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Some of them stood to gain protection in an alternative design: a single, 5-mile-long barrier between Sandy Hook, N.J., and the Rockaway Peninsula in Queens, estimated at $5.9 billion.

Both approaches were designed to block a 25-foot storm surge but had navigational locks or other mechanisms to let water and ship traffic flow under normal conditions. Some designs featured visible walls or berms above the waterline, but one envisioned a wall that would lie flat underwater and rise into position when needed.

Advocates note that an 1821 hurricane flooded what’s now Manhattan’s financial district _ and that experts estimate the city could face a surge as high as 25 feet and a 3 million-person evacuation if threatened by a storm as strong as a notorious 1938 hurricane that sawed through nearby Long Island. Moreover, the city projects global warming could boost sea levels by up to 4 1/2 feet by the end of the century, making flooding a growing threat.

Troubled by the projections, retired newspaper publisher and community activist Robert Trentlyon started broaching storm-surge barriers with local organizations and officials about two years ago.

Then came Irene, a 500-mile-wide hurricane that weakened to a tropical storm with 65-mph winds just before its center made landfall at Brooklyn’s Coney Island.

“There’s absolutely no question: From the time of Irene, for the next six months, people were more concerned,” said Trentlyon, who now makes his rounds with a growing file of supportive statements.

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

The City Council’s Environmental Protection Committee heard from barrier advocates, among others, at a hearing in December, and Chairman James Gennaro would like the barriers to be among ideas getting further review by a city climate-change task force.

The council passed a proposal Wednesday to expand the group’s scope to assess how heat, storms and flooding affect various aspects of the city.

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Follow Jennifer Peltz at http://twitter.com/jennpeltz