Numerical model study on ice impact on Lake Superior outflow limit

Improved regulation of the wintertime flow from Lake Superior is needed to improve the balance of water levels of Lake Superior and Lake Michigan-Huron to decrease the frequency of extreme levels without unduly affecting Lake Superior interest. The wintertime outflow limit is set as 2410 m(3)/s by Lake Superior Regulation Plan 1977-A as a result of ice jam flooding during the 1916-1917 winter. This paper presents a numerical model study on the ice conditions in the St. Marys River to assess the maximum allowable Lake Superior wintertime outflow. Freeze-up, frazil transport and accumulation, and breakup were simulated with a thermal-ice dynamic model. The highest potential for flooding exists during ice breakup and simulations were run to determine a safe discharge limit for the breakup period. Simulations indicated the winter flow limit may be increased to 2690 m(3)/s if flow regulation is managed with care to prevent premature ice cover breakup.