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Damaged homes on the shoreline

Extensive shoreline erosion near homes at Mokulēiʻa on Oʻahu’s north shore. Credit: Brad Romine.

By including models of dynamical physical processes such as erosion and wave run-up, a team of researchers from the University of Hawaiʻi at Mānoa and the state Department of Land and Natural Resources determined that Hawaiʻi land areas vulnerable to future sea-level rise may be double what was previously estimated.

The study was published September 27 in the Nature journal, Scientific Reports.

“It’s important that we identify land areas vulnerable to sea level-related hazards because, if left unmanaged, flooding, wave inundation and erosion will continue to encroach upon coastal lands that are typically heavily developed,” said Chip Fletcher, study co-author and associate dean and earth sciences professor at the UH Mānoa School of Ocean and Earth Science and Technology (SOEST). “Preparing for these effects will be very costly and take a long time to implement. With these results, stakeholders of all types are now able to establish empirically based adaptation policies.”

Previously, a widely used approach for assessing impacts of sea level rise was the “bathtub” model, in which a static sea-level surface is projected onto a terrain model.

“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 Hawaiʻi‘s high-energy coasts,” said lead researcher Tiffany Anderson, a faculty member in the Department of Earth Sciences at SOEST.

As sea level rises, several processes are at work. Coastal erosion results in permanent land loss, but is also essential for preserving beaches; annual wave flooding rapidly escalates past a critical point; and groundwater inundation and storm-drain backflow create new wetlands and cause urban flooding. These render coastal protection insufficient as an adaptation strategy.

Developing a more realistic prediction

Beach with tarps

Shoreline erosion near homes. Credit: Sam Lemmo.

“Our more comprehensive assessment reveals important realities that can be overlooked with other methods,” said Fletcher. “Critically, the ‘bathtub’ approach alone ignores 35–54 percent of the total land area exposed to one or more of these hazards, depending on location and sea level rise scenario.”

The team also found that typical elevations of Hawaiʻi‘s low-lying coastal plains create thresholds of flood levels, above which rapid increases in flooding occur. As sea level rises, coastal lands are exposed to higher flood depths and water velocities.

The prevalence of low-lying coastal plains leads to a rapid increase in land exposure to hazards when sea level exceeds a critical elevation of about 1 to 2 feet, depending on location.

The team had identified this phenomenon in previous research and named it a “critical point.”

“Additionally, a large portion of lands at risk of flooding is not in direct proximity to the shoreline,” said Anderson. “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 one to two miles inland from the coastline.”

Preparing for the future

The modeling presented in this study was conducted to support the creation of the Hawaiʻi Sea Level Rise Vulnerability and Adaptation Report, which is the basis for further government planning initiatives.

Anderson and team are currently incorporating rainfall into the computer model to determine how sea level-related flooding might be exacerbated during rainfall events that occur during high tides. Hawaiʻi Sea Grant, also located within SOEST, and Tetra Tech Inc. are helping to guide state and county agencies in considering this new data in future planning.

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