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Major flood at border forecast

A lot of snow, slow melt blamed

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A major flood on the Red River at the Canada-U.S. border is all but certain this spring, based on the latest number-crunching by U.S. flood forecasters.

The U.S. National Weather Service's advanced hydrologic prediction service said Thursday there is a 95 per cent chance of major flooding at Pembina, N.D., at some point during the next two months. The prediction is based on conditions observed Monday.

"We're looking at a 2011-type flood," said Mike Lukes, a National Weather Service hydrologist based in Grand Forks. Although overshadowed by the Assiniboine River flood of 2011, the Red River flood during the same spring resulted in ring-dike closures at 15 Manitoba communities in the Red River Valley, the closure of Highway 75 south of Morris and the second-highest flood crest in Winnipeg since the Flood of the Century in 1997.

A flood of similar magnitude is probable this year despite dry weather in 2012. Heavy rains last fall saturated the top level of the soil, which froze solid early in the winter, Lukes said. The heavy snowpack on top of this crust and the delayed thaw have combined to increase the flood threat. The snowmelt, usually underway by now, has yet to commence.

"There's a topsoil layer of wet, frozen ground. In order for the meltwater to penetrate that, it has to be melted or thawed," Lukes said. "The delayed thaw puts us into the territory where rapid melts could happen, with the possibility of rain on top of that."

Manitoba flood forecasters issued similar opinions earlier this week. They are now incorporating the U.S. data into Manitoba's next flood outlook, due next week, a provincial spokesman said.

The National Weather Service also predicted a 50 per cent chance of major flooding at Grand Forks and a 98 per cent chance of major flooding at Fargo, where 750,000 sandbags have already been filled in preparation for a record Red River crest.

But what happens at Pembina is far more relevant for the Manitoba flood outlook. That's partly because the Red has a much smaller capacity at Fargo, but it's mainly because Pembina sits right at the 49th parallel, downstream from two of the Red's largest tributaries -- the Red Lake River, which enters the Red at Grand Forks, and the Pembina River, which drains a portion of south-central Manitoba before flowing into the Red in North Dakota.

In Manitoba, ring-dike closures and a Highway 75 closure are possible again this year, provincial officials said. A 2011-scale Red River flood would also trigger precautionary sandbagging of low-lying Winnipeg properties, although the city, like the province, is only data-crunching at this point.

What is clear is Manitoba's stretch of the Red River Valley is heading into its third major flood in five springs. Although climate-change models predict more extreme weather for mid-continental areas such as the Canadian Prairies, hydrologists say it's too soon to attribute the recent string of very large floods to human-induced changes to long-term precipitation and temperature patterns.

"Floods do come in bunches. This still could be a random grouping," said University of Winnipeg geography Prof. Bill Rannie. "Whether this is a cluster in an otherwise trendless situation or (the result of climate change), I don't know. If I knew and had some way of proving it, I would be writing about it."

Southern Manitoba has a highly variable climate in terms of temperature and precipitation. A prolonged period of cool, sunny weather could vastly reduce the severity of the Red River flood this spring. A massive blizzard in April could put the Red River Valley in a position similar to that of 1997, but that prospect is remote.

It's not clear yet what will happen this spring along the Assiniboine River, although some flooding is expected. The province has raised the possibility of protecting First Street and 18th Street in Brandon with a row of super-sandbags, a provincial spokesman said.

Recent floods

Red River crests in Winnipeg during the past two decades, expressed in feet above normal winter ice level at the James Avenue monitoring station (with crest dates in parentheses):

1995 (March 28): 17.9 feet

1996 (April 28): 19.4 feet

1997 (May 3): 24.5 feet

1998 (March 28) 17.2 feet

1999 (April 10): 17.4 feet

2000* (July 8-9): 15.6 feet

2001 (April 7): 18.5 feet

2002* (June 15): 17.3 feet

2003: No flooding

2004 (April 4): 19.0 feet

2005* (July 3-4): 20.2 feet

2006 (April 7): 20.4 feet

2007 (April 11): 17.9 feet

2008: No flooding

2009 (April 16): 22.6 feet

2010: No flooding

2011 (April 9): 20.7 feet

2012: No flooding

Normal summer level: 6.5 feet

Assiniboine Riverwalk: 8.5 feet

* Flood due to summer rain, not spring snowmelt


-- source: City of Winnipeg

Republished from the Winnipeg Free Press print edition March 22, 2013 A3

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