Variability of Longshore Currents

Simultaneous measurements were made of the offshore directional spectra of gravity waves, and longshore currents within the surf zone. The goal was to test theories which suggest a direct relationship between mean longshore currents ($\overline{\text{V}}$ ) in the surf zone and offshore values of the offaxis component of radiation stress (S_XY). Seventeen minute averages of both S_XY and $\overline{\text{V}}$ showed considerable temporal variation, and little or no tendency to vary together. There was also considerable longshore spatial variability of the longshore" current. Attempts to measure gradients of S_XY in the surf zone failed because of small errors in instrument orientation. The measurements suggest that considerable temporal and spatial averaging will generally be required to obtain a representative picture of longshore currents, even if no rip currents are present, due to the presence of “eddy” motions or long edge waves.