Turbulence modulation in clay-rich sediment-laden flows and some implications for sediment deposition

Clay-rich flows are common in many sedimentary environments, ranging from rivers with high suspended sediment concentrations and fluid muds on the continental shelves to deep-ocean density currents. Such sediment-laden flows have fluid-dynamic characteristics which are radically different from their clear-water counterparts. Laboratory experiments demonstrate that an increasing concentration of kaolinite results in a distinct change in velocity and turbulence structure in flows transitional between Newtonian and non-Newtonian behavior. Such transitional flows develop a lower region of reduced velocity that is separated from the overlying flow by a distinct shear layer. Large-scale Kelvin-Helmholtz instabilities are generated along this shear layer and dominate both turbulence production and fluid mixing. These experimental results are used to interpret the formation of parallel laminae in turbiditic muds and propose a model for sediment sorting in a range of clay-rich flows. Non-Newtonian flows with low concentrations of kaolinite (∼ 4% by volume) are devoid of both turbulent and cohesive strength at the shear velocities investigated. Deposition of coarser, noncohesive particles in such flows would be expected to be controlled entirely by differences in settling velocity, and the deposits would therefore be expected to be graded and massive.