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Grain Size Controls on the Internal Structure of Sediment Gravity Currents and Their Implications for Depositional Properties; a Novel Application of CT imagery


In recent years turbidity currents have been the focus of intensive laboratory studies with the aim of characterizing fluid-particle interactions. However many fundamental questions remain, in part because of technical challenges associated with highly concentrated flows. Unfortunately, conventional methods for obtaining direct measures of suspended sediment concentration along the current axis introduce flow disturbances and lack the spatio-temporal resolution to properly characterize many features in the flow, and while indirect measures such as pixel coloration from video analysis provide relative measures of sediment concentration, results are limited to the near-wall region and direct comparison of multiple runs is problematic, especially if grain color changes. Similarly, acoustic techniques are often not viable due to signal attenuation and it is not clear if saline density currents are reliable proxies for sediment gravity flows. This study overcomes this challenge by using Computed Tomography (CT scanner) to measure current density across the width of the flume (0.3m) and 0.18m in the downflow direction. Here we focus on four experimental currents with distinct, uniform grain distributions (d50: 70, 150, 230, 330 μm) of equal initial slurry concentrations (∼18% by mass) and flow velocities (∼0.7 m/s). This data is supplemented with PIV imagery to investigate fluid-particle processes operating at spatial and temporal scales beyond the limits of the CT scanner. Results show that grain size plays a fundamental role in the character of sediment gravity flows and how they interact with the ambient fluid. Specifically, many of the smaller mixing features are replaced by larger, stronger mixing structures with increasing grain size. We believe the reasons for this are twofold. First, the increased particle mass necessitates higher impulse to change particle trajectory and thus are less sensitive to weak or brief fluctuations in fluid stress. Second, the higher settling velocities promote flows that are highly stratified between the base and top of the current, promoting the development of large rather than small Kelvin-Helmholtz instabilities. This is supported by the CT data, which demonstrates that concentration profiles in the coarse grained runs decrease exponentially away from the bed, whereas they become more linear and then vertically uniform with decreasing grain size.