Mixing Processes and Hydraulic Control in a Highly Stratified Estuary

Daniel MacDonald, Ph.D., 2003
W. Rockwell Geyer, Advisor

This thesis utilizes field data from the Fraser River Estuary (British Columbia, Canada) to investigate the nature of mixing processes in a highly stratified environment. During the late ebb, a stationary front exists at the Fraser mouth. Although streamwise densimetric Froude numbers at the front are supercritical, the front is oriented such that Froude numbers are equal to one in a reference frame perpendicular to the front. This represents a robust extension of established two-layer hydraulic theory to three dimensions, and implies similarity with trans-sonic flows. Mixing processes were evaluated at the mouth using a control volume approach to isolate mean vertical entrainment processes from turbulent processes. Observed turbulent dissipation is on the order of 10-3 m2s-3. Flux Richardson numbers are confined between 0.15 to 0.2, with gradient Richardson numbers between 0.2 and 0.25. These results are consistent with previous laboratory studies, but represent energetic conditions several orders of magnitude higher. In the estuarine channel, mixing variability was investigated using control volume and overturn scale methods. Spatially, mixing was observed to be more intense near a width constriction on the order of 25%. Temporally, more dominant mixing was observed during ebbs, due to increases in both vertical shear and stratification.