Coherent Doppler sonar: Sediment flux and turbulent velocities in a wave flume

Observations of vertical sediment flux and velocity structure are made under prototype scale waves in the Wave Research Flume at the National Research Council (NRC) in Ottawa. Observations were made under regular waves of 3.5 s period with heights ranging from 20 to 70 cm. Direct measurement of sediment flux is made possible using 1.7 MHz pulse-to-pulse coherent sonar which determines concentration from acoustic backscatter levels and velocity using acoustic Doppler. Operating over a 0.8 m range, velocity profiles with 1.4 cm range resolution and a 0.5 cm s-1 vertical velocity accuracy can be made at a rate of 30 profiles per second. We find that there is an apparent balance between the mean downward flux of sediment and the upward flux due to turbulent motions. The component of (upward) vertical flux caused by wave action is small compared to the turbulent and mean components. Profiles of turbulence intensity are provided by the vertical velocity fluctuations, these profiles show a rapid rise to a peak value within 5 cm of the bottom and then a subsequent decrease. The (near bottom) peak value of root-mean-square vertical, velocity fluctuations are equal to the friction velocity characteristic of the bottom boundary layer. The decrease in turbulence with height above the bottom shows behavior consistent with the decrease in grid generated turbulence but appears sensitive to the length scales of the bed-forms rather than sand grain roughness.