--> Vertical Momentum Controls on the Run-Out Distances of Turbidity Currents

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Vertical Momentum Controls on the Run-Out Distances of Turbidity Currents

Abstract

It is well known that many turbidity currents originate on the upper continental slope, accelerate downslope and then deposit much of their sediment load on the deep basin floor. What's less clear is how these currents are able to achieve run-out distances of hundreds to thousands of kilometers along the basin floor under virtually zero grade conditions. Although numerous researchers have suggested that this is related to the internal momentum of the flow, obtaining simultaneous high-resolution velocity and density datasets of sediment gravity currents is notoriously difficult. Consequently, many experimental studies employ saline density currents as surrogates for particle gravity flows, even though it is unclear how suitable they are as proxies for explaining run-out distances since they omit the effects of varying particle settling velocities and fluid-particle and particle-particle interactions, all which must have played some, if not major role in governing the internal characteristics of the turbidity currents. 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 sediment volume %). Results show that compared to coarser-grained flows, finer-grained flows are less density stratified, have a more bulbous velocity profile and the high velocity core is positioned higher above the bed. Reduced density stratification, in addition to reduced grain settling velocity and increased particle-particle interactions, controls the shape of the velocity profile, which in fine-grained flow leads to a more symmetric (“plug-like”) profile between the bed and the top of the boundary layer. It is this more vertically uniform density structure in fine-grained flows, rather than the velocity profile, that controls the local momentum gradient, and as a consequence reduces mixing between the current and the ambient fluid. Reduced mixing allows these flows to retain more of their initial momentum, and accordingly, promotes longer run-out distance across a virtually horizontal deep basin floor.