Mixing processes and hydraulic control in a highly stratified estuary

Abstract

This thesis utilizes field data from the Fraser River Estuary, a highly stratified system located in southwestern British Columbia, Canada, to investigate the nature of mixing processes in a highly stratified environment, and to extend two-dimensional hydraulic theory to a three dimensional environment.During the late ebb, a stationary front exists at the Fraser mouth. Although densimetric Froude numbers in the vicinity of the front are supercritical in a frame of reference parallel to the local streamlines, the front itself is oriented such that the value of the Froude number is equal to the critical value of unity when taken in a frame of reference perpendicular to the front. This observation presents a robust extension of established two-dimensional, two-layer hydraulic theory to three dimensions, and implies similarity with trans-sonic flows, in that a Froude angle can be used to identify critical conditions in a manner similar to the Mach angle.Mixing processes were evaluated at the mouth during the late ebb using a control volume approach to isolate mean vertical entrainment processes from turbulent processes, and quantify the vertical turbulent salt and momentum fluxes. Observed turbulent dissipation rates are high, on the order of 10 -3 m 2 s -3 , with vertical entrainment velocities on the order In the estuarine channel, the variability of mixing processes was investigated through the tidal cycle 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. Mixing is active and important throughout the tidal cycle, and was found to be the dominant process responsible for removing salt from the estuarine channel during the ebb. AcknowledgementsThis work could never have been accomplished without assistance and encouragement from a large number of people. It has truly been a pleasure to work with my advisor, Rocky Geyer, over the last four years. His enthusiasm for his work has been inspiring and I have learned a lot from him. I also appreciate the timely advice and input I have received from the other members of my committee, John Trowbridge, Eric Adams, and Ole Madsen, and I would like to thank Jim Ledwell for chairing my defense.The Fraser River cruises would not have been possible without help from David Jay, Philip Orton and Doug Wilson. The crew of the R/V Clifford A. Barnes, Ray McQuin and Nikki Hix, was fantastic to work with. Alex Horner was a tremendous help during both cruises, particularly with the collection of the data described in Chapter 4, and its subsequent analysis. Here at WHOI, Terry Donoghue and Jim Irish provided a tremendous amount of assistance in preparation for the two cruises.The Coastal and Estuarine GFD course at Friday Harbor Labs during the summer of 1999 was a great experience and provided me with a good foundation for the research presented here. I'd like to thank...

Turbulent energy production and entrainment at a highly stratified estuarine front

“…It is the flux in this lower region that supplies most of the salt to the lift-off zone [MacDonald, 2003] and is most likely to influence the salt balance and thus impact the turbulence calculations.…”

“…A separate analysis of data from the 2000 freshet, as discussed by MacDonald [2003], has suggested that the flow in this region of the channel is nearly two-dimensional, particularly above the 20 psu isohaline, based on a three-dimensional salt balance and the relative contribution of lateral (southward-directed) salt flux to the overall salt budget in the region. However, due to the complexities of the interactions between entrainment and turbulence in generating the vertical transport, there are multiple pathways through which lateral fluxes may impact the calculations.…”

Role of mixing in the structure and evolution of a buoyant discharge plume

“…Certainly, the scales of river plumes are typically large enough that the effect of the Earth's rotation becomes important in the far field [e.g., Fong, 1998]. In the near field, most river plumes are narrow enough that Coriolis effects are negligible [e.g., Wright and Coleman, 1971;MacDonald, 2003], and in this region the most useful comparisons between industrial plumes and geophysical plumes can be drawn.…”

Relating River Plume Structure to Vertical Mixing

“…Here, the estuary mouth acts as a hydraulic constriction for the upper layer (Armi and Farmer 1986). The accelerating flow soon becomes unstable, and strong shear mixing occurs in the near field (Wright and Coleman 1971;MacDonald 2003). Beyond this region, mixing is primarily caused by wind stress through a mechanism described by Fong and Geyer (2001).…”