Response of Stratification Processes to Tidal Current Alteration due to Channel Narrowing and Deepening

Stratification in estuaries has received much focus due to its importance in estuarine hydrodynamics and material transport. By utilizing a well-calibrated numerical model, in this work we investigate the changes in stratification in the deepened and narrowed North Passage of the Changjiang Estuary. Before channel narrowing and deepening, lateral straining, generated by the interaction between vertical shear in lateral flow and transverse salinity gradient, is the dominant factor that controls stratification. A two-layer structure of the lateral flow strains the isopycnal transversely, resulting in rapid stratification from late flood to early ebb tide. Thus, maximum stratification occurs during the early ebb. Then, the stratification was suppressed by the vertical mixing and the less stratified water advected from upstream, even the vertical shear in along-channel flow continued to strain the isopycnal. After channel deepening and narrowing, the salinity in the upper water column experienced a sharp vertical gradient during the entire tidal cycle, while the transverse salinity gradient and lateral flow are profoundly reduced. The impact of lateral straining on stratification becomes minor. The enhanced stratification results in a sharp decrease in turbulent mixing within the pycnocline. The water movement in the upper layer is in a free-stream status and the tidal current speed increases significantly. The alteration of the vertical current structure enhances the along-channel tidal straining and stratification is most vigorous on late ebb tide.