A new study by the University of Hawai‘i underscores the urgent need for flood preparedness in certain coastal areas where land subsidence is occurring much faster than sea level rise.
Some parts of Hawai‘i are sinking faster than others, setting the stage for an accelerated timeline of coastal flooding. This vital discovery, revealed recently in a study by researchers at the University of Hawai‘i (UH) at Mānoa, signals that infrastructure, businesses and communities in these low-lying regions are at risk of flooding much sooner than previously predicted.
The study, published in Communications Earth & Environment, combines nearly two decades of satellite radar data with a high-resolution digital elevation model to measure vertical land motion and map coastal topography.
The findings emphasize that localized subsidence — a factor often overlooked in flood risk assessments — is drastically altering the expected impact of sea level rise.
“Our findings highlight that subsidence is a major, yet often overlooked, factor in assessments of future flood exposure,” lead author Kyle Murray, a researcher with the Climate Resilience Collaborative (CRC) at UH Mānoa’s School of Ocean and Earth Science and Technology (SOEST), said in a news release. “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.”
As the Hawaiian Islands drift away from the volcanic hotspot beneath the Big Island, they slowly sink due to their own weight.
However, the research unveiled alarming subsidence rates on O‘ahu’s south shore, where some areas are sinking at a rate of over 25 millimeters per year — nearly 40 times faster than the island-wide average of 0.6 millimeters per year.
“Much of the urban development and infrastructure, including parts of the industrial Mapunapuna area, is built on sediments and artificial fill,” added Murray. “We think the majority of subsidence is related to the compaction of these materials over time.”
This discovery is particularly crucial as the rate of land subsidence in these areas outpaces the long-term sea level rise in Hawai‘i, which has been about 1.54 millimeters per year since 1905.
“This rate of land subsidence is faster than the long-term rate of sea level rise in Hawai‘i, which means those areas will experience chronic flooding sooner than anticipated,” added co-author Phil Thompson, the director of the UH Sea Level Center in SOEST. “In places like the Mapunapuna industrial region, subsidence could increase flood exposure areas by over 50% by 2050, while compressing flood preparedness timelines by up to 50 years.”
The implications of these findings are substantial. Hawai‘i’s coastal regions are pivotal to the state’s economy, housing critical infrastructure, businesses and communities. Persistent subsidence will continuously exacerbate flooding, affecting urban planning and coastal adaptation strategies if not appropriately addressed.
“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,” added co-author Chip Fletcher, the 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.”
As coastal subsidence continues to reshape the landscape, it’s a stark reminder of the urgency needed in planning and preparedness efforts to safeguard Hawai‘i’s future from the rising tides.
Source: University of Hawai’i at Mānoa