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UH study: Maui more susceptible to erosion from rising sea levels

Erosion, wave run-up models show greater risk

Maui lands vulnerable to erosion are more than double previous projections, according to a new University of Hawaii study released last month.

UH researchers and the state Department of Land and Natural Resources determined that lands statewide are increasingly exposed to future sea-level rise. That assessment comes after the completion of models of erosion and wave run-up, which had not been used in prior projections.

Published in the Nature journal Scientific Reports, the study expounds upon on a widely used approach on sea-level rise impacts called the “bathtub” model. The model uses land elevation to see at what point it gets flooded because it is below sea level.

“The bathtub method provides a good first look at low-lying, flood-prone areas but underestimates the full extent of potential damage due to sea-level rise, particularly on Hawaii’s high-energy coasts,” said lead researcher Tiffany Anderson, a faculty member in the Department of Earth Sciences at UH’s Ocean and Earth Science and Technology school.

By including erosion and wave run-up models, Maui was projected to have the largest increase of land exposed to erosion in the state with 2.4 times more than previous data, according to the study. Oahu and Kauai rose 1.9 and 1.8 times, respectively.

“Our more comprehensive assessment reveals important realities that can be overlooked with other methods,” said study co-author Chip Fletcher, associate dean and earth sciences professor at UH. “Critically, the ‘bathtub’ approach, alone, ignores 35 to 54 percent of the total land area exposed to one or more of these hazards, depending on location and (sea-level rise) scenario.”

Maui and Oahu are expected to see the largest increase in land area exposed to waves due to lower dune elevations, according to the study. The lower elevations may be due to flattened dunes on Oahu resulting from high coastal urbanization and less dune building wind and wave energy on Maui — where most beaches are located on the leeward side of the island with less-persistent winds and in the shadow of nearby islands that buffer incoming wave energy.

On undeveloped coasts, erosion can lead to total loss of a beach when there are limited sand deposits inland of the existing beach such as on the north shore of Maui. There, clay deposits back narrow beaches, according to the study. Researchers said previous studies relying only on passive flood models would likely underestimate these impacts, resulting in insufficient planning.

In a phone call Friday, Fletcher said that erosion “doesn’t mean the automatic end of the beach,” noting that it only needs another sand source. He reiterated the challenges for north shore beaches that may not have extra sand deposits.

“Any place eroding into sand dunes is very likely to continue having a beach, but other places where stream deposits and clay or volcanic rock exist — erosion is going to be a more negative experience for the beach,” he said. “Instead, they’ll be eroding into gravel and mud.”

Members of the public can identify Hawaii shorelines with sand or clay deposits for themselves online using the Hawaii Sea Level Rise Viewer. Modeling in the new UH study supported the creation of the viewer and the Hawaii Sea Level Rise Vulnerability and Adaptation Report, which is the basis for further government planning initiatives.

To view the Hawaii Sea Level Rise Viewer, visit www.pacioos.hawaii.edu/shoreline/slr-hawaii/.

Fletcher said beaches have retreated since the Ice Age, when sea levels were 400 feet lower and rose as ice melted. He said policymakers will need to decide if they want to “allow sea walls to be built or let land erode.”

“If the land has a lot of sand and you want there to be a beach, let it erode because beaches are perfectly happy migrating landward,” he said. “It’s when you build a wall that beaches will disappear or if they run out of sand. That can happen if your shoreline is right up against Haleakala on the north or east side where its all solid rock cliffs. Then you might not have a beach.”

While sea-level rise continues to erode beaches statewide, it also has exposed more areas to flooding.

Researchers found that the prevalence of low-lying coastal plains leads to a rapid increase in land exposure to hazards when sea level exceeds a “critical point” of about 1 to 2 feet, depending on location. As sea level rises, coastal lands are exposed to higher floods and faster floodwaters.

“A large portion of lands at risk of flooding are not in direct proximity to the shoreline,” Anderson said. “Instead, they are low-lying areas where sea-level rise causes the groundwater table to rise up to the surface. These areas can be located 1 to 2 miles inland from the coastline.”

Anderson and her team are incorporating rainfall into the computer model to determine how sea level-related flooding might be exacerbated during rainfall events that occur during high tides. UH’s Hawaii Sea Grant and Tetra Tech Inc. also are guiding state and county agencies in considering the new report in future planning.

* Chris Sugidono can be reached at csugidono@mauinews.com.