Turbulent structure of high-density suspensions formed under waves

Abstract

We performed a series of laboratory experiments on the interactions between turbulent wave boundary layers and a predominantly silt-sized sediment bed. Under a wide range of wave conditions similar to those observed on storm-dominated midshelf environments we produced quasi-steady high-density benthic suspensions. These suspensions were turbulent, while containing large near-bed concentrations of suspended sediment (17–80 g/L), and were separated from the upper water column by a lutocline. Detailed measurements of the vertical structure of velocity, turbulence, and sediment concentration revealed that the wave boundary layer, while typically >1 cm thick in sediment-free conditions, was reduced substantially in size, often to <3 mm, with the addition of suspendible sediment. This likely resulted from sediment-induced stratification that limited vertical mixing of momentum. Despite boundary layer reduction the flows were able to support high-density suspensions as thick as 8 cm because turbulent energy was transported upward from this thin but highly energetic near-bed region. Standard formulations of the Richardson number for shear flows are not applicable to our experiments since the suspensions were supported from transported rather than locally produced turbulence.

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