Coupling between suspended sediment distribution and turbulence structure in a laboratory turbidity current

A detailed laboratory experiment reveals the coupling between flow and suspended sediment properties (velocity, turbulence, concentration, and granulometry) in a low-density, depositional turbidity current. A model is proposed that explains spatial variation in the coupling through size segregation and mixing controlled by the interplay between upward directed turbulence and gravitational settling of suspended particles. The head of the flow is strongly turbulent, relatively well mixed, dilute, and coarse grained because upward directed turbulence, related to frontal shear and mixing with ambient water, generally outbalances particle settling. The body is grain size and density stratified, with strong turbulence confined to upper and lower flow boundaries. Here settling is generally more important than turbulence. Contrasting dynamics of different particle sizes modify the general patterns recognized in the head and body. Thus preferential settling of the coarsest particles produces normal grading in the lower head. In the lower body, highly turbulent, relatively low downstream velocity fluid packages, interpreted as upward moving, basal shear-induced ejections, alternate with less turbulent, higher downstream velocity fluid packages. Relatively coarse grains react to these flow changes by alternate vertical mixing and preferential accumulation near the flow's base, whereas the distribution of finer grains remains unaffected. Kelvin-Helmholtz waves generated by upper boundary shear penetrate the upper body. The zone of maximum flow velocity is typically weakly turbulent and appears to form a barrier to turbulence-induced particle exchange between the lower and upper flow regions. The coupling model entails periodic changes in grain size measures near the flow's base, which may explain the formation of lamination and banding in natural turbidites.