight polyethylene bladder on the inside, a protective layer of Vectran in the middle, and a heavy-duty web of 2-inch Vectran strips on the outside. John Fortune, project manager for the DHS’s Science and Technology Directorate, likens the outer shell to a picnic basket.

3. Once deployed, the balloons fully inflate in about three minutes. Water or pressurized air is then added to the bladder to help hold the balloons in place against high-pressure floodwaters (balloons filled with regular air might let water leak past). “We think the balloons will be very effective,” Fortune says. “I just hope there’s no need to use them.”

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