Over the past two decades the application of acoustics to the measurement of small-scale sediment processes has been gaining increasing acceptance within the sedimentological community. This has arisen because acoustics has the potential to measure non-intrusively, with high temporal and spatial resolution, profiles of suspended sediment size and concentration, profiles of flow, and the bedform morphology. In the present article we review the capability of acoustics to deliver on its potentiality to make a valuable and unique contribution to the measurement of small-scale sediment processes. The article introduces the reasons for using acoustics, the physics underlying the approach, a series of examples illustrating collected data, a discussion on some of the difficulties encountered when applying acoustics and finally a look to the future and possible new developments.

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A review of acoustic measurement of small-scale sediment processes

The Glenn and Grant [1987] continental shelf bottom boundary layer model for the flow and suspended sediment concentration profiles in the constant stress layer above a noncohesive movable sediment bed has been updated. The Reynolds fluxes for sediment mass and fluid momentum are closed using a continuous, time-invariant linear eddy viscosity modified by a continuous stability parameter to represent the influence of suspended sediment-induced stratification throughout the constant stress region. Glenn and Grant [1987] use a less realistic discontinuous eddy viscosity and neglect the stratification correction in the wave boundary layer. For typical model parameters the two models produce currents above the wave boundary layer that are in better agreement than the suspended sediment concentrations. Within the wave boundary layer the differences are much greater for both the current and the sediment concentration. This leads to significant differences in the sediment transport throughout the constant stress layer. Sensitivities of the updated model were examined on the basis of observed wave and current data acquired during storms on the inner continental shelf. Comparisons between the stratified and neutral versions of the updated model indicate that the stratified version produces a total depth-integrated sediment transport that can be 2 orders of magnitude less than, and time-averaged shear velocities that can be nearly half of, that predicted by the neutral version. Sensitivities to grain size distributions indicate that even a small amount of finer sediment can stratify the storm-driven flows. Sensitivities to closure constants within the range of reported values also produce up to an order of magnitude variation in sediment transport, illustrating the need for dedicated field experiments to refine further estimates of these parameters.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JC900115

Modeling stratified wave and current bottom boundary layers on the continental shelf

Seabed-hugging flows called turbidity currents are the volumetrically most important process transporting sediment across our planet and form its largest sediment accumulations. We seek to understand the internal structure and behavior of turbidity currents by reanalyzing the most detailed direct measurements yet of velocities and densities within oceanic turbidity currents, obtained from weeklong flows in the Congo Canyon. We provide a new model for turbidity current structure that can explain why these are far more prolonged than all previously monitored oceanic turbidity currents, which lasted for only hours or minutes at other locations. The observed Congo Canyon flows consist of a short-lived zone of fast and dense fluid at their front, which outruns the slower moving body of the flow. We propose that the sustained duration of these turbidity currents results from flow stretching and that this stretching is characteristic of mud-rich turbidity current systems. The lack of stretching in previously monitored flows is attributed to coarser sediment that settles out from the body more rapidly. These prolonged seafloor flows rival the discharge of the Congo River and carry ~2% of the terrestrial organic carbon buried globally in the oceans each year through a single submarine canyon. Thus, this new structure explains sustained flushing of globally important amounts of sediment, organic carbon, nutrients, and fresh water into the deep ocean.

Newly recognized turbidity current structure can explain prolonged flushing of submarine canyons.

For the first time, detailed measurements of sediment concentrations and grain velocities inside the sheet flow layer under prototype surface gravity waves have been carried out in combination with measurements of suspension processes above the sheet flow layer. Experiments were performed in a large-scale wave flume using natural sand. Sand transport under high waves in shallow water is mainly contained within the so-called “sheet flow layer,” a thin layer (10–60 grain diameters) in which the volume concentration of sand decreases by an order of magnitude from a value near 0.6 at the stationary bed. The thickness of the layer varies over a wave cycle and the maximum thickness increases with increasing peak Shields stress. The concentrations within the sheet flow layer vary approximately synchronously with the orbital velocity measured by an Acoustic Doppler Velocimeter (ADV) located 0.1 m above the bed, with typical phase lags of 0–π/5. In contrast, the suspended sediment concentrations a few centimeters and higher above the bed exhibit larger phase lags. Grain velocities were successfully measured in the middle and upper portions of the sheet flow layer around the time of their maximums. These velocities increased weakly with elevation from approximately 50% to 70% of the velocity outside the wave boundary layer. The observations are compared to previous experimental work and are found to be mainly consistent with observations in steady unidirectional flows and in oscillating water tunnels (OWTs), although differences in the suspended sediment concentration and the total sediment transport rate are apparent. Observations are also compared to two very different models: a 1DV suspension model for oscillatory flow with enhanced boundary roughness and a two-phase collisional grain flow model for steady unidirectional flow. While the suspension model describes the velocity profile fairly well and the collisional model describes the concentration profile well, neither model accurately predicts both the velocity and the concentration and therefore the sediment flux over the full vertical extent of the sheet flow.

/pdf/sheet-flow-dynamics-under-monochromatic-nonbreaking-waves-1bcugcuqkf.pdf

Sheet flow dynamics under monochromatic nonbreaking waves

[1] Measurements of suspended sediment concentration, and the associated bedform morphology, were made beneath regular and irregular waves in a large-scale flume. The bed forms were measured using an acoustic ripple profiler, and the suspended sediments were measured using an acoustic backscatter system, together with pumped sampling. Using the measured bed form dimensions and the flow as inputs, standard approaches have been used to predict the reference concentration and vertical distribution of suspended sediment concentration. Nielsen's [1986] empirical expression for the reference concentration, C0, shows reasonable overall agreement with the present measurements in respect of the dependence of C0 on the Shields parameter (skin friction); but the formula somewhat overestimates the measured concentrations. To analyze the form of the measured mean concentration profiles, comparisons have been made with the simple one-dimensional (vertical) formulations, and extensions thereof, proposed by Nielsen [1992] on the basis of pure diffusion, pure convection, and combined convection and diffusion. It is concluded that in a near-bed layer of thickness about two ripple heights (i.e., the layer dominated by vortex formation and shedding above ripples), pure diffusion characterized by a height-independent sediment diffusivity provides a good representation of the measured profiles. Above this, Nielsen's [1992] convection-diffusion solution provides a better representation. It is shown, however, that by use of pure turbulent diffusion modeling concepts, the same profile can also be obtained by the use of a height-varying, “constant + linear,” sediment diffusion model. This diffusivity represents the enhanced mixing in the outer part of the oscillatory boundary layer caused by the breakdown of coherent vortex structures into random turbulence. The relative merits of convection and diffusion schemes are discussed.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2001JC000988

Suspended sediments under waves measured in a large-scale flume facility

Models for the vertical distribution of suspended sediment under waves and currents are compared with field observations The models include a diffusion-based model, a convection-based model, and a combined convection-diffusion model The field observations were carried out at Vilano Beach, Florida, in a water depth of approximately 3 to 4 m, with a median grain diameter of 014 mm The rms wave height ranged from approximately 04 to 12 m Under low waves, the seabed was covered with very small ripples, and, under the higher waves, sheet flow conditions were present The diffusion-based model was found to best describe the observed data under high wave conditions, but the convection model was more accurate under low wave conditions Over all conditions, the combined model was the most accurate These results are consistent with the different mechanisms for mixing suspended sediment over flat versus rippled beds

T. H. Lee

Papers

Comparison of field observations of the vertical distribution of suspended sand and its prediction by models