Three-Dimensional Nearshore Currents and Eddies on an Alongshore-Variable Barred Beach

Abstract

Three-dimensional circulation in the nearshore region is critical for contaminants, sediment, and larvae dispersion along the coast and transport between the surf zone and the inner shelf. In the surf zone, eddies generated by short-crested wave breaking, wave-group forcing, and shear instabilities are associated with energetic vortical motion at very low frequencies (f ~ 0.005 Hz), but the horizontal length scales and vertical structure of these vortical motions are poorly understood. Here, a three-dimensional phase-resolving numerical model, Simulating WAves till SHore (SWASH), simulates wave propagation and three-dimensional circulation patterns on a barred beach near Duck, North Carolina. Modeled wave statistics and nearshore currents are consistent with field measurements collected in Oct. 2015. The model accurately simulates the cross-shore wave height gradient, mean alongshore currents, and offshore-directed meanders within the surf zone for a range of offshore wave conditions. Simulated eddy length scales, quantified using the alongshore wavenumber spectra of vorticity, suggest that large-scale eddies (O(100) m) occur for both alongshore-variable (observed) and alongshore-uniform (alongshore mean of observed) bathymetry, and may be enhanced by surfzone bathymetric variability. Small-scale eddies (O(10) m) generated by short-crested breaking and wave-group forcing have similar eddy variance for simulations with alongshore-uniform and alongshore-variable bathymetry. The depth dependence of the alongshore wavenumber spectra of vorticity magnitude and peak length scales varies with cross-shore location and bathymetric variability. The complex vertical structure of the energy, coherence, and phase of very low-frequency vortical motion for both alongshore-variable and alongshore-uniform bathymetry may be associated with the vertical structure of eddy injection, instabilities, mean shear, and bottom boundary layer dynamics.