Observations of coastal infragravity wave characteristics under swell-dominated conditions

Abstract

This study analyzed the characteristics of infragravity (hereafter IG) waves under a microtidal and swell-dominated wave climate based on the field observations collected in the nearshore waters (water depth: ∼ 13 m) of southern Sri Lanka. The main focus is on the dependence of nearshore IG waves on the incident short wave parameters, with particular attention to the influence of the short wave spectral shape on the IG wave characteristics. The observation results indicate that IG waves are primarily locally excited by incident swell wave energy, and accurate prediction of the IG wave height can be achieved using the parameter HS2Tm−1,0, which represents the short wave energy flux. Bound IG wave energy is much smaller than the free IG wave under conventional sea states but becomes significant under high-energy swell sea states.Higher levels of IG wave energy are found to be forced under narrower spectral shapes and smaller directional spreading short wave conditions, and these conditions typically correspond to stronger coupling strengths in the bound IG wave theory. Compared to the single-peaked spectra wave condition, the forced IG wave energy decreases under the double-peaked spectra wave condition. This is likely mainly because the latter tends to have a broader spectrum and a larger directional spreading. On the other hand, under the narrow-banded spectral short wave conditions, IG wave energy is inclined to be distributed over a lower and smaller frequency range. This is noteworthy because lower-frequency IG waves tend to be more difficult to break. In addition, the bound IG wave energy was found to increase at low tide, but for the intermediate water depths in which the observation site is located in this study, the influence of the small tidal range in the study area on the IG wave energy was not significant.