Surface Drift Currents Induced by Waves and Wind in a Large Tank

The empirical features of surface drift currents induced by both mechanical and wind waves are presented. The measurements were made by using surface floats in a large tank with dimensions of 32.5 x 1 x 2m(3). Three cases were studied: (i) regular (narrowband) mechanical waves, (ii) irregular (wideband) mechanical waves, and (iii) wind waves. The measured surface drift currents induced by mechanical waves U-d are compared with the Stokes drift at the surface U-St estimated by a well-known formula with an integral over the wave spectrum. In this case, the ratio U-d/U-St varies in the range of 0.5-0.93 and slightly increases with decreasing wave steepness. No visible dependence on the breaking intensity is observed. In the case of wind waves, the wind-induced part of the surface drift U-dw is compared with the friction velocity u(*). In our measurements, the ratio U-dw/u(*) varies systematically in the range of 0.65-1.2. Considering the percentage ofwave breaking Br, the wave age A, and the wave steepness., the parameterization of U-dw was obtained in the form U-dw= (Br + (sic)A)u(*), which corresponds to the observations with a mean error of 10%. For the first time, this ratio provides the dependence of windinduced drift on the surfacewave parameters. The obtained results and problems related tomeasuring surface drift currents are discussed.