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 (V) in the surf zone and offshore values of the off axis component of radiation stress (S ). Seventeen minute — xv averages of both S and V showed considerable temporal xv variation, and little or no tendency to vary together. There was also considerable longshore spatial variability of the 1ongshore"current. Attempts to measure gradients of S 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.