Effects of Grain Size and Sediment Concentration on Turbulence and Sediment Transport Dynamics in Unsteady Turbidity Currents
Turbidity currents are a special type of density current where the density contrast is the result of suspended sediment within the flow, and are the primary means of transporting coarse sediment from the continental shelf into the deep marine. Consequently, a clear understanding of fluid-particle processes is needed to better predict the internal architecture of their deposits. Indeed, one of the most noticeable features of these flows are large-scale Kelvin-Helmholtz instabilities, and it has been suggested that fluid turbulence is the sole process responsible for the generation of long-lasting sediment gravity currents. Yet due to instrument limitations, most studies have relied on saline rather than sediment transporting gravity currents, focusing on one-dimensional time-averaged turbulence statistics rather than discrete, coherent turbulent structures. In this study, we investigate the turbulent properties and sediment transport dynamics of unsteady sediment gravity currents. We present four laboratory flows using two grain sizes and two sediment concentrations, which represent the end members of a larger dataset. High resolution velocity and concentration profiles were measured using a three-dimensional ultrasonic Doppler velocity profiler (UDVP 3D) and a Computed Tomography (CT) scanner, respectively. The non-obtrusive nature of this setup allowed the distance between their sampling positions to be minimized. All runs were characterized by non-uniform flow deceleration, and deposition appears to result from a lack of flow capacity rather than competence. Preliminary results indicate that all flows are populated by vertically coherent fluid structures that span the entire flow depth, and whose intensity and duration depends on particle size and concentration. CT results show that the flow field contains alternating pulses of high and low sediment concentration fluid that also span the entire flow depth, and which importantly can be correlated with coherent turbulent structures in the velocity data. All these observations, therefore, challenge the generally held opinion that the high velocity core represents a momentum and sediment exchange barrier, and that the upper and lower regions act as decoupled flow units. Rather, these results suggest that, much like rivers, turbidity currents operate as a single, vertically continuous flow system, at least up to sediment concentrations of 8% by volume.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California