Influence of flow discharge and minibasin shape on the flow behavior and depositional mechanics of ponded turbidity currents

Abstract

The interplay between seafloor sediment-laden density-driven flows, turbidity currents, and topography helps to shape continental margins. However, these interactions are poorly understood, especially those within enclosed depressions termed minibasins. In this study, novel experiments quantify the three-dimensional (3-D) dynamics of turbidity currents interacting with a range of minibasin geometries that, for the first time, scale within the parameter space of natural systems. Controls on flow dynamics are quantified by measuring the evolving velocity and sediment transport fields, in addition to maps of bathymetry. This study focuses on three aspects of turbidity current interactions with minibasins. First, the results suggest that sediment transport and deposition in minibasins is likely dominated by evolving flow conditions. Contrary to earlier studies in two-dimensional (2-D) flumes, this study supports a time-to-flow equilibrium in minibasins that scales with the time to replace ambient fluid with turbid influx, and this replacement time likely takes days to achieve in many field-scale minibasins. Second, in all experiments, horizontal flow circulation is observed, which is critical for distributing sediment throughout minibasins. However, the strength of the horizontal circulation reduces as the ratio of minibasin length to width increases, which leads to stagnant or even upstream-directed flow near the bed, elevated height of the velocity maximum in flows, the lowering of near-bed shear stresses, and more homogeneous deposits through a reduction in bed reworking. Finally, the results indicate that fluid detrainment from minibasins significantly reduces sediment fall velocities, severely lowering the sediment-trapping efficiency for small or light particles. This reduction in effective fall velocities of sediments suggests a mechanism that fractionates fine particulates (e.g., clays), nutrients (e.g., organic carbon), and pollutants (e.g., microplastics) along transport paths down topographically complex margins.