Now Mr. Reynolds, a technician with the United States Geological Survey, had reached the line of dunes that runs the length of the 31-mile barrier island. To complete the elevation profile of this slice of the beach-dune system, he needed to run the equipment to the top.
“When we came out here before the storm, we could walk right up the dune,” Mr. Reynolds said. But that storm — Hurricane Sandy — had obliterated much of the dune, leaving a steep scarp face about 12 feet high. So a colleague from the National Park Service, Jordan Raphael, scrambled to the top, threw down a rope and hauled the pole and backpack up, taking the final measurements.
The work — in all, Mr. Reynolds and Mr. Raphael took about 10 profiles at intervals along the island on two brisk and sparkling days last week — is part of an effort by scientists to take advantage of a storm like no other in recent memory to learn more about how Fire Island, and all barrier islands, respond to and recover from major natural events.
“We have the experiment of a lifetime sitting in our laps,” said Cheryl J. Hapke, a geological survey scientist who has studied Fire Island since 2006.
Already, Dr. Hapke and others have learned a lot about how the narrow island behaved during the storm — pretty much as expected, they say. And in the seven weeks since the storm they are already seeing signs of natural recovery, as waves remold the beaches and windblown sand builds up at the foot of the dune scarps.
What the scientists learn will help government agencies, including the Army Corps of Engineers, in their lengthy effort to determine how best to maintain parts of Fire Island, to protect the private vacation communities on its western half, which were hit hard by Hurricane Sandy, and to ensure that the island continues to do what it did during the storm — safeguard the Long Island mainland a few miles north across the Great South Bay.
“Barrier islands are supposed to take the brunt of storms,” said Chris Soller, superintendent of the Fire Island National Seashore. “And Fire Island did that.”
Dr. Hapke, a geologist, will be analyzing the latest elevation profiles, comparing them with ones taken on Oct. 28, just before the storm hit, and at various times since, and other data, including aerial surveys of the island made using laser range-finding equipment called lidar. (The geological survey is currently analyzing lidar data for New Jersey and other affected coastal areas as well.)
For Dr. Hapke, the data, and her trained eyes, already are telling her that the storm wrought major changes.
“Sandy completely flattened this beach,” she said, looking around as the survey team took a break at this pristine spot in the national seashore east of Davis Beach, one of the worst hit communities. The beach is now about eight feet lower, an elevation loss that is typical.
The scarped dunes extend as far as the eye can see up and down the island. In many places 50 feet or more of dune are gone, exposing heavier and redder garnet sand that separated into layers over time since being deposited as glacial ice retreated thousands of years ago.
At many low spots along the dunes, the surge from the storm completely overtopped them. In three locations on the eastern half, the surging waters carved new channels, breaches that allow water to flow between the ocean and bay. At other locations the island remained intact but the waters pushed enormous amounts of sand inland, raising elevations on the bay side.
Overall, Mr. Soller said, the storm shifted so much sand inland that Fire Island probably migrated north, toward the mainland, by about 65 to 85 feet.
“This was an amazingly powerful storm that reshaped the island dramatically and moved it,” he said.
But such migration was not unexpected. “Storms are the driver” that shape and move barrier islands, Dr. Hapke said.
Many factors influenced how Hurricane Sandy altered the island, including its size, strength, track and duration, the height and timing of tides, and the height and composition of the dunes. Although beaches and dunes across the island lost about two-thirds of their sand in the storm, Dr. Hapke said, the losses were greater in some of the developed areas, where homes that once were sitting atop dunes on timber piles now seem stranded in midair.
These areas may have lost more sand because they had more to lose — their beaches and dunes were replenished three years ago, by pumping sand from offshore. But another reason, Dr. Hapke suggested, may be that natural dunes, which become well consolidated as they slowly build over time, are better able to withstand erosion than dunes that consist largely of bulldozed sand.
Underwater features just offshore also played a role in determining the storm’s impact, increasing or reducing the wave energy reaching the island. “The pattern of the response to storms is directly linked to geology,” Dr. Hapke said.
In fact, she said, Fire Island is in some ways a creature of offshore conditions that create disparities between the western and eastern halves.
Just offshore near the island’s midpoint, she said, is a huge delta of rock and sand that washed out of the glaciers thousands of years ago. West of this delta lies a series of undersea ridges of sand, which may have come from the glacial outwash.
The western half of the island has never had a shortage of sand — research shows it has gained elevation in the last 500 to 1,000 years — and the undersea ridges may be the source. The ridges also reduce some of the energy hitting the shoreline; there is no historical record, for instance, of a breach ever occurring in the western half.
But the eastern half has no such sand supply, and the nearby seafloor is relatively deep and flat. “A lot higher wave energy reaches the coastline on the eastern side,” Dr. Hapke said.
In its long-term planning for the island, which calls for replenishing beaches and dunes to protect developed areas, the Army Corps has identified the offshore sandy ridges as potential sources of sand, or “borrow” areas. But Dr. Hapke said that taking sand from these areas might reduce the amount available naturally to the island, and changing the underwater landscape might affect the power of future storms. “If you mine too much sand, you alter the wave action,” she said.
Christopher Gardner, a spokesman for the New York district of the Army Corps, said that “there’s a lot that goes into the selection process for borrow areas.” The plan, called the Fire Island to Montauk Point Reformulation Study, is still being prepared.
Regardless of what people do, the island will continue to reshape itself. Barrier islands are dynamic places, and post-storm, waves and wind have already been at work.
Two of the breaches were closed by the Army Corps, but the third, near a place called Old Inlet, is being left open for up to 60 days and monitored. Breaches are a concern because they may raise water levels in the bay, which could worsen mainland flooding in the event of another storm. But breaches, at least temporary ones, can also provide benefits by flushing stagnant water from the bay.
Last week, as water rushed through the 100-yard-wide channel, there were signs that the breach might be closing on its own, as sand was building up from east to west on the ocean side. Scientists and officials from the various agencies involved will meet before the end of the year to decide how to proceed.
The beaches are evolving; sandbars appeared along some stretches in the days immediately following the storm as the seas calmed, and waves are currently fighting to reclaim them. But the island’s northward migration is permanent; the beaches and dunes may at some point resemble how they looked before the storm, but they will not be in the same place.
“As much as everyone would like it to be exactly as it was, what we’re going to have is a different Fire Island,” Mr. Soller said. “That’s the reality.”
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