A study by University of Hawaii researchers has found a third of the state’s shorelines are vulnerable to coastal hazards as the waves and storms that hit the islands become intensified by the growing impacts of sea level rise.
Published in the journal Ocean and Coastal Management, the study examined the shorelines of seven of the eight main Hawaii islands and determined their vulnerability to coastal hazards, ranking each area from low to high risk.
High-risk zones on Oahu include the North Shore from Haleiwa to Kahuku, which is exposed to high wave energy, and Ewa Beach, which has low elevation, little reef protection or natural habitat and greater vulnerability to sea level rise.
Other areas at risk on Oahu include stretches of Kailua, Waimanalo and Waianae.
Primary author Yaprak Onat from UH’s Department of Ocean Resources and Engineering accounted for a range of factors in determining coastal hazard exposure, including topography, underwater landscape, erosion, wave energy, vulnerability to surge and sea level rise.
Onat also factored in the effects of natural coastal habitats, defensive structures like sea walls and revetments, plus human activities.
The variables for each coastal area were fed into a specially designed computer model that determined relative comparisons between shoreline segments and identified the most hazardous locations in the state.
While the good news is that “the average exposure index of the islands is at the low to medium vulnerability level, an alarming 34 percent of the state has moderate to high vulnerability,” the study said.
Most of those areas have a significant population, with homes, roads and other assets at risk.
Maui, Oahu, and Kauai are the top three most vulnerable islands, according to the study. While the coastal landscape is most important in influencing vulnerability on Oahu and Kauai, sea level rise and surge potential are the biggest factors on Maui, which is sinking under the relative newness of its volcanic formation.
And although high wave energy affects all the Hawaiian Islands, Lanai and Kauai are particularly influenced by wave exposure, while Oahu has the most eroded shorelines. Oahu’s risk scores were generally lowered by having plenty of bays with relatively sheltered water.
Hawaii island, meanwhile, wasn’t listed among the most vulnerable islands because there are more rocky shorelines, higher-elevation coastal regions and lower population density.
The study calculated that coastal defense structures — breakwaters, sea walls and other hardened barriers — reduce erosion vulnerability by 30 percent.
“That doesn’t mean you put concrete everywhere. That’s not what we’re talking about,” she said in an interview.
Building a sea wall, for example, can have big consequences for beaches down the coast, she said. Thorough environmental assessment and engineering design is essential before construction of a coastal structure is considered, Onat said.
Onat said the study is designed to assist policy makers with the tough decisions they are likely to make in the years to come in preventing the loss of lives and assets.
“You can see which factor is contributing most to the vulnerability of an area, and it allows us to pinpoint where we should divert our resources,” she said.
Onat’s co-authors include UH researchers Michelle Marchant, Oceana Francis and Karl Kim.
Francis, an assistant professor of civil engineering, said the results of the coastal hazards study are already being used to help guide priorities in a study she is working on for the state Department of Transportation. The study, expected to be completed next year, is looking at risks to coastal roads statewide.
“Yaprak’s work here has very very big implications for what we’re doing for this project,” Francis said. “Her work will really impact more studies coming out, too.”