Mixing processes in a highly stratified tidal flow

Mixing processes in estuaries remain a very important, but relatively poorly understood, mechanism affecting estuarine circulation. This is especially the case in highly stratified estuaries where mixing is localized in interfacial and near-bed regions. Currents in tidally forced highly stratified estuaries are probably the most difficult to model precisely because of problems associated with the parameterization of mixing. Lack of experimental data on these processes also contributes to this problem. In this study, measurements of turbulent dissipation rates, buoyancy, momentum, and sediment fluxes are conducted and analyzed to better understand turbulent mixing processes in highly stratified tidal flow. The site of the field study is the Columbia River estuary.; The theory of the spectral form of turbulent velocity fluctuations (Kolmogorov's theory) is used to estimate turbulent dissipation rates and a correlation technique is used to measure fluxes. The measurements are interpreted in terms of scaling arguments, and mixing efficiency is estimated. Mixing efficiency is found to be consistent with or slightly higher than measured in lower Reynolds number ocean and laboratory flows, but less than those measured in high Reynolds number tidal front turbulence. Next, an iteration technique is developed to estimate the along channel (horizontal) pressure gradient from data collected from the stationary vessel. This allows an examination of the vertical structure of the momentum balance, stress, and mixing rates during ebb mixing events. The turbulent kinetic energy dissipation rates estimated with the iterative technique compare favorably with measured values, offering mutual validation of the measurements and calculation.; Measurements of near-bed turbulence were made and analyzed to examine the effects of stratification and time-dependence on bed friction. Results suggest that stratification causes deviation of bed friction from neutral theory but time dependence do not, except around slack tide when acceleration is high. A few stress profiles were selected to apply a boundary layer mixing length turbulence closure with stratification correction, and compare with the use of an eddy viscosity model incorporating direct measurements of turbulent kinetic energy and turbulent kinetic energy dissipation rates. Results suggest that the stratified boundary mixing length closure does not adequately capture the effects of diffusion and advection. Finally, a calibration of the acoustic backscatter signal of the velocity sensor was used to estimate sediment fluxes. This novel use of the backscatter strength of the velocity sensor allows direct measurement of Reynolds sediment fluxes in a 1 $cm3$ sampling volume.