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