Little by little, Hawaii’s iconic beaches are disappearing.
Most beaches on the state’s three largest islands are eroding, and the erosion is likely to accelerate as sea levels rise, the United States Geological Survey is reporting.
Though average erosion rates are relatively low — perhaps a few inches per year — they range up to several feet per year and are highly variable from island to island and within each island, agency scientists say.
The report says that over the last century, about 9 percent of the sandy coast on the islands of Hawaii, Oahu and Maui has vanished.
That’s almost 14 miles of beach.
Beaches like Waikiki face accelerating erosion, a new report says.
The findings have important implications for public safety, the state’s multibillion-dollar tourism economy and the way of life Hawaiians treasure, said Charles H. Fletcher, who led the work for the agency.
“This is a serious problem,” said Dr. Fletcher, a geologist at the University of Hawaii at Manoa.
Sea level does not rise uniformly around the world, and so far, Dr. Fletcher and other geologists said in interviews, Hawaii has escaped some of the rise that has occurred elsewhere as earth’s climate warms.
But that situation is unlikely to continue, the report says.
Hawaii’s geological history also leaves it unusually vulnerable.
The islands formed, one by one, as a tectonic plate carrying them moved to the northwest over a “hot spot,” where a plume of molten lava pushes through the seafloor.
Over the millenniums, this material cools, accumulates and eventually rises above the waves. (Loihi, an underwater — for now — mountain southeast of the island of Hawaii, is the latest to undergo this process.)
But once the slow plate movement carries an island away from the hot spot, its volcanic material begins to compress, causing the island to start to sink, worsening its erosion prospects.
The new analysis, “National Assessment of Shoreline Change: Historical Shoreline Change in the Hawaiian Islands,” is the latest in a series of reports the geological survey has produced for the Atlantic and Gulf Coasts, California and some of Alaska.
Over all, their findings are similar: “They all show net erosion to varying degrees,” said Asbury H. Sallenger Jr., a coastal scientist for the agency who leads the work.
He said the studies aimed to establish a baseline from which scientists could “really assess what sea level rise actually does in the future to our coasts.”
S. Jeffress Williams, another scientist with the agency, said researchers had over the years produced a number of studies of Hawaii’s shorelines, using various methods of data collection and analysis. “Many were well done, but it is sort of mixing apples and oranges,” he said, referring to the need to adopt standard study methods.
The new work aims to allow researchers to compare data from states around the country.
And though it seems self-evident that erosion must be tied to rising seas, “you have to document it,” Mr. Williams said.
“Sea level rise is only one of the driving forces that control what happens at the shoreline.”
For example, he said, on a beach that receives a steady influx of sand, “you can have marginal erosion or stable or even accreting shorelines.”
But that is not ordinarily the case in Hawaii, where the typical response to erosion has been to protect buildings with sea walls and other coastal armor.
“It’s the default management tool,” Dr. Fletcher said.
But in Hawaii, as nearly everywhere else this kind of armor has been tried, it results in the degradation or even loss of the beach, as rising water eventually meets the wall, drowning the beach.
He suggested planners in Hawaii look to American Samoa, where, he said, “it’s hard to find a single beach. It has been one sea wall after another.”
But the most common alternative approach, replenishing beaches with pumped-in sand, is difficult in Hawaii, where good-quality sand can cost 10 times as much as it does on the East Coast, Mr. Williams said.
Dr. Fletcher said he believes the answer lies in encouraging people to move buildings and other infrastructure away from the shoreline, a strategy coastal scientists call retreat.
“If we want beaches we have to retreat from the ocean,” he said.
But, he added. “It’s easy to say retreat; it’s much harder to implement it.”
Dr. Sallenger said he hoped the work in Hawaii and elsewhere would help policy makers.
“We don’t define what rules and laws are written about coasts and exactly how they are managed,” he said, “but this is information that can be factored into that process.”