Water particle velocities measured under ocean waves

Abstract

Simultaneous measurements of waves and two orthogonal water particle velocities were made at six elevations in 19 m of water by using a penetrating wave staff and an electromagnetic flowmeter. Moderate swell and low-wind conditions prevailed during the experiment. The measured wave-induced velocities were 2–4% greater than those calculated by using linear wave theory. Coherence of the wave height and wave-induced velocities in the significant energy-density range was computed to be over 0.85, indicating that the motion was almost totally wave induced. At higher frequencies it was apparent that the motion was primarily turbulence. Phase spectra computed for the measured wave heights and orbital velocities compared very well with linear theory. Measured frequency distributions were compared with both Gaussian and Gram-Charlier distributions by using the chi-squared goodness-of-fit test. Qualitatively, the Gram-Charlier distribution gave the better fit to the data suggesting a weakly nonlinear system.