MS Plan A Defense: Morphodynamic changes due to calm-moderate wave forcing: A case study of Waikiki Beach

Julianne Kalksma
Graduate Student
Department of Ocean and Resources Engineering
University of Hawaiʻi at Mānoa

Sea level rise, erosion, and the wave climate influence Waikiki Beach on the South Shore of Oahu which is a popular beach in metropolitan Honolulu. In response to recent erosion events and ongoing beach nourishments, weekly surveys have been collected for the past 3 years, from April 2018 through December 2020, to better understand coastal morphology. Local studies found detailed two-dimensional morphological structures; however, no direct relationships between the offshore driving ocean conditions and Waikiki Beach have been established. Other previous works relate the wave climate to beach state (e.g. bar or berm). Consequently, the purpose of this study is to relate the offshore wave conditions to sand movement. We use a wave hindcast to quantify relationships between the sand volume and various wave parameters. We find that the two dominant wave parameters driving changes in the sand volume are the peak wave direction and Easterly wind wave height relative to other parameters of the wave climate. Overall, there is only a small seasonal cycle in both the wave climate and sand volume. We find the antecedent wave condition influences the beach state and the previous 50 weeks might impact the present beach state. The spatial relationship between the wave parameters demonstrates clear geophysical oscillations in the sand motion which supports that the peak wave direction and wind wave height are influencing the nearshore dynamics and resultant beach morphology.

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