Circulation and mixing processes in the St. Lawrence Estuary are examined using both newly acquired and historical observations, as well as numerical modeling. In the Summer of 1997 a field experiment was undertaken to verify the prediction of a three-dimensional numerical model on the occurrences of shear instabilities within the upper estuary. The instabilities were found as predicted and documented from acoustic imaging, current profiler and density measurements. The instabilities, as in the case of Kelvin-Helmholtz instabilities, first develop in the form of wavelike disturbances before they break. The unstable waves have wavelength of around 140--150 m and a vertical extent of between 10 and 25 m. It is shown that the modeled gradient Richardson numbers near the pycnocline are reasonably well reproduced and appear to be useful for the prediction of flow instabilities in such a complex environment. / The seasonal variability of the general circulation and mixing is examined with a newly developed laterally averaged numerical model. The model takes into account tidal forcing and a realistic topography while forced with a large seasonal change in the river discharge. The model is able to reproduce important processes associated with the tidal forcing and mean conditions (density field, residual circulation). The numerical results show that mixing in the St. Lawrence Estuary is not evenly distributed. Mixing is found to be large near the sill at the head of the Laurentian Channel, as previously thought, as well our new results show that equivalent levels of mixing are also found near English Bank and near Ile-aux-Coudres. During the spring freshet, when the river discharge is 40% higher than normal, the limit of salt intrusion is moved seaward by approximately 12 km, the residual circulation is intensified by approximately 10%, and the turbulent vertical mass flux increases throughout owing to greater availability of freshwater. The results also suggest that during high runoff periods, the vertical stability of the water column increases in the upper estuary. / The model is also used to assess the impacts of climate change on the estuarine dynamics. (Abstract shortened by UMI.)
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.37871 |
| Date | January 2001 |
| Creators | Bourgault, Daniel. |
| Contributors | Lin, Charles A. (advisor) |
| Publisher | McGill University |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Format | application/pdf |
| Coverage | Doctor of Philosophy (Department of Atmospheric and Oceanic Sciences.) |
| Rights | All items in eScholarship@McGill are protected by copyright with all rights reserved unless otherwise indicated. |
| Relation | alephsysno: 001847858, proquestno: NQ75609, Theses scanned by UMI/ProQuest. |
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