The 2011 flood event in the Assiniboine River Basin: causes, assessment and damages

Abstract

In spring and summer of 2011, the Assiniboine River and its tributaries underwent a flood of unprecedented proportions. It was, by several measures, including the computed return period and duration of flooding, a more extreme event than the 1997 Red River “Flood of the Century,” and may have been the most severe flood experienced in the history of Canada. The 2011 flood was the largest recorded in the over 100 years that flow records have been kept on the Assiniboine River. As reported in the 2013 Manitoba 2011 Flood Review Task Force Report, antecedent conditions, winter snowpack and summer rains combined to produce one of the largest floods ever experienced in Manitoba. The spring of 2011 was preceded by a very wet fall in 2010 when precipitation averaged between 150 and 200% of normal over a wide area in Manitoba and Saskatchewan This produced extremely high regional antecedent moisture conditions whereby soil moisture levels were 100 to 250% of the long-term average. Added to this, the winter snowpack was relatively high, varying geographically in the range of 90 to 130% of normal snow water content and lower-than-normal winter temperatures, causing significant frost penetration. The combination of high soil moisture in the previous summer/fall and deep frost penetration reduced the soil’s ability to absorb spring meltwater and increased runoff volumes contributing to the magnitude of the spring peak. The severe winter was followed by a series of rainfall events that resulted in unprecedented rainfall volumes in the region throughout the months of May, June and July, when some areas received rainfall amounts that were 350% of normal and produced up to two additional flood peaks at some locations. Flooding on the Assiniboine River near Brandon lasted for 120 days. Large volumes of floodwater were diverted to Lake Manitoba due to the use of the Portage Diversion to mitigate flooding downstream of the City of Portage La Prairie. This large volume of inflow resulted in a wind-effect-eliminated Lake Manitoba level that peaked at 249.1 m in late July of 2011, which was close to 1 m higher than flood stage and almost 1.5 m higher than the top of the desirable range. Peak outflow from Lake Manitoba into the Fairford River was almost double historic maxima and resulted in Lake St. Martin levels that peaked in July of 2011 at 245.55 m, which was more than 0.6 m higher than the historic peak that occurred under natural conditions in 1955, and 1.4 m above flood stage. The damage estimate for this flood event has not been finalized but is likely in excess of CAD $1 billion. The majority of flood damages were incurred by communities and infrastructure along major tributaries like the Souris and Qu’Appelle Rivers, and the communities on Lake Manitoba and Lake St. Martin. The Lake St. Martin First Nations community was virtually destroyed, and as of 2014 some First Nations people from the communities along Lake St. Martin are still evacuated and waiting for their homes and communities to be rebuilt. Temporary flood control works were largely effective in preventing infrastructure damage to communities along the Assiniboine River, such as Brandon, although the agricultural damage and damage to individual properties were extensive.