Numerical modeling of low-frequency waves on a reef island in the South China Sea during typhoon events

Abstract

Low-frequency waves with infragravity and very-low-frequency bands have been found to induce coastal flooding and erosion on low-lying reef islands under storm conditions. The aim of this study is to investigate the dynamics of low-frequency waves around an atoll reef island in the South China Sea, Dongsha Island, during a large typhoon event. A Boussinesq-type wave model, FUNWAVE-TVD, is applied to reproduce nearshore wave dynamics that caused severe erosion on the southern coast of the island. The interactions between waves with different incident directions and the localized atoll topography are examined. Modeled alongshore significant wave heights, wave setups, infragravity waves and very-low-frequency waves on the southern coast are first examined. The results show that alongshore low-frequency waves increase, and wave setup decreases as incident wave angles decrease. Eastward low-frequency edge waves are dominant in all incident wave directions. Comprehensive two-dimensional wave variations are further used to illustrate the interplay between incident waves and the localized atoll geometry. Infragravity and very-low-frequency waves are energetic around the eroded position of the shoreline with nearly shore-normal incident waves. Specifically, the shapes of very-lowfrequency waves are highly asymmetrical, suggesting that very-low-frequency waves play a crucial role in affecting erosion and sedimentation during the typhoon events.