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His firm’s proposal is to build a barrier in the Verrazano Narrows between Brooklyn and Staten Island, shielding Upper New York Bay. It would be supplemented by two smaller barriers, one between Staten Island and New Jersey and the other on the East River. Such a barrier would have protected Manhattan and much of Brooklyn and Staten Island from Sandy, but left southern Brooklyn and Kennedy Airport exposed.

Robert Trentlyon, a New York community activist who has been advocating for storm-surge barriers, sees the one-two punch of Hurricane Irene in 2011 _ which came within a foot of flooding subway stations in southern Manhattan _ and Sandy as a sign that the time has come.

“Having had two storm surges within one year, and their both being major ones, I just find it very difficult to think the city could not go ahead and act,” the retired local newspaper publisher said by phone Sunday from his Manhattan apartment, which was left without power. His Chelsea neighborhood, though not his building, was among those that flooded.

In August, U.S. Rep. Jerry Nadler urged city officials to take a comprehensive look at storm-surge barriers, bulkheads and other flood-fighting devices.

After the storm, reactions from the government have been mixed, as the region battles to recover from the storm rather than looking at how to prevent the next disaster.

“We cannot build a big barrier reef off the shore to stop the waves from coming in,” Bloomberg said Monday. But Gov. Andrew Cuomo opened the door to new ideas Tuesday, saying that the government has a responsibility to think about new designs and techniques to protect the city in the face of what look like increasingly frequent megastorms.

One doesn’t have to go to Europe or New Orleans to find examples of massive sea barriers: The city of Providence, R.I., has been protected by a 3,000-foot gated barrier since 1966. Construction was prompted by two devastating hurricanes in 1938 and 1954. The barrier has prevented flooding of the low-lying parts of the city several times since then, including during Sandy.

“This is not far-out science or engineering,” Bowman said. “This is easy to do.”

“Easy” doesn’t mean it would be something that could be put in place quickly. Even after politicians line up behind the project, funding, permitting and environmental studies are likely to take years.

“It could take 20 years before people even start pouring concrete,” Bowman said.