Analysis of the contributing factors to infragravity swash based on long-term observations

Abstract

Predicting the runup heights from the offshore waves and beach characteristics has been one of the most important issues in coastal engineering. Various parameters are considered potential contributing factors, but some of them have not been verified in the field. This study discusses the characteristics of infragravity (IG) swash and its relationship with the offshore wave parameters by analyzing the nine-month field observation data of waves and their runup at a sandy beach in Japan. In particular, the contribution of the frequency spread and directional spread was investigated using field data. IG waves developed as bound waves in a shoaling zone, while they caused frequency-dependent dissipation in the surf and swash zone, which affected the IG swash. The lower frequency components of incoming IG waves generated a larger swash than the higher frequency components when IG wave dissipation was not negligible, but the latter was higher without the dissipation. The integrated IG swash was roughly proportional to H0L0, where H0 is offshore wave height and L0 is offshore wavelength. Furthermore, the inclusion of the frequency spread and directional spread improved the IG swash prediction. An increase in the frequency spread and decrease in the directional spread tended to increase the IG swash, which is in qualitative agreement with the bound wave characteristics. The contribution of the directional spread becomes prominent under energetic conditions. The definition of the offshore wave period also affected the swash prediction, and the spectral mean period gave better performance than the peak period during multimodal wave conditions.