Current Status of My Thesis Work

1997 November 12

The Columbia River Entrance channel

My observations in the Columbia River entrance channel reveal a partial breakdown of the traditional inviscid model of two-layer exchange. An along-channel section of measured density shows internal hydraulic control at a lateral constriction in the channel (dotted vertical line in figure 2a). The pycnocline, whose finite thickness is not part of two-layer theory, is distinctly thinner at the constriction. Observed early flood currents, which are strongest at mid-depth show a greater departure from two-layer theory (figure 2b). Whereas the classical model suggests that flood currents should be strongest in the lower layer, observed flood currents migrate upward through the water column, due to a combination of interfacial mixing and bottom friction. The mixing thickens the pycnocline, while the friction retards bottom currents, so the fastest currents are seen at or above mid-depth.

Figure 2: Observed Along-channel Section of Density and Salinity
Full color plot

I developed a time dependent numerical model of three layer along-channel circulation in an estuarine channel, with the specific goal of recreating the observed vertical distribution of flood currents. Model development closely followed Helfrich's (1995) work on a rigid lid two layer exchange model. The model is driven by an imposed barotropic transport, which may be steady or vary tidally. The middle layer is assumed to grow due to turbulent entrainment from the other two layers, with a Richardson number dependent entrainment velocity borrowed from Ellison and Turner (1959). Here I show results of a model run using tidally-varying currents through a constricted channel, with significant vertical mixing and bottom friction. The pycnocline is thinner at the constriction (figure 3a); the model constriction is less sharp than the real constriction, which is reflected in the more gradual thinning of the pycnocline. The figure represents conditions at early flood, and the middle layer is moving faster, as observed (figure 3b). This landward jet is thinner than observed, because the layer model cannot represent both a thin pycnocline and a thick velocity layer. Due to an initial condition on the vertical shear, the seaward end of the surface layer is ebbing; this non-physical result will be addressed in later model tests.

Figure 3: Along channel section of density and velocity from a three-layer model


Ellison and Turner, 1959, Turbulent entrainment in stratified flows, Journal of Fluid Mechanics, 6, 423-448
Helfrich, 1995, Time-dependent two-layer hydraulic exchange flows, Journal of Physical Oceanography, 25, 359-373