Headland Rip Very Low Frequency Fluctuations and Surfzone Eddies During High-Energy Wave Events

Abstract

Abstract A wave-group-resolving model is used to investigate the driving mechanisms and the spatiotemporal variability of very low frequency (VLF) fluctuations of a headland deflection rip, measured during a 4-m oblique wave event. Surfzone eddies (SZE) occurring in the presence of a strongly sheared longshore current V at a longshore-uniform beach are first modeled. The spectral signature and the variability of SZE are displayed and compared with the literature. The model is then used to explore the dynamics of vorticity in the surf zone and against a headland under energetic oblique wave conditions. The resulting weakly sheared V is found to host large-scale SZE propagating toward the headland at a speed decreasing seaward. Vorticity animations and spectral diagrams indicate that VLF fluctuations of the deflection rip are driven by the deflection of the upstream SZE. In line with measurements, periods from 40 min to 1 h dominate the spectrum hundreds of meters from the headland at low tide. At high tide, vorticity spectra in the rip are much narrower than in the surf zone, suggesting that the headland enforces the merging of SZE. This mechanism is further analyzed using idealized simulations with varying headland lengths, aiming at extending traditional deflection patterns at the VLF scale. Finally, we discuss the existence of a continuum in SZE driving mechanisms, going from fully wave-group-driven to both wave-group- and shear-instability-driven SZE for weakly and strongly sheared V, respectively. This continuum suggests the importance of wave groups to produce SZE under energetic wave conditions.