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Example of chronic coastal erosion threatening a building on Maui. (Photo by Zoe Norcross-Nuu)

Sea-level rise has been isolated as a principal cause of coastal erosion in Hawaiʻi. Differing rates of relative sea-level rise on the islands of Oʻahu and Maui remain as the best explanation for the difference in island-wide shoreline trends (that is, beach erosion or accretion) after examining other influences on shoreline change including waves, sediment supply and littoral processes, and anthropogenic changes.

Researchers from the University of Hawaiʻi at Mānoa’s School of Ocean and Earth Science and Technology (SOEST) and the state Department of Land and Natural Resources recently published a paper showing that sea-level rise is a primary factor driving historical shoreline changes in Hawaiʻi and that historical rates of shoreline change are about two orders of magnitude greater than sea-level rise. “Are beach erosion rates and sea-level rise related in Hawaiʻi?” was published in Global and Planetary Change.

Knowing that sea-level rise is a primary cause of shoreline change on a regional scale allows managers and other coastal zone decision-makers to target sea-level rise impacts in their research programs and long-term planning. This study is confirmation that future sea-level rise is a major concern for decision-makers charged with managing beaches.

“It is common knowledge among coastal scientists that sea-level rise leads to shoreline recession,” said Brad Romine, coastal geologist with the Sea Grant College Program. “Shorelines find an equilibrium position that is a balance between sediment availability and rising ocean levels. On an individual beach with adequate sediment availability, beach processes may not reflect the impact of sea-level rise. With this research, we confirm the importance of sea-level rise as a primary driver of shoreline change on a regional to island-wide basis.”

Globally averaged sea-level rose at about 2 mm per year over the past century. Previous studies indicate that the rate of rise is now approximately 3 mm per year and may accelerate over coming decades. The results of the recent publication show that sea-level rise is an important factor in historical shoreline change in Hawaiʻi and will be increasingly important with projected sea-level rise acceleration in this century. “Improved understanding of the influence of sea-level rise on historical shoreline trends will aid in forecasting beach changes with increasing sea-level rise,” said Charles Fletcher, associate dean and professor of geology and geophysics at SOEST.

Results of island-wide historical trends indicate that Maui beaches are significantly more erosional than beaches on Oʻahu. On Maui, 78 percent of beaches eroded over the past century with an overall (island-wide) average shoreline change rate of 13 cm of erosion per year, while 52 percent of Oʻahu beaches eroded with an overall average shoreline change rate of 3 cm of erosion per year.

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

Read the UH Mānoa news release for more.

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