Winnipeg Free Press - PRINT EDITION

Flood risk low to normal, for now

'Dry' snowpack reduces likelihood

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Old Man Winter might have southern Manitoba in a vise-like grip, but that hasn't stopped the province's flood experts from issuing a generally sunny forecast for rivers and lakes this spring.

With normal weather from here on out, there will be no need to close Highway 75 this year, and the flood risk will be low to moderate in most of southern Manitoba, they said Friday.

But there was still a question nagging at the province's flood-forecasting team as it provided its first outlook of 2014: What will happen if the frigid temperatures persist, followed by a rapid melt atop frozen soils?

The answer might be ice jams and frozen culverts that would trigger localized flooding.

Southern Manitoba is experiencing one of its coldest winters in decades, and the outlook for March is for colder-than-normal temperatures.

The frost extends to between a metre and a metre-and-a-half below the surface of the soil, said Steve Topping, the province's executive director of hydrologic forecasting and water management.

"What's interesting this year is the temperature of the soil is much colder than normal. It's -7 C, and generally it tends to be closer to -2 C," Topping said.

"(With) a fast runoff and strong ice, we could have a predominance of ice jamming in the province," he said.

 

-- How is the province dealing with the ice? Manitoba's fleet of Amphibex ice-cutters has already been out on the Red River to weaken the ice in a bid to prevent jams this spring. That work is continuing. Ice-jam mitigation will also be carried out on the Assiniboine, Icelandic, Brokenhead and Fisher rivers as well as at the Portage Diversion. The province also has 61 heavy-duty steamers to thaw culverts.

 

-- We've had a huge amount of of snow. Doesn't that pose a flood risk? You've heard of a dry cold? Well, we've had a "dry" snowpack this winter, said Infrastructure and Transportation Minister Steve Ashton. The fact the moisture content of the snow is relatively low lessens the chances of flooding.

 

-- Are there areas of concern? Parts of the Souris River basin and the areas around The Pas have above-average flood potential as the soil moisture content at freeze-up was high in these areas.

 

-- What's the state of our lakes and rivers? Several lakes are higher than normal for this time of year, including Lake St. Martin (3.5 feet above normal), Lake Winnipegosis (two feet) and Shoal Lake (4.2 feet). However, Lake Manitoba, at 811.8 feet above sea level, is within its normal range, while Lake Winnipeg is only slightly above normal elevation for this time of year. Flows along the Red and Souris rivers are generally normal or near normal for this time of year. Assiniboine River flows are slightly above normal due to efforts to draw down the Shellmouth Reservoir, which is not expected to overflow this spring, given average weather.

 

-- So there are no worries? The potential for flooding this spring still depends on the weather between now and the snowmelt. Will we get more than average precipitation? Will the thaw be greatly delayed? Who knows? "One of the challenges is, obviously, if we get any kind of rainfall while the ground is still frozen," said Ashton.

larry.kusch@freepress.mb.ca

Republished from the Winnipeg Free Press print edition March 1, 2014 A13

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