Winnipeg Free Press - PRINT EDITION

'We don't want too rapid a melt'

Freezing night temperatures are needed, Ashton says

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OUR winter that refuses to die?

If you live in the flood zone, hope that it dies a slow death, a slow, slow, really slow death.

Otherwise much of southern Manitoba and parts of northern Manitoba could see an increased risk of moderate to major spring flooding, mostly because of the heavy snowpack that sits unbudged on the Prairie region, if temperatures rise too fast and it melts too quickly or worse, if it rains, too.

"I hate to say this, but most Manitobans over the next couple of weeks, if you look at the flood situation, you should actually be hoping that it goes below freezing or close to freezing overnight," Infrastructure and Transportation Minister Steve Ashton said Wednesday during the province's third flood-outlook briefing of the spring.

"We don't want too rapid a melt."

A slow, gradual snow melt allows the ground to thaw during the day and absorb that moisture rather than it melting too quickly and cascading over frozen ground into streams and rivers.

"That will dissipate the snowpack," Steve Topping, the province's executive director of hydrologic forecasting and water management, said of a slow melt.

That's the kind of weather Environment Canada is forecasting, at least for the next week. Temperatures are expected to climb above freezing during the day and go back to below-freezing temperatures at night. However, rain or flurries are anticipated early next week.

"When we get three days in a row that evening and night temperatures are above freezing, that's when the runoff gets going based on my experience," Topping said, adding this year's runoff is expected to start early next week.

Based on the delayed melt, and more snow in Saskatchewan, Manitoba flood fighters have revised their spring flood forecast, but only slightly. They're now calling for moderate to major flooding for the Assiniboine and Souris rivers. The revised update says with unfavourable conditions, the towns of Melita and Souris will need additional flood protection.

There could also be major flooding through the Assiniboine Valley to Brandon with unfavourable weather conditions, although it's expected to be at levels much lower than those of 2011.

The Portage Diversion will also be used this spring, but nothing like it was in 2011 when Lake Manitoba flooded, the province said.

The diversion will be needed to manage ice jams on the Assiniboine River east of Portage and to provide flood protection to Winnipeg. The flow range is expected to be between 9,400 cubic feet per second (cfs) and 18,500 cfs, depending on weather conditions.

The flow range was 35,000 cfs during the height of the 2011 flood.

To the west, the Shellmouth Reservoir has already been drawn down to one foot below what it was going into the 2011 flood on the Assiniboine River to retain what's coming from Saskatchewan.

Plus, based on information from American forecasters, the province said the revised outlook indicates the flood risk has increased to moderate to major from minor to moderate for the Roseau and Pembina rivers.

The flood risk hasn't changed dramatically for the Red River Valley. Along the Red River, the risk remains for a moderate-to-major.

Flood forecasters are still calling for a flood on par to what Manitobans experienced in 2009 when ice jams along the Red were major factors worsening the flood fight. However, in 2009, the ice on the Red broke up in the last week of March.

Topping said ice-breakers have smashed 29 kilometres of river ice from Selkirk to Lake Winnipeg to aid in the breakup.

On the Red south of Winnipeg, the province says Highway 75 will be closed just as it was in 2009. The main route south to the United States was closed for 36 days in 2009 and 44 days in 1997. No Red River Valley communities are expected to lose road access and no Manitoba communities face evacuation at this point.

The detailed flood outlook is available on the province's website.

bruce.owen@freepress.mb.ca

Republished from the Winnipeg Free Press print edition April 11, 2013 A3

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