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

Data were acquired continuously during the 19-day DELILAH nearshore ex periment with a specific objective of examining variability of the lon gshore current at tidal frequencies. It is hypothesized that breaking wave heights inside the surf zone are strong functions of the depth wh ich 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 m ean longshore current variations for moderate wave height conditions, indicating that the tide is a dominant mechanism associated with longs hore current variability. Simulations of the magnitude and phase of th e longshore current variability with tide elevation using the model by Thornton and Guza (1986) are used to explain observations. The measur ed tidal elevation and longshore current are in phase in the inner sur f zone and out of phase in the outer surf zone as predicted by the mod el, verifying the hypothesis.