Directional Characteristics of Infragravity Waves During Storms in the Nearshore Coastal Region

Nose, T.; Babanin, A., and Ewans, K., 2024. Directional characteristics of infragravity waves during storms in the nearshore coastal region. Journal of Coastal Research, 40(2), 353–365. Charlotte (North Carolina), ISSN 0749-0208. Presented is an infragravity wave analysis of field observations that were made using acoustic Doppler wave gauges for nearshore waters with depths typically less than 10 m. The study found a correlation between total infragravity wave energy with the wind-generated waves and the relative depth, which was robust for high waves during storms. Directional distributions of nearshore infragravity waves were derived from Acoustics Doppler gauges (Acoustic Waves and Currents (AWACs)); it was found that bimodal directional peaks emerged during storms. Previous infragravity wave studies suggest dominant energy is in the propagation sector outward from the coast, i.e. reflected waves, but the analysis here showed that the dominant infragravity wave energy was in the propagation sector incident to the coast. A numerical model that models free infragravity wave evolution from the bound infragravity wave liberation was used to reproduce the observed bimodal peaks during several storm events; however, in contrast to the observations, the dominant energy of the modelled infragravity waves is in the propagation sector outward from the coast. The AWAC directional analysis and its comparison with the model indicate that the breakpoint generation mechanism may contribute to the shoreward propagating infragravity wave energy during storm events. Using the correlation obtained from the AWAC data between the total infragravity wave energy and the relative depth parameter, an empirical model was developed to simulate the directionally discretised infragravity wave heights. The feasibility was demonstrated with reasonable skill values for the total and reflected model infragravity wave heights, which may be a computationally inexpensive tool to provide bulk infragravity wave statistics for preliminary engineering designs. This study demonstrated that AWAC data are useful for infragravity wave studies in nearshore coastal waters.