MS Plan A Defense: The Relationship Between Hydrodynamic and Morphologic Changes at Sunset Beach

Merritt Shepherd
Master’s Student
Department of Ocean and Resources Engineering
University of Hawai’i at Mānoa

**This defense will be held over Zoom**
Meeting ID: 970 3827 6831
Passcode: MerrittMS
https://hawaii.zoom.us/j/97038276831

In recent years, Sunset Beach on O’ahu’s North Shore has experienced multiple erosion events. Consequently, the US Army Crops of Engineers has collected datasets describing Sunset Beach through aerial photographic surveys and beach cameras to resolve the coastline variability from 2020-2021. The aerial and coastline imagery were processed to determine beach area and volume time series. The constructed beach time series indicates large seasonal variations and year-to-year variations, driven by energetic winter waves and calm summer waves. Analysis of adjacent beach sections shows high variability and often opposing beach behavior consistent with long-shore sediment transport. This study investigates the relationship between ocean hydrodynamics and beach morphology using a wave hindcast. The relationship, quantified by correlation coefficients, between the offshore wave conditions and the beach area is weak (r=0.4). The effect of the antecedent beach condition was found to have a large range from 4 to 53 days and it does not drastically enhance the relationship between the offshore wave conditions and beach response. Overall, the shoreline observations reveal a high spatial variability across the beach which might be due to the heterogeneous nature of the offshore reefs, nearshore rock structures, or important sub-littoral cell circulation patterns. The observations demonstrate that the beach min/maximum sizes lag the wave seasonality by two months and the summer-time beach recovery greatly varies (area difference between the two summers: 1500 m² in the 1.5-year time series.