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How to keep New York afloat

With sea levels rising, once-a-century floods may become once-in-20-years events. One solution: huge storm-surge barriers.

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Much of this city of 8 million, the largest and most densely populated major city in the US, is only 10 feet above sea level. The potential 30-foot storm surge accompanying a Category 3 hurricane would flood large swaths of south Brooklyn, parts of Queens, Staten Island, and Manhattan below Canal Street, including the World Trade Center site – 100 square miles total. As happened during a 1992 northeaster, floodwater might pour into the city's tunnels and subway system, many of whose entrances are but 10 feet above sea level, short-circuiting public transportation and stopping traffic. The city's wastewater treatment plants – all 14 of which lie at the water's edge and have outfalls at mean tide level – could back up, sending raw sewage into basements and bathrooms citywide.

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Klaus Jacob, a special research scientist at the Lamont-Doherty Earth Observatory at Columbia University in New York, estimates the cost of such an event up to $100 billion. That's one-tenth of the $1 trillion gross regional product of the New York metropolitan area, embracing three states and 22 million people. (Some estimate that Katrina will cost Louisiana and Mississippi up to $150 billion.)

Rather than individually shoring up the city's many vulnerabilities, the better solution is to use the region's topography, say engineer Douglas Hill and Malcolm Bowman, head of the Storm Surge Research Group at Stony Brook University. Three barriers placed at strategic "choke points" – the Verrazano Narrows, Throgs Neck, and the Arthur Kill – would protect all of Manhattan and half the entire flood-prone area, they say.

Similar smaller barriers already protect Providence, R.I., New Bedford, Mass., and Stamford, Conn. Completed at a cost of £535 million in 1982 ($2.1 billion in today's dollars), the Thames River Barrier, about the size of the one proposed for the Arthur Kill, has been raised more than 90 times. Italy plans to finish its MOSE project, a series of inflatable pontoons to protect the Venice Lagoon, by 2011.

And then there's the Netherlands: Half the nation is below sea level. Its colossal Eastern Scheldt barrier, nearly two miles long and often called the "eighth wonder of the world," most resembles the one proposed for the mile-wide Verrazano Narrows.

Human nature being what it is, Mr. Bowman doesn't see construction beginning any time soon. Without exception, the aforementioned barriers were built after – not before – major floods. The British and Dutch barriers were built after a 1953 North Sea storm caused major loss of life in both countries. The New England barriers rose after the "Long Island Express" hurricane of 1938.

In fact, in the 1960s the Army Corps of Engineers proposed something similar to block storm surges from Lake Pontchartrain, which abuts New Orleans. Never built, the barriers "might have made enough of a difference" during hurricane Katrina, says Bruce Swiren, a mitigation specialist at the Federal Emergency Management Agency. The disaster has led him to reconsider Bowman's idea. "I used to think that it was a complete pie in the sky," he says. "After Katrina, I'm starting to think maybe it's not such a crazy idea after all."

But Klaus Jacob, author of several papers on New York's vulnerability to flooding, opposes such large-scale solutions not on engineering but on philosophical grounds. They lend an "illusion of protection" that will only prove catastrophic in the end, he says. "The higher the defenses, the deeper the floods that will follow," he says.

Commonsensical preparations such as raising houses; putting electrical infrastructure in the attic, not the basement; and formulating clear contingency plans will go much further. In the end, however, Jacob sees only one viable, long-term option: Retreat from low-lying areas.

"That's the lesson learned," he says, the "price to be paid for pumping CO2 into the atmosphere."

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