LONGSHORE-CURRENT AND WAVE HEIGHT MODULATION AT TIDAL FREQUENCY INSIDE THE SURF ZONE

Abstract

Data were acquired continuously during the 19-day DELILAH nearshore experiment with a specific objective of examining variability of the longshore current at tidal frequencies. It is hypothesized that breaking wave heights inside the surf zone are strong functions of the depth which are modulated by the tidal variations, and since radiation stress is a function of the wave height, longshore currents are forced at the tidal frequency inside the surf zone. The measured longshore current variations at tidal frequency are the same order of magnitude as the mean longshore current variations for moderate wave height conditions, indicating that the tide is a dominant mechanism associated with longshore current variability. Simulations of the magnitude and phase of the longshore current variability with tide elevation using the model by Thornton and Guza (1986) are used to explain observations. The measured tidal elevation and longshore current are in phase in the inner surf zone and out of phase in the outer surf zone as predicted by the model, verifying the hypothesis.