UH study finds some parts of Hawaiʻi sinking faster than others

Some parts of Hawaiʻi are sinking faster than others.

That discovery, published recently in a study by researchers at the University of Hawaiʻi at Mānoa, also highlights that as sea level rises, the infrastructure, businesses and communities in these low-lying areas are at risk of flooding sooner than scientists anticipated — particularly in certain urban areas of Oʻahu.

“Our findings highlight that subsidence is a major, yet often overlooked, factor in assessments of future flood exposure,” said Kyle Murray, lead author of the study and researcher with the Climate Resilience Collaborative at the UH-Mānoa School of Ocean and Earth Science and Technology. “In rapidly subsiding areas, sea level rise impacts will be felt much sooner than previously estimated, which means that we must prepare for flooding on a shorter timeline.”

Subsidence is the gradual caving in or sinking of an area of land. As the islands move farther from the hot spot beneath Hawaiʻi Island, they very slowly sink because of their weight.

This islandwide subsidence rate is low on Oʻahu, about 0.6 millimeters — or about the thickness of 10 sheets of printer paper — each year.

However, researchers found localized areas on the south shore of Oʻahu, where land is sinking nearly 40 times faster, exceed 25 millimeters per year.

The rate and localized nature of the sinking surprised the research team.

“Much of the urban development and infrastructure, including parts of the industrial Mapunapuna area, is built on sediments and artificial fill,” Murray said. “We think the majority of subsidence is related to the compaction of these materials over time.”

He and and his study co-authors analyzed nearly 2 decades of satellite radar data to measure vertical land motion throughout the Hawaiian Islands. They also developed a high-resolution digital elevation model to accurately map coastal topography.

By combining these datasets, they modeled how sea level rise and ongoing subsidence will exacerbate future flooding.

Phil Thompson, one of the study’s co-authors and director of the University of Hawaiʻi Sea Level Center in the School of Ocean and Earth Science and Technology, said the rate of land subsidence is faster than the long-term rate of sea level rise in Hawaiʻi, which has been 1.54 millimeters per year since 1905.

“In places like the Mapunapuna industrial region, subsidence could increase flood exposure area by over 50% by 2050, while compressing flood preparedness timelines by up to 50 years,” said Thompson.

The shoreline plays a vital role in sustaining Hawaiʻi’s coastal communities, economy and infrastructure.

Researchers found sinking rates of certain coastal regions remained consistent during the past 2 decades, suggesting subsidence will persistently exacerbate flooding for parts of the island.

If subsidence is not accounted for, urban planning and coastal adaptation strategies could underestimate the urgency of mitigation efforts.

Chip Fletcher, another of the study’s co-authors, director of the Climate Resilience Collaborative and interim dean of the School of Ocean and Earth Science and Technology, said the work researchers are doing directly serves the people of Hawaiʻi because it ensures local adaptation strategies are based on the best available science.

That ultimately helps protect homes, businesses and cultural areas.

“Our research provides critical data that can inform state and county decision-making, helping to improve flood exposure assessments, infrastructure resilience and long-term urban planning,” said Fletcher.