Coherent Doppler profiler measurements of near-bed suspended sediment fluxes

Abstract

Vertical profiles of vertical turbulence intensity and vertical sediment fluxes were collected by an acoustic Coherent Doppler Profiler during two experiments: at Queensland Beach, NS and SandyDuck97. Measurements are investigated as a function of bedstate from low energy vortex ripples to high energy flat bed. Ensemble-average vertical turbulence intensity profiles exhibit a peak within the wave boundary layer at heights of O (1 cm) above bottom for all bedstates. Peak average turbulence intensities are relatively independent of bedstate, varying by no more than 50% despite a factor of 7–10 variation in average wave energy. This otherwise remarkable observation can be understood from the corresponding decrease in the physical roughness of the bed, associated with the different observed bedstates. Estimated wave friction factors are highest for low energy rippled beds and smallest for flat bed conditions. Estimates of the vertical suspended sediment flux partitioned into mean, wave, and turbulent components, show that in general, there is a balance between upward fluxes and downward settling; except immediately above the bed, and except for the case of a stationary ripple field. The suspended sediment flux coherence indicates enhancement at incident wave frequencies, with the largest coherence for flat bed conditions very near the bed.