High frequency (weekly) carbonate beach width, volume, and nearshore sand cover variability using Unmanned Aerial Systems (UAS): Waikīkī beach, Hawai`i

The Royal Hawaiian Beach in Waikīkī is chronically eroding. To inform the development of management policies, we conduct drone surveys of the beach and nearshore sand cover. Using SfM, 45 weekly aerial and ground surveys (January 2019 to February 2020) were used to build high-resolution digital terrain models (DTM) of the beach and an orthomosaic of the nearshore. We track sub-aerial beach characteristics including surface area, volume, foreshore slope, berm crest, and the low-water mark. Typically clear water allows the application of a binary classification (sand/no sand) to the nearshore. These datasets reveal connectivity between beach and nearshore sand reservoirs. Results show multiple event-based processes at work in both environments, as well as longer term trends. Individual swell events can drive significant changes in overall beach volume on the order of ±1300 m³. Contrary to previous studies, we find an overall decrease in beach volume (-2034 ± 518 m³), and high variability (55-72%) in nearshore sand cover.