Wave particle velocities measured with a Doppler current meter

A newly developed three-dimensional Doppler current meter is described and the results of preliminary field experiments are presented where simultaneous measurements of surface elevation and water velocity associated with wave orbital motion were made. The phase difference between the surface elevation and the vertical velocity measured at 1.0 and 0.45 meters below the mean water level is found to be approximately 90, in accord with the theory for surface waves of infinitesimally small amplitudes. The spectral (frequency) density distribution for velocity is also found to agree with that we would expect from the linear theory for the observed frequency distribution of surface elevation. However, the amplitude of velocity is consistently smaller (about 10 %) than that we would expect. This reduction of amplitude is more pronounced in cases where waves are high and the water depth is shallow.