Modeling of Free Infragravity Wave Using WAVEWATCH III

Long waves excited by incident short waves, named infragravity (IG) waves, are known to play an important role in coastal hazard during storms. Previously, locally excited IG waves have attracted a lot of attentions, but recent studies pointed out that freely propagating IG waves from remote beaches can give non-negligible contributions to the coastal hazard. From an engineering perspective, it is required to establish an efficient method to predict the free IG waves. In this study, directional spectra of free IG waves were reconstructed by a novel method which has been developed recently, and they were used to test a skill of a widely used wave hindcasting model in predicting free IG waves during stormy conditions. Wave parameters of short waves were reproduced precisely by the hindcasting model, but significant errors occurred for free IG waves. The magnitude was overestimated in most cases, and unnatural peaks appeared in modeled directional distributions possibly because reflection from a breakwater was overestimated by the present model. Based on the validation results of free IG waves, which have been difficult to conduct due to the lack of observation data, a way to improve the modeling method was discussed further.