The interaction between stratification, circulation, and sediment transport in a partially-mixed estuary

Detailed field observations from the York River estuary, Virginia are used to examine the processes governing vertical density stratification and to evaluate the importance of spatial and temporal variations in turbulent mixing on estuarine dynamics and sediment transport. Contrary to previous findings that suggest wind stress acts predominantly as a source of energy to mix away stratification, this study demonstrates that the wind can play a more important role in "straining" the along-channel estuarine density gradient. As a result, down-estuary winds enhance the tidally-averaged vertical shear, which interacts with the along-channel density gradient to increase stratification. Conversely, up-estuary winds tend to reduce, or even reverse the vertical shear, reducing stratification. While wind straining can play a dominant role in governing the overall degree of turbulent mixing at sub-tidal time scales, tidal straining of the along-channel density gradient can result in asymmetries in turbulent mixing at the tidal time scale. In estuarine systems with channel-shoal morphologies, tidal straining can lead to asymmetries in turbulent mixing near the deeper channel while the neighboring shoals remain relatively well-mixed. These temporal and spatial variations in turbulent mixing result in a barotropically-induced estuarine residual flow that favors inflow over the shoal regions and outflow over the channel. This pattern of residual circulation can offset, or even reverse, the pattern of residual circulation typically associated with baroclinic estuarine circulation. These tidal asymmetries in mixing have the opposite influence on the patterns of sediment flux. The higher values of eddy viscosity that occur during the less-stratified flood tide resuspend sediment higher in the water column, favoring up-estuary pumping. The presence of strong density stratification significantly damps turbulence in the upper water column, and the lateral dynamical balance is largely geostrophic at tidal time scales. Even though friction does not contribute at lowest order to the lateral balance, the lateral circulation is frictionally-driven by Ekman transport in the bottom boundary layer. The interaction of the lateral circulation and the stratification acts to limit the strength of the lateral circulation and as a result, significantly stronger lateral circulation occurs during less stratified conditions.