Exploring Wind-Driven Subsurface Water Flow with an Acoustic Doppler Velocity Profiler

In this study, we report on experiments carried out in a large wind-wave tank to investigate the potential of the acoustic Doppler velocity profiler for determining the structure of the subsurface water boundary layer. This flow located just beneath the air-water interface forms whenever wind blows. The profiler is first tested for a steady flow generated by pumps beneath a flat water surface. Measurements of the velocity field at different stages of development of the wind-induced shear flow, from laminar to fully turbulent, are then analyzed. The best way to obtain reliable data under these flow conditions is thoroughly examined. Despite the inherent difficulty of seeding acoustic tracers homogeneously in such a boundary layer, the profiler has the major advantage of providing records of the instantaneous profiles of the subsurface velocity field referenced to surface elevation. This feature makes it possible to estimate statistical properties of the water motions at various scales in a wave-following coordinate system, and thus greatly increases the physical significance of the measured quantities. The variation with fetch of the main characteristics of the mean drift current, orbital wave motions, and turbulent flow disturbances estimated in this coordinate system is then presented and discussed in detail.