Linear modeling of infragravity waves during Delilah

Abstract

The generation of infragravity waves by directionally spread shortwaves incident on an alongshore uniform beach is investigated. Two mechanisms responsible for the generation of infragravity waves are considered: the release of the bound infragravity waves associated with changes in the spatial variation of the incident shortwave energy and the forcing of trapped waves by obliquely incident directionally spread shortwaves. The infragravity wave response is examined using linear shallow water equations, taking into account the presence of bottom friction, setup of the mean water level, rollers, and longshore current. Summing all infragravity contributions due to difference interactions between pairs of shortwave spectral components with the same frequency difference results in infragravity energy density spectra. Calculated spectra are compared with measured spectra obtained during the Delilah field experiment for 11 consecutive days, including calm, mild, and storm conditions. In the comparison, attention is focused on the frequency distribution of the infragravity energy density throughout the surf zone and the transformation of the root mean square infragravity wave height. The measured infragravity response in the frequency range from 0.01 to 0.06 Hz is reproduced by the computations, with relatively broad spectra offshore and a clear nodal structure closer to shore. Typically 80% of the infragravity wave height variability is explained by the model of which 30% or less is due to bound infragravity waves. Differences occur mainly during storm events in which the infragravity energy is underpredicted in the outer surf zone and overpredicted in the inner surf zone.