MORPHODYNAMIC CHANGES DUE TO CALM/MODERATE WAVE FORCING: A CASE STUDY OF WAIKĪKĪ BEACH

Abstract

Sea level rise, erosion, and the wave climate influence Waikīkī Beach on the South Shoreof O‘ahu which is a popular beach in metropolitan Honolulu. In response to recent erosion events and on-going 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 Waikīkī Beach have been established. 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 wave direction from swells generated in the Southern Ocean (Dps) and Easterly wind wave height (Hse) relative to many other parameters of the wave climate. Both wave sources are active throughout the year and we are unable to discern seasonal beach changes. We find the antecedent wave condition influences the beach state and the previous 50 weeks might affect the present beach state. The spatial relationship between the wave parameters demonstrates clear geophysical oscillations in the sand motion which supports that Dps and Hse are influencing the nearshore dynamics and resultant beach morphology.