The ocean batters away at the beach near Seaview on...

The ocean batters away at the beach near Seaview on Fire Island. (Oct. 29, 2012) Credit: Lori Mattiasen

The beaches and dunes of Fire Island lost more than half of their pre-storm volume of sand during superstorm Sandy, significantly changing the structure of the barrier island and leaving it vulnerable to future storms, according to a federal study released Tuesday.

The U.S. Geological Survey study shows stretches of beach are narrower and the elevation of the island is lower, said Cheryl Hapke, an agency research geologist and lead author of the report.

Sandy "profoundly altered the shape and position of the barrier island, shifting it landward and redistributing large amounts of sand," she said.

The report shows that the beaches and dunes lost 54.4 percent of their pre-storm volume and the dunes experienced overwash along 46.6 percent of the island, dramatically changing the island's shape, Hapke said.

Erosion was more severe in some areas where a 2009 beach nourishment project added 1.82 million cubic yards of sand to Fire Island. Areas near Ocean Beach, Watch Hill and Davis Park lost 72 percent of pre-storm sand volume.

That increase is likely attributable to man-made dunes not having the resiliency of naturally made ones, Hapke said. Artificial dunes "don't have that same internal structure" as natural dunes, she said. "It's sort of a looser pile of sand that can be more prevalent to erosion."

A January report by Coastal Planning and Engineering of NY concluded that 2.2 million cubic yards of sand would be needed to return the beaches that were part of the nourishment program to their 2009 condition. The estimated cost is $71 million.

During the past winter, the shoreline position shifted as much as 189 feet inland.

Sand is starting to naturally return to the washed-out areas. The USGS surveyed through April and found that about 18 percent of pre-Sandy volume was back.

After a strong storm season in the 1990s, the island took more than a decade to recover. It's likely that the same amount of time, if not longer, will be needed to reach pre-Sandy levels, Hapke said.

The study will help scientists predict the future and evaluate areas that may be vulnerable to flooding. The Fire Island National Seashore, which did not respond to a request for comment, had previously said the results would be used for planning purposes.

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