UAS surveys reveal high spatiotemporal variability in carbonate beach morphology including subcell sand exchange and accretion during swell events: Waikīkī, Hawaiʻi

Abstract

More than 70 percent of Hawaiian beaches are chronically eroding due to both natural and anthropogenic causes. Small unmanned aerial systems (sUAS) provide an efficient way to reveal processes controlling the morphology of these sandy shorelines so that they can be more effectively managed. One of Hawaiʻi’s most popular tourist destinations, Waikīkī’s Royal Hawaiian Beach, suffers from chronic erosion and requires regular nourishment to prevent complete beach loss. To evaluate the efficacy of using consumer-grade sUAS to monitor subaerial sand volume and processes that drive beach morphodynamics, we conducted weekly aerial and ground surveys from April to November 2018 from which high-resolution point clouds, digital elevation models, and orthomosaics were generated. Our observation period brackets the season of high swell thought to largely control annual beach behavior in Waikīkī. Using empirical orthogonal function (EOF) and surface variability analyses, we describe subcell behavior within the greater littoral system that has not previously been observed. Despite being characterized as a chronically eroding beach, net gains of surface area and sand volume were observed over the course of the 8-month monitoring period (708.5 ± 43.5 m2 and 1384.8 ± 102.2 m3, respectively). These gains were due to seasonal swell activity and a relatively active hurricane season. We also quantified and compared shorter-duration volume gains and losses due to swell events, hurricanes, and wind variability. Considering its economic value and the expected exacerbation of erosion as sea level rises, information provided through sUAS surveys will likely become integral to coastal zone managers as they develop strategies to preserve Waikīkī’s Royal Hawaiian Beach.