Velocity profile measurements in the crest of a breaking wave using the BASS rake acoustic velocity sensor

The BASS Rake, a differential travel time, velocity profile sensor with multiple sample volumes, was originally designed for use in thin, oscillatory, bottom boundary layers. Recent proposals have suggested using the instrument to measure full water column and near-surface profiles in the swash and surf zones on a beach. Other technologies cannot reliably provide the detailed imaging of velocity structure under field conditions that is possible with the BASS Rake. Unfortunately, entrained air bubbles are ubiquitous in the swash and surf zones and their presence is known to decrease sound speed and increase attenuation under some circumstances. Either of these effects might preclude the use of this acoustic technology. The experiments described in this paper were conducted in several wave tanks and flumes of Cornell University's DeFrees Hydraulics Laboratory, specifically to determine the possible extent of this problem. Our results show that, for the BASS acoustic wavelength, approx. 0.86mm, and the bubble size range generated in the wave tank and flumes, ≤8mm, there is no measurable reduction in sound speed nor any measurable increase in attenuation. We did observe an increase in measurement dropout with increasing bubble fraction, however, velocities were reliably and robustly acquired in all but the highest bubble densities.