During superstorm Sandy, a swollen New York Bay overflowed into Manhattan, flooding subways, tunnels, and a major power substation.
But what if the storm surge had been stopped by a five-mile-long barrier outside the harbor? Or what might have happened if New York had built marshes and oyster beds at the tip of Manhattan that had absorbed some of a storm surge's energy?
In the wake of New York's worst natural disaster in modern times, city and state officials are beginning to consider longer-term solutions to prevent a recurrence of the flooding.
"Climate change is a reality," Gov. Andrew Cuomo (D) said during one of his post-Sandy briefings. "Given the frequency of these extreme weather situations we have had, for us to sit here today and say this is a once-in-a-generation [storm] and it's not going to happen again, I think would be shortsighted."
Even before Governor Cuomo's pronouncement, Halcrow Group, a British company that works on infrastructure projects worldwide, in 2009 proposed a five-mile fixed barrier stretching from Sandy Hook, N.J., to Breezy Point in the Big Apple borough of Queens.
Meanwhile, "Rising Currents: Projects for New York's Waterfront," presented by the city's Museum of Modern Art (MoMA) two years ago, among other things asked architects to imagine new ways of using "soft" methods to cope with rising sea levels.
And New York City has had engineers examining ways to address the prospect of rising sea levels for a city with many low-lying areas.
The first step in deciding what to do should be the formation of a harbor protection commission, says Vishaan Chakrabarti, an architect and professor of real estate development at Columbia University here. "We need to include all levels of government," he says. "We need to get the right business and civic voices involved so we can get a broad consensus."
Initially, it appears that Mayor Michael Bloomberg is dubious about the idea of building dikes to hold back the sea.
"I don't think there's any practical way to build barriers in the oceans," he said on Nov. 1. "Even if you spent a fortune, it's not clear to me that you would get much value for it."
Others say New York – the financial capital of the world and home to at least 8 million people – is already very late in acting.
"The rest of the world has been doing it for about 50 years or so," says Robert Yaro, the influential head of the Regional Plan Association, an independent group that looks at ways to improve the quality of life in New York, New Jersey, and Connecticut.
Mr. Yaro points to the Dutch, who keep building higher and higher dikes to keep the North Sea at bay. "They have gone from preparing for the worst storm in 10,000 years to preparing for the worst storm in 100,000 years," he says. "They are also building so they have more redundancy and so they function better with natural systems."
One proponent of some form of sea wall is Malcolm Bowman of the State University of New York at Stony Brook, who heads the school's group that predicts and models storm surges for the New York area.
Professor Bowman uses as an example the Halcrow Group plan, which involves a long, tall causeway that would be built outside the harbor in water about 20 feet deep. Huge gates would allow ships in and out but would close during powerful storms to keep out the surge. Additional barriers would be built in western Long Island Sound and Arthur Kill, a waterway in Staten Island.