Complex drivers of reef-fronted beach change

Royal Hawaiian Beach in Waiki over bar ki over bar 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. Our results show 12 months of subaerial accretion (+2400 +/- 59 m(3)) followed by 10 months of erosion (-3090 +/- 51 m(3)) for a net loss of 690 +/- 51 m(3), and document that interannual variations in beach width and volume overprint seasonal pat-terns. Notably, a seasonal signal is recorded in the topographic structure of the beach. We test the relationship of beach volume and width to variations in wind, water level, wave energy flux generated from southern hemi-sphere swell, and wave energy flux from locally generated trade-wind waves. We identify three beach segments and three nearshore sand fields that form a sand-sharing, source-sink network, yet operate quasi-independently. Our analysis reveals that individual beach segments and their adjacent sand fields experience coherent (simul-taneous) gains and losses of sand, suggesting that alongshore sediment exchange is dominant over cross-shore exchange. The main drivers of beach change are variations in water level and wave energy flux. Beach volume and width both vary with nearshore sand cover, indicating that free exchange with nearshore sources is intrinsic to beach variability. Our results suggest that rising sea level and extreme El Nino-Southern Oscillation events will contribute to Royal Hawaiian Beach destabilization, which may amplify erosional events and increase the cost of future beach maintenance.

Automated summary

This study focuses on the Royal Hawaiian Beach in Waiki and examines the drivers of beach change over a two-year period. The results indicate that variations in water level and wave energy flux are the main factors influencing beach volume and width. Furthermore, the study identifies a sand-sharing network between beach segments and nearshore sand fields. The findings suggest that future beach maintenance costs may increase due to rising sea levels and extreme El Nino-Southern Oscillation events.