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.