Three-dimensional Unstable Littoral Currents Analyzed with An Eulerian Phase-averaged Primitive Equation Based on A Vortex Force Formalism

Abstract

Littoral flows are analyzed with an Eulerian-averaged primitive equation for slowly-evolving oceanic flows based on a vortex-force formalism coupled with a WKB spectrum-peak wave model (ROMS-WEC; Uchiyama et al., 2010). Two surf zone problems on a realistic topography are analyzed: 1) shear instability associated with longshore currents driven by obliquely incident waves, and 2) normal mode instability of offshore-directed rip currents under a near-normal incident condition. The coupled wave-current model successfully reproduces 3-D shear waves during the SandyDuck field measurement. We found in 3-D rip-induced coherent eddies that littoral currents have significant depth dependency leading to vorticity stretching/titling effects and to faster decay of enstrophy and kinetic energy than 2-D rip eddies.