Turbulence Dynamics at the Shoal–Channel Interface in a Partially Stratified Estuary

Turbulence observations at the shoal–channel interface in South San Francisco Bay are described and analyzed in this work. Profiles of turbulent kinetic energy (TKE) shear production and dissipation rate are estimated from ADCP beam velocities over a nine-day period during winter 2009 when the estuary was partially stratified. The TKE shear production profiles are consistent with a tidally driven bottom boundary layer, except late in the ebb when shear production intensifies and the region of maximum production is located in the upper half of the water column. A simple scaling for the depth-averaged TKE shear production based on bed friction performs well except during this late ebb period when it fails to predict the peak in shear production, which suggests that a mechanism other than bottom drag drives turbulence dynamics during this period. The profiles of TKE dissipation rates also display a late ebb peak. A lateral circulation develops at the shoal–channel interface during this late ebb period, and its effects on turbulence dynamics are investigated. An analytical framework is developed to identify and quantify these effects. Four mechanisms through which the lateral circulation can impact stability are identified, and two are found to be important during the late ebb period in South San Francisco Bay: the straining of the lateral density and velocity gradients in the lateral circulation. These results suggest that the lateral circulation acts to reduce the gradient Richardson number and is driving the late ebb peak in turbulence production and dissipation.