Historic Flood - 2011

To date, the 2011 flood represents the largest flood of record for the Assiniboine River. The flood was caused by the following factors: above normal soil moisture at freeze-up (130% above normal), above normal winter snow pack (150% above normal), and above normal spring precipitation (200-300% above normal). Major storms in May and June within the Souris River Basin caused high flows and multiple peaks on the Souris River. A total of 16 hydrometric gauging stations reported record peak flows or water levels during the 2011 summer flood event.

Planning and preparation for the flood of 2011 began in November of 2010 when the unprecedented high flows on the Assiniboine River were two times greater than those in 1975, leading up to the flood 1976.

In mid-May, Provincial staff forecasted a peak flow of 1585 cms (56,000 cfs) on the Assiniboine River upstream of Portage La Prairie for unfavourable future weather conditions. This peak flow was well in excess of the maximum safe capacity of the Assiniboine River channel downstream of the Portage Diversion (510 cms (18,000 cfs)) plus the design capacity of the Portage Diversion (708 cms (25,000 cfs)).

In response to the forecasted flows, work was undertaken to increase the capacity of the Portage Diversion to approximately 960 cms (34,000 cfs). These works included fortifying the dikes, re-enforcing the channel drop structure walls, placing rip-rap around bridge crossings including rail bridge crossings and raising the levels of the dikes.

A contingency plan also had to be developed to handle any flow in excess of sum of the maximum capacity of the Assiniboine River dikes and the temporarily upgraded Portage Diversion. The selected plan involved a controlled breach of the Assiniboine River dikes at the Hoop and Holler bend. A controlled release at this point meant that anticipated excess flow could be diverted safely away from the Assiniboine River, southeast towards the Elm River which then flows east into the La Salle watershed/River and then empties into the Red River south of Winnipeg.

The maximum recorded daily inflow into the Portage Reservoir ended up being 1,504 cms (53,100 cfs) on May 14. The maximum recorded daily flow in the Portage Diversion was 983 cms (34,700 cfs). Flow through the controlled breach at the Hoop and Holler cut was maintained between 5.66 cms (200 cfs) and 11.3 cms (400 cfs). A total of 3.4 square kilometers (1.3 square miles) were inundated by floodwaters flowing through the controlled release.

Map of Diverted Flows from the Hoop and Holler Controlled Release Site, May 19, 2011 that affected 3.42 square kilometres. (Source: Manitoba Infrastructure)

In 2011, the Portage Diversion contributed a total of 5,870,000 cubic decametres (4,762,000 acre-feet) of water into Lake Manitoba. This contribution was the largest volume of water contributed to the lake by the Portage Diversion in the diversion’s history. As 2011 was also a year of unprecedented flooding in the Lake Manitoba Basin, this record contribution for the Portage Diversion resulted in 38% of the annual volume of water inflowing into the lake.

 

 

Major reinforcement of lower Assiniboine River dikes in early 2011	Hoop and Holler Cut, May 2011
Assiniboine River and Portage Diversion at Southport, May 2011	Portage Diversion at first drop structure, May 2011
Portage Diversion dike maintenance, May 2011	Portage Diversion second drop structure maintenance, May 2011
Helicopters deliver sandbags to Assiniboine River dike reinforcement. May 2011	Overland Flooding, Assiniboine River north of St. Eustache, May 2011
Long Lake Drain at Assiniboine River, May 2011	Lower Assiniboine, Overland Flooding, May 2011
Portage Diversion dike failure, May 2011	Portage Diversion, 3 km upstream of Lake Manitoba, May 2011
Assiniboine River at P.R. 240, May 2011	St. Francois Xavier May 2011
Assiniboine River, 9 km East of Poplar Point, May 2011	Caron Drive, St. Francois Xavier, May 2011
Assiniboine River at Headingley, May 2011	P.R. 26 Adjacent to Assiniboine River, May 2011