University of Hawaii researchers used drone surveys of Waikiki to figure out how sand moves around its beaches over time, and their findings suggest that sea-level rise and more “extreme” waves will exacerbate beach erosion.

In one of two related studies published this year and authored by UH researchers, surveys of the beach fronting the Royal Hawaiian hotel were conducted from 2018 to 2020. The study, published in the journal Marine Geology, found that sand moves from one part of the beach to another, driven primarily by changes in water level and wave energy.

The study found that the beach accumulates sand over time before experiencing an extended period of erosion. For 12 months the beach accrued sand, the study found, but it lost even more sand in the following 10-month period — a pattern that was more significant than the typical seasonal pattern that affects beach morphology.

“Instead of seeing high volumes of sand in summer, and low volumes in winter, we saw consistently increasing beach volume the first 12 months of the study and then erosion of the beach the following 10 months,” said Anna Mikkelsen, a UH geospatial analyst and co- author of the study, in a news release.

The other study, published in the journal Remote Sensing, surveyed the beach from April to November 2018, allowing data to be collected during the Central Pacific hurricane season and a season of elevated southerly swell, both of which increase sand movement in the area.

“We found a clear relationship between increases in south swell and beach accretion (gradual accumulation), and, on the flipside, increased trade swell was associated with beach erosion,” said UH geospatial analyst Kristian McDonald, who led the study, in the release. “In addition, the hurricane activity of 2018 generally increased the surface area and volume of this beach due to the associated increase in south swell wave energy.”

The studies provide insight on the chronically eroding beaches in Waikiki and how they might be affected in the future. Rising sea levels and extreme El Nino-related events could amplify beach erosion and increase the cost of beach maintenance, they suggest.

Beach erosion in Waikiki, the center of tourism in Hawaii, is mostly addressed by occasional sand replenishment projects.