Manitoba Infrastructure forecasts high risk of major spring flooding

Portage Diversion channel drop structure. Manitoba Infrastructure

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Manitoba Infrastructure Minister, Ron Schuler, announced today that there is a major risk of spring flooding in the province’s Red River Valley.

“Early forecasting data shows that we expect to see major flooding along the Red River, with both the Red River Floodway and the Portage Diversion pressed into service,” Schuler said. “March remains a crucial month in terms of snowmelt and weather conditions, and how that will affect the flood forecast going forward.”

According to Manitoba Infrastructure, with normal weather conditions, levels on the Red River south of the floodway are expected to be near 2011 levels. Unfavourable weather conditions, including late-season snow, spring rains and a more rapid snowmelt, would bring Red River levels comparable to those of 2009, which resulted in extended closures of PTH 75 and other roads, and even evacuations in some areas.

Based on long-term weather forecasts, below-normal temperatures are expected in March and April, further delaying snowmelt, with the risk of spring rains occurring at the same time.

Furthermore, ice is expected to be thicker than normal, which would increase the risk of ice-jam flooding. Manitoba Infrastructure is currently collecting ice thickness samples across Manitoba basins.

“Manitoba Infrastructure continues to gather data and work with counterparts in the United States and Saskatchewan to ensure timely information is available,” Schuler added. “We expect to update Manitobans with more information as updated forecast data becomes available.”

Manitoba’s major lakes are expected to remain within their respective operating ranges. There is a low risk of overland flooding in the Interlake, upper Assiniboine and Whiteshell lakes areas.

Flows on the Assiniboine and Souris rivers are also expected to be high. However, peak water levels will be below flood protection levels. Flows may produce some flooding of farmland and low-lying areas. The Shellmouth Dam will be operated to store a portion of the spring run-off, thereby reducing downstream river flows.

Further outlooks will be issued as updated forecast data becomes available. More information about flooding and how to prepare for an emergency situation is available at www.gov.mb.ca/flooding.

Portage Diversion background

The Portage Diversion is a 29 km long channel located just west of Portage la Prairie that diverts water from the Assiniboine River northward into Lake Manitoba. The Portage Diversion is one of the major flood control infrastructure components that were recommended in the 1958 Report of the Royal Commission on Flood Cost Benefit.

Construction of the Portage Diversion started in 1965, and was completed in 1970 for a total cost of $20.5 million. The Portage Diversion was first put into operation in the spring of 1970 and provides flood protection to the city of Winnipeg and to communities along the Lower Assiniboine River, which include the rural municipalities of Portage la Prairie, Cartier, St. Francois Xavier and Headingley.

The Portage Diversion project includes the following components:

-a 29 km (18 mile) long diversion channel
-embankment dikes adjacent to the diversion channel
-two drop structures within the channel and an outlet drop structure at Lake Manitoba to reduce flow velocities in the channel
-an inlet control structure at the entrance of the diversion channel to control flow entering into the diversion.
-a river control structure that acts as a dam and spillway along the Assiniboine River downstream of the diversion channel

The Portage Diversion was designed to convey up to 708 cms (25,000 cfs) from the Assiniboine River when the Portage Reservoir is at full supply level of 234.4 m (769.0 ft). The last five kilometers of the diversion channel passes through Delta Marsh and this portion of the diversion channel has a lower capacity of only 425 cms (15,000 cfs). Any flows in excess of 425 cms (15,000 cfs) spill laterally from the channel into Delta Marsh through a section of the west embankment.

According to Manitoba Infrastructure, operation of the Portage Diversion is guided by the following objectives:

-provide maximum benefits to the City of Winnipeg and areas along the Assiniboine River downstream of Portage la Prairie.
-minimize ice jams forming along the lower Assiniboine River.
-not to increase the water level in Lake Manitoba beyond the maximum regulated level of 247.763 m (812.87 feet), if possible.
-prevent overtopping of the failsafe section in the Portage Diversion, if possible.

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