Boussinesq modeling of typhoon-induced infragravity oscillations in Hualien Harbor, eastern Taiwan: Influence of the adjacent coast

Abstract

Large infragravity-wave oscillations inside harbors are generally induced by typhoon-generated swell waves. The interaction between infragravity waves and harbor geometry can significantly amplify wave oscillations when the specific frequencies match the natural resonance frequencies of the harbor. Additionally, the neighboring coastal morphology can enhance the influx of infragravity energy. In this study, we investigated infragravity oscillations in Hualien Harbor, eastern Taiwan, during the passage of a strong typhoon. The harbor geometry comprises outer and inner basins aimed at providing a well-sheltered area. However, the extreme infragravity wave height of 2.0 m was observed at the innermost basin. A wave-resolving Boussinesq model was applied to reproduce harbor oscillations and was validated against the measured data within the port. The accuracy of the simulated significant wave heights was sensitive to the direction of the incident wave, but not that of the simulated infragravity waves. Furthermore, lower and higher resonance frequencies prevail in the inner and outer basins, respectively. The simulated spatial distributions of wave heights in the resonance periods demonstrated that these waves were present both within the harbor and along the adjacent coast. Additionally, the refraction effects of the adjacent coast were investigated through numerical simulations. Our experiments revealed that the infragravity energy generated on the adjacent coast contributed to an increase in infragravity wave heights inside the harbor up to 47 %. Moreover, we found that reducing the long-period infragravity waves significantly altered the distribution of short-period swell waves in the outer harbor basin.