Sediment Suspension by Straining-Induced Convection at the Head of Salinity Intrusion

The tidal straining can generate convective motions and exert a periodic modification of turbulence and sediment transport in estuarine and coastal bottom boundary layers. However, the evidence and physics of convection and sediment suspension induced by tidal straining have not been straightforward. To examine these questions, mooring and transect surveys have been conducted in September 2015 in the region of the Yangtze River plume influence. Field observations and scaling analyses indicate an occurrence of convective motions at the head of saline wedge. Theoretical analyses of stratification evolution in the saline wedge show that unstable stratification and resultant convection are induced by tidal straining. Vertical turbulent velocity and eddy viscosity at the head of saline wedge are both larger than their neutral counterparts in the main body, largely enhancing sediment suspension at the head of saline wedge. Moreover, sediment suspension in both neutral and convection-affected flows is supported by the variance of vertical turbulent velocity, rather than the shearing stress. Finally, the stability correction functions in the Monin-Obukhov similarity theory can be simply derived from the local turbulent kinetic energy balance to successfully describe the effects of tidal straining on turbulent length scale, eddy viscosity, and sediment diffusivity in the convection-affected flow. These recognitions may provide novel understanding of estuarine turbidity maxima, and the dynamical structure and processes for coastal hypoxia.