UNIDIRECTIONAL AQUEOUS FLOW 555 Figure 13 Schematic model for flows with increasing clay concentrations Each model depicts the characteristic velocity profile and nature of flow, a vertical profile of the rms u, representative time series at several heights, and a representation of sediment settling rms refers to the root mean square value of the downstream velocity component, or level of turbulence, within the flow, and indicates the principal regions of turbulence generation within each profile (A) Turbulent flow with a logarithmic velocity profile and turbulence generation near the bed At low clay concentrations, drag reduction may begin to occur Coarser sediment is supported through turbulence, and sedimentary structures can develop (B) Transitional flows with a developing shear layer, which separates a lower region of high velocity gradient from an upper layer of reduced shear Sketch of flow depicts streamlines Turbulence is strongest in the shear layer, along which Kelvin Helmholtz instabilities are developed with a distinctive velocity signature (see inset sketches) Sediment entrained into the basal region is trapped, and parallel lamination may be produced by the variable shear stresses induced by the shear layer instabilities (C) Laminar plug flow without turbulence and with low cohesive strength The cohesive strength of the flow is unable to support coarser sediment, which settles to the bed (D) Laminar plug flow with high cohesive strength is able to support coarser sediment suspended within the flow Reproduced from Baas JH and Best JL (2002) Turbulence modulation in clay rich sediment laden flows and some implications for sediment deposition Journal of Sedimentary Research 72: 336 340 SEPM (Society for Sedimentary Geology) of sediment requires turbulence within the flow, but a feedback is exerted where at some point the turbulence begins to be modified by the sediment in suspension This complex feedback mechanism is poorly understood, with factors such as the concentration of sediment and ratios of grainsize: turbulent length and time scales thought to be important in causing either a decrease or increase in turbulence (turbulence attenuation and enhancement respectively) within the flow (Figure 12) Turbulence modulation is thus a key feedback mechanism within many unidirectional flows, acting to both enhance and suppress turbulence production, and is also known to change the downstream velocity profiles significantly with subsequent implications for sediment transport and sorting (Figure 13)