Nonlinear evolution of shear instabilities of the longshore current: A comparison of observations and computations

The time dependent nearshore circulation field during 3 days of the SUPERDU CK field experiment is simulated. We consider the generation of nearshore c urrents due to obliquely incident breaking waves: damping effects due to bo ttom friction, and diffusion effects due to lateral momentum mixing caused by turbulence and depth-varying current velocities. Because of uncertaintie s in the friction and lateral mixing: coefficients, numerical simulations a re carried out for a realistic range of values for these coefficients. The resulting shear instabilities of the Longshore current exhibit unsteady lon gshore progressive vortices with timescales of O(100 s) and length scales o f O(100 m) and longer. The time dependent flow involves the strengthening, weakening, and interaction of vortices. Vortex pairs are frequently shed of fshore. During this process, locally strong offshore directed currents are generated, We find that a stronger mean current and faster and more energet ic vortex structures result as the friction coefficient is decreased. Howev er, the longshore length scales of the resulting: flow structures are not a ltered significantly. An increase in the mixing coefficient causes relative ly small variations in the propagation speeds. However, the resulting flow structures are less energetic with larger longshore length scales. Sheer in stabilities are found to induce significant horizontal momentum mixing in t he surf zone and affect the cross-shore distribution of the mean longshore current. Mixing due to the presence of the instabilities is found to be dom inant over mixing caused by more traditional mechanisms such as turbulence. For values of the free parameters that reproduce the propagation speed of the observed motions, the frequency range within which shear instabilities are observed as well as the mean longshore current profile are predicted we ll.