Shear Instabilities in the Longshore Current: A Comparison of Observation and Theory

Abstract

Abstract Low-frequency (<0.01 Hz) oscillations in the surf zone longshore current with wavelengths too small (<300 m) to be surface gravity waves were observed during the 1986 SUPERDUCK experiment at Duck, North Carolina. The observations suggest that these oscillations are dynamically linked to the mean longshore current in the surf zone, leading Bowen and Holman to propose that the observed oscillations are manifestations of a shear instability in the longshore current. In this paper, field data from both the SUPERDUCK experiment (a barred beach) and the l980 NSTS experiment, at Leadbetter Beach, Santa Barbara, California (a plane beach), are used to compare quantitatively the model of Bowen and Holman (which is extended to include the effects of dissipation due to bottom friction) with observation. Observed frequency-cyclic wavenumber (f-K) spectra (constructed from alongshore arrays of velocity measurements made in about 1.5–2 m of water in the trough of the bar at SUPERDUCK, and in about 1 m at NSTS)...