Modelling ice cover formation of a lake–river system with exceptionally high flows (Lake St. Martin and Dauphin River, Manitoba)

In 2011, Manitoba experienced extreme flooding on a wide extended scale that affected many large watersheds, in particular the Upper Assiniboine, QuʹAppelle and Souris River watersheds. In order to protect Winnipeg from excessive flooding, some of the flood waters were diverted via the Portage Diversion into Lake Manitoba and Lake St. Martin, causing extreme high water levels and flooding of these lakes. The outlet of this entire drainage basin is the Dauphin River flowing from Lake St. Martin to Lake Winnipeg. The riverʹs conveyance capacity was exceeded by the flood and the lakes could not be drained fast enough to avoid damage from high flood levels and shore line erosion and property damage from accentuated wind setup (seiche) and wave uprush (runup) of the lake water. An Emergency Channel was constructed to compensate for this constriction, which now conveys additional water from Lake St. Martin towards Lake Winnipeg. the extreme flooding, the Dauphin River had unprecedented record flows in the autumn of 2011. Ice cover formation during winter freeze-up of these waters could potentially exacerbate the flooding situation for communities alongside Lake St. Martin and the Dauphin River. Hence, the river ice model RIVICE was implemented to determine flood protection elevations to which existing dikes needed to be raised and extended. The modelling takes into consideration the potential frazil ice generation that can raise water levels by up to 3 m from open water levels. High discharge and water level data was available to calibrate the model both for an open-water and an ice-covered case. Three scenarios were then simulated to determine ice-covered water levels of the river for early, best-estimate and late freeze-up events to determine flood protection levels along the Dauphin River and Lake St. Martin. The model was successfully validated using data collected during a freeze-up event in December 2011.