Rising sea levels in Long Island Sound could threaten dozens of waterfront properties in Nissequogue and Head of the Harbor villages and drastically change conditions in Stony Brook Harbor, a Stony Brook University scientist warns.

New York State scientists project sea-level rise at 2 to 10 inches in the 2020s, and 15 to 72 inches by 2100. Rising waters will accelerate erosion, undercut the Nissequogue bluffs and collapse front yards into the Sound, said Lawrence Swanson, interim dean of the School of Marine and Atmospheric Sciences and a former Head of the Harbor trustee now advising its officials.

Head of the Harbor Mayor Douglas Dahlgard said in an interview he asked Swanson for advice earlier this year, prompted by a growing concern about sea-level rise impacts on his village of about 1,475 people.

“This is going to be something that’s eventually universal on the North Shore and South Shore of Long Island,” Dahlgard said. “Our hope is that by thinking far enough in advance, that maybe we can do something.”

State environmental officials are working on guidance for agencies, planners and local governments covering the impact of changing sea levels, with draft reports expected to be released this summer.

For now, Swanson said, the strategies available to tiny Head of the Harbor and its slightly larger neighbor are limited. The villages can stiffen laws that already bar structures within 100 feet of the landward edge of a bluff. They can discourage erosion control structures such as sea walls, which protect individual properties, but damage others by deflecting wave energy and interfering with the natural flow of sediment needed to replenish beach areas.

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Another strategy involves spraying sediment on top of the harbor islands to keep pace with rising sea levels. But spraying, which state officials said has shown some promise in Jamaica Bay, would likely cost millions, far more than the villages could afford. Operating budgets for the two villages are about $2 million each.

“Sea-level rise at an accelerating rate is a reality,” said Swanson, co-author of a book, “Between Stony Brook Harbor Tides.” “People who live around the harbor are going to have to debate what is the management practice they want.”

Rising sea levels also would change the interior of Stony Brook Harbor, which would further put communities at risk, Swanson said.

Portions of Nissequogue’s Long Beach, the spit that has protected the harbor for thousands of years, flooded 30 inches during Tropical Storm Irene. At the high extreme of projected sea-level rise, it could turn into an island, imperiling about 40 homes there and opening a new channel from the Sound, Swanson said.

That would expose the harbor’s tiny islands and hundreds of acres of salt marsh, which would drown if sea level rises faster than the bluffs recede.

The marshes are a breeding ground for fish and wildlife. But they also provide what Swanson called a “frictional surface” that breaks up waves. Losing them could further intensify erosion of the harbor shoreline, which, like the portion of Nissequogue that fronts the Sound, is lined with high-priced homes.