Accelerated Subsidence in Hawaii Escalates Coastal Flood Risks

In Hawai’i, the impacts of increasing sea levels are becoming increasingly evident. These include beach loss, coastal erosion, and flooding resulting from rising ocean waters, backed-up storm drains, and groundwater levels.

Vertical land motion (VLM), especially land subsidence, can also intensify the consequences of sea level rise.

Subsidence is making some parts of Hawaii sink faster than others, making the impacts of sea levels worse and increasing the risk of flooding in low-lying areas. O’ahu’s low-lying areas, Hawaii’s third-largest island, are at risk of flooding from land subsidence.

In O’ahu, particularly in urban Honolulu and various coastal areas, flooding resulting from sea level rise and land subsidence poses a significant threat, putting over $12.9 billion in infrastructure at risk. This situation could lead to major disruptions in tourism, coastal development, and the natural environment.

A study by researchers at the University of Hawai‘i (UH) at Mānoa, “Coastal land subsidence accelerates timelines for future flood exposure in Hawaii,” analysed nearly 20 years of satellite radar data to track vertical land movement across the Hawaiian Islands.

They created a detailed digital elevation model to map the coastal landscape precisely. Combining these datasets, they modelled how rising sea levels and continued land sinking will worsen future flooding.

The authors highlight that as the sea level rises, the infrastructure, businesses, and communities in these low-lying areas risk flooding sooner than scientists anticipated, particularly in certain urban areas of O‘ahu.

According to the study, much urban development, including parts of the industrial Mapunapuna area, is built on artificial fill. In some places, this fill can be as deep as 10 metres and comprises various materials like soil, rocks, trash, organic matter, coralline sand, gravel, concrete rubble, and other debris.

The way the fill was placed varied greatly. Sometimes, it was dumped with little planning or compacting, and there is limited information about when it happened or what it contains. The fill is inconsistent, with soft or hard areas, and it includes materials like clay, silt, sand, gravel, and even large rocks. Some parts are well-compacted, while others are loose and unstable.

Over time, organic material in the fill can break down, which weakens the ground, and some materials may compress or settle. This fill type is increasingly seen as prone to sinking and compacting over time. The authors attribute the compaction of these materials over time to subsidence.

The authors modelled potential flood risks by combining this elevation data with subsidence rates. Their findings show that while Oahu’s overall subsidence rate is around 0.6 mm per year, the south shore is sinking much faster, at more than 25 mm per year.

This sinking rate is much higher than the island’s long-term sea level rise, which has been about 1.5 mm per year since 1905 and could increase flooding risks by as much as 53% by 2050 in areas like the Mapunapuna industrial region. Taking subsidence into account shortens the timeline for flood preparedness by up to 50 years, highlighting the importance of considering these factors in planning and policies for sustainable development and flood prevention.

According to the study, subsidence must be considered when assessing the impacts of rising sea levels, and adaptation strategies should address both sea level rise and land sinking together.

In the future, these results can be combined with other factors that worsen flood risks, such as wave run-up, storm drain backflow, groundwater flooding, coastal erosion, rising tides, and storm-related flooding. The combined effects of these factors can lead to much higher flood risks.

Learn more about the study by browsing the links below.