Model-data comparisons of shear waves in the nearshore

Abstract

Observations of shear waves, alongshore propagating meanders of the mean alongshore current with periods of a few minutes and alongshore wavelengths of a few hundred meters, are compared with model predictions based on numerical solutions of the nonlinear shallow water equations. The model (after Özkan-Haller and Kirby (1999)) assumes alongshore homogeneity and temporally steady wave forcing and neglects wave-current interactions, eddy mixing, and spatial variation of the (nonlinear) bottom drag coefficient. Although the shapes of observed and modeled shear wave velocity spectra differ, and root-mean-square velocity fluctuations agree only to within a factor of about 3, aspects of the cross-shore structure of the observed (∼0.5–1.0 m above the seafloor) and modeled (vertically integrated) shear waves are qualitatively similar. Within the surf zone, where the mean alongshore current (V) is strong and shear waves are energetic, observed and modeled shear wave alongshore phase speeds agree and are close to both V and C lin (the phase speed of linearly unstable modes) consistent with previous results. Farther offshore, where V is weak and observed and modeled shear wave energy levels decay rapidly, modeled and observed C diverge from C lin and are close to the weak alongshore current V. The simulations suggest that the alongshore advection of eddies shed from the strong, sheared flow closer to shore may contribute to the offshore decrease in shear wave phase speeds. Similar to the observations, the modeled cross- and alongshore shear wave velocity fluctuations have approximately equal magnitude, and the modeled vorticity changes sign across the surf zone.