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The Importance of Density Structure on Vertical Momentum Exchange and its Implication for Turbidity Current Run-out Distances and Depositional Geometry

Abstract

Be it through hydrodynamic instabilities or turbulence, it's well accepted that all natural geophysical flows are characterized by a range of variously sized, superimposed coherent fluid structures. More fundamentally these structures are created by a momentum gradient, which in turn controls the very nature of mixing and related energy loss. The momentum gradient consists of two parts, a vertical velocity shear component and a density shear component. However many geophysical flows are only weakly stratified, and because of the difficulty of obtaining sufficiently resolved datasets of these two properties, the density gradient term is often assumed to be negligible, and momentum exchange is equated exclusively to velocity shear. Yet in the case of highly stratified flows, like turbidity currents, this assumption can potentially lead to erroneous conclusions regarding the nature of mixing in these flows, including the vertical extent of Kelvin-Previous HitHelmholtzNext Hit instabilities and the presence or absence of a vertical momentum exchange barrier across the high velocity core. Collectively these can have important implication on the internal character of the flow, including its ability to suspend sediment, degree of ambient fluid entrainment, and ultimately their run-out distance and architecture of their deposits. Here we report on a series of experiments that paired a three-dimensional ultrasonic Doppler velocity profiler (UDVP-3D) and a medical grade computed tomography (CT) scanner to simultaneously examine the velocity and density structure of sediment gravity currents across a range of particle sizes (d50: 70, 150, 230, 330 μm) and sediment concentrations (∼5-18% by mass; 2-8 volume sediment %). Results show that the density shear is of the same order as the velocity shear, suggesting that density structure plays a first order control on the nature of mixing. Moreover, in the coarse grained flows (230 and 330 um) the momentum barrier is displaced closer to the bed and away from the velocity core, which then permits the growth of larger Kelvin-Previous HitHelmholtzTop instabilities that now more closely approach the thickness of the flow. These conditions result in increased mixing (i.e. drag) between the current and the ambient fluid, and ultimately shorter run-out distances and more lobate deposits.