Beach erosion hitting Maui at quickest pace, study finds

Sea level rise is a primary factor in the changing size and shape of Hawaii’s shorelines, but Maui is losing beaches to erosion far faster than Oahu, according to a study by researchers at the University of Hawaii and the state Department of Land and Natural Resources.

"It is common knowledge among coastal scientists that sea level rise leads to shoreline recession," said Bradley Romine, a coastal geologist with the University of Hawaii Sea Grant College Program, in a statement Thursday.

But not all beaches are created equal.

"Shorelines find an equilibrium position that is a balance between sediment (sand) availability and rising ocean levels," Romine said.

If an individual beach has a sufficient sand supply, it may not reflect the effects of sea level rise, Romine and colleagues say.

But on average Hawaii’s beaches are eroding, especially Maui, they said in a paper published in the journal Global and Planetary Change.

On Maui, 78 percent of beaches eroded over the past century with an overall, island-wide average shoreline change rate of 5 inches of erosion per year, while 52 percent of Oahu beaches eroded with an overall average shoreline change rate of 1.2 inches of erosion per year.

Kihei, Maui, has the worst rate, with 82 percent of beaches eroding.

On Oahu, the North Shore has the worst rate, with 63 percent of beaches eroding.

Around Oahu, sea levels are rising on average 0.4 to 0.8 inches per year, the researchers said.

But Maui is experiencing a relative rise of 0.8 to 1.2 inches per year, perhaps due to other factors, including the bending of the sea floor under the weight of a growing Big Island. Variations in density of the upper levels of the ocean could be another factor, the researchers said.

Earlier studies show that the global average rate of sea level rise is about 1.2 inches per year, up from 0.8 inches over the past century. And the rate may accelerate in the coming decades, the scientists warn.

Other factors affecting the change in a certain shoreline are wave conditions, exposure to tradewinds, the shape and geology of the coast, and human alterations like seawalls, groins and sand mining.

The scientists calculated islandwide and regional historical shoreline trends using aerial photographs and topographic and hydrographic survey charts provided by the National Ocean Service. These were manually digitized for analysis.

"The research being conducted by SOEST provides us with an opportunity to anticipate (sea level rise) effects of coastal areas, including Hawaii’s world-famous beaches, coastal communities and infrastructure," said Sam Lemmo, administrator of the state Office of Conservation and Coastal Lands. "We hope this information will inform long-range planning decisions and allow for the development of (sea level rise) adaptation plans."

Others involved in the research are Matthew Barbee, Tiffany Anderson and Neil Frazer of the UH Department of Geology and Geophysics.