WEEKLY UAS MONITORING OF A WAIKĪKĪ BEACH REVEALS SEDIMENT CONNECTIONS AND DRIVERS OF BEACH CHANGE

Royal Hawaiian Beach in Waikīkī plays an essential role in Hawai‘i's tourism-based economy. To inform development of management policies, we conduct two years of weekly ground and aerial surveys (April 2018 to February 2020) to track change on this chronically eroding beach. We use multiple linear regressions, Self- Organizing Maps (a form of cluster analysis), remotely sensed nearshore sand fields, hydrodynamic modelling, and monitoring of key physical processes to identify the principal drivers of beach change. We identify three beach segments and three nearshore sand fields that form a sand-sharing, source-sink network, yet operate quasi-independently. Our results reveal that (1) interannual variations in beach width and volume overprint seasonal patterns, (2) alongshore sediment exchange appears dominant over cross- shore exchange, (3) different environmental conditions facilitate distinct sand-movement patterns, and (4) water level and wave energy flux are primary drivers of beach change. Overall, ground and UAS surveys integrated with nearshore sand fields provide an improved understanding of beach dynamics to inform development of management policies.