This study demonstrates observations of turbulence dissipation rate on the lutocline and the role of sediment-induced stratification and velocity shear. The turbulence dissipation rate at 1.3 mab is on average greater during flood, characterized by well mixing and rapid entrainment, than that during ebb, when the water column is clearly stratified. The dissipation rate in ebb is more related to velocity shear than stratification, especially in late ebb, when a secondary high shear region appears in the water column. During the formation of fluid mud, the shear layer rises with the fluid mud layer, resulting in the appearance of low Ri and high dissipation rate above the lutocline. Subsequently, the buoyancy effect caused by stratification increases and shear decreases, leading to the dissipation rate decreasing rapidly with increasing Ri. There is a turbulence transition zone around the lutocline, where the dissipation rate is almost constant and Ri always varies around the critical value of 0.25 due to the balance of buoyancy frequency with velocity shear. These data present turbulence conditions in a hyperturbid estuary where fluid mud layers commonly exist, contributing to the understanding of the dynamics of fluid mud formation and entrainment.
 

fluid mud, lutocline, turbulence, stratification