Coherent Doppler Profiler measurements of near‐bed suspended sediment fluxes and the influence of bed forms

Abstract

This paper reports on remote acoustic observations of vertical turbulence intensity and vertical suspended sediment flux profiles on a planar beach in 3–4 m water depth. The measurements of suspended sediment concentration and velocity are colocated and simultaneous and extend through the wave bottom boundary layer to the bed with 0.7 cm vertical resolution. Normalized cospectra of the suspended sediment flux and the vertical velocity for different bed states (irregular ripples, cross ripples, linear transition ripples, and flat bed) indicate a small but significant peak at incident wave frequencies but are otherwise rather flat, with weak redness. Estimates of the vertical flux components indicate a general balance between upward fluxes due to waves and turbulence and downward settling. Two exceptions to this balance are found immediately above the bed and for nonmigrating irregular ripples. The contribution from the high‐frequency turbulent component is small. Wave phase averages for low‐energy bed states exhibit near‐bed peaks in the suspended sediment flux following wave phase reversal. Wave phase averages for the high‐energy cases do not exhibit a diffusive signature. Observed vertical profiles of turbulence intensity for different bed states reveal that the near‐bed turbulence levels are relatively independent of bed state. Friction velocity predictions from presently available models, including a bed stress model and a sediment eddy diffusion model, are compared to measured values of near‐bed turbulence intensity. Reasonable agreement is found between measured and predicted bottom friction velocities when wave friction factors from Tolman [1994] are used.