Some parts of Hawai'i are sinking faster than others. That discovery, published recently in a study by researchers at the University of Hawai'i (UH) 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 O'ahu.
"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 (CRC) at the UH Mānoa School of Ocean and Earth Science and Technology (SOEST). "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."
Murray and co-authors analyzed nearly two decades of satellite radar data to measure vertical land motion across 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.
As islands in the Hawaiian chain move farther from the hotspot beneath the Big Island, they very slowly sink due to their own weight. This island-wide subsidence rate is low on O'ahu, around 0.6 millimeters, about the thickness of 10 sheets of printer paper, each year. However, the researchers found localized areas on the south shore of O'ahu where land is sinking nearly 40 times faster, exceeding 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," said Murray. "We think the majority of subsidence is related to the compaction of these materials over time."
"This rate of land subsidence is faster than the long-term rate of sea level rise in Hawai'i (1.54 millimeters per year since 1905), which means those areas will experience chronic flooding sooner than anticipated," said Phil Thompson, study co-author and director of the UH Sea Level Center in SOEST. "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."
The shoreline plays a vital role in sustaining Hawai'i's coastal communities, economy, and infrastructure. The researchers found that sinking rates of certain coastal regions have remained consistent over the past two decades, suggesting that subsidence will persistently exacerbate flooding for parts of the island. If subsidence is not accounted for, urban planning and coastal adaptation strategies may underestimate the urgency of mitigation efforts.
"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 Chip Fletcher, co-author, director of CRC, and interim dean of SOEST. "This work directly serves the people of Hawai'i by ensuring that local adaptation strategies are based on the best available science, ultimately helping to protect homes, businesses, and cultural areas."
