| Abstract | Unlike in most other countries, rivers in Canada freeze during the colder months of the year. Hence, in addition to flooding events in open water, which are a challenge on their own, those involving ice introduce an additional set of issues which are also difficult to foresee and prepare against, given the acknowledged complexity of river ice dynamics. The purpose of this report is to examine the influence of surface ice on water levels in the St. Lawrence River, with the premise that an improved understanding of related phenomena would ultimately increase the reliability of the well-known stage-discharge relationships in winter conditions and provide additional insights into floods induced by ice. Fundamental to any endeavor aimed at monitoring water levels are basic principles in hydrology and hydraulics, including the various field methods used to gather these data – these are summarized. The basis for the relationship between stage and discharge is the Manning formulation. When ice is present, however, that relationship is no longer reliable. Instead, the presence of ice can lead to very high-water levels at relatively low discharge, which is caused by channel constriction by ice keels below the water surface.
Lake St. Louis, a river segment along the St. Lawrence Seaway next to Montreal, is the target area for a site-specific analysis presented in this report. Historical stage and discharge data for up to 50 years were downloaded from ECCC’s website – these were generated at two hydrological stations: Pointe-Claire and Lasalle. Indirect evidence of the influence of ice on stage is indicated by the difference in daily water level patterns: they fluctuate more in the winter than in the summer. Discharge, which is derived from water levels, is also more stable in the summer than in the winter (bearing in mind it is considered unreliable for icy conditions). A full-ice cover is associated with low water levels, which may be caused by non-uniform flow, leading to a variation of water depths in the flow direction observed between the two stations. This is consistent with observed discrepancies in water levels at the stations. Additional insights could be obtained with more detailed information about the ice cover and its dynamic behavior, as well as about discharge from the Ottawa River and that resulting from dam operations. |
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