Internal tides in a partially mixed estuary

Muir, Langley Russell (1981) Internal tides in a partially mixed estuary. University of Southampton, Doctoral Thesis.

Abstract

In 1974, 1975 and 1977 measurements of temperature salinity and velocity were carried out on the Middle Estuary of the St. Lawrence River, in order to provide information on the primary physical processes controlling the circulation of this partially-mixed estuary. An examination of the T - S relationship shows that mixing is conservative over one tidal cycle, but that the properties of the end members change sufficiently rapidly that there is no generally applicable T - S relationship. The tidally averaged depth-density relationship may equally well be described by any one of three functions and the horizontal and vertical variability may be separated. The 44 current metre records show that the circulation cannot be explained by a simple theory of barotropic tides, but that baroclinic motions are also important. The presence of horizontal density gradients in the Estuary does not affect the horizontal propagation of the internal tides, but these horizontal density gradients are advected back and forth by the tidal currents. A set of prediction equations is developed which allows the computation of the instantaneous density and velocity profiles that would be measured by a fixed instrument in the estuary. The results of applying these equations to a hypothetical channel, somewhat like the St. Lawrence, are very like the density and velocity profiles which are observed in partially-mixed estuaries. It is not possible to ignore either the baroclinic tides or the horizontal density gradients and still get results that look like the observations. The inverse problem is not soluble in general since the nonlinear, transcendental equations do not have a unique solution. The interaction of all of the physical processes controlling the circulation of the Middle Estuary of the St. Lawrence is very complicated and discussed at some length.

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