A simple model of mixing in stratified tidal flows

We use a simple model of vertical mixing in a stratified tidal flow to study flows that might be observed in shallow estuaries. In the model, flow is driven by a specified surface pressure gradient due to barotropic tidal motions. The model incorporates two means for accomplishing vertical mixing: (1) shear instability and (2) bottom mixed‐layer deepening by turbulence produced at the estuary bottom and in the shear layer at the top of the mixed layer. We discuss model results that show how the combined action of shear instability and tidal “stirring” accomplishes vertical mixing. The interaction between the instabilities that develop because of the substantial tidal timescale velocity shear caused by stratification is the key feature of mixing. Shear instability weakens the stability of the mixed layer, allowing stirring to efficiently entrain lighter fluid into the mixed layer. The importance of shear is found to depend on the stability of the water column and on the relative importance of frictional forces.