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2D Finite Element Modeling of Red River Floods
Ninel Gonzalez, 1999

The flood protection works (Shellmouth Reservoir, Portage Diversion and the Red River Floodway) were designed to protect the City of Winnipeg from a flood event with a peak discharge of 169,000 cfs. Control of Red River flows through Winnipeg is provided by forcing Red River waters to flow into the floodway channel. Records show that the peak discharge of a flood event (in Winnipeg) has always been due to a combination of high flows from the Assiniboine and Red Rivers. However, Red River flows have been historically higher (greater than 50%) than the contribution made by the Assiniboine River. The Red River Floodway is the component of the flood control works system with the greatest capacity and the one offering immediate flood relief to the City from the Red River floodwaters.

After the flood of 1997 the adequacy of the floodway to provide protection against a larger flood event became questionable. The role the floodway plays in providing flood protection to the City of Winnipeg cannot be emphasized enough. The safety of the City depends on the adequate and reliable performance of this channel.

A numerical model of the floodway channel has been established using the one-dimensional HEC-RAS and the two-dimensional FESWMS programs. The models were calibrated using water level measurements collected during the 1997 flood. The calibrated models were then used to evaluate the performance of the floodway channel under high flow conditions. Of particular interest were the flow corresponding to the maximum design discharge of the channel (Q = 100,000 cfs) and the flow associated with an 1826 flood event. The results determined the current maximum capacity of the channel and identified spill locations along the east and west dykes. The existing floodway channel does not have the capacity to convey the maximum design discharge. Alternatives to confine the flow within the channel and increase its capacity were also examined. These include raising of the dykes and use of composite cross sections throughout the channel. Results indicated that although some of these alternatives help confine the flow within the channel, they create higher water levels throughout, in particular at the inlet. The removal of bridges is an option that helps reduce water levels. However, the freeboard provided under such conditions is not considered to be adequate and therefore the possibility of dyke overtopping due to wind set up exists.

It is recommended that the design of the floodway channel be reconsidered in order to accommodate for higher flows and provide adequate flood protection to the City without sacrificing the areas upstream of the floodway.

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