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Assiniboine's assault surprises experts

Peak flows hugely underestimated

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A week can seem like an eternity when you're fighting a flood.

Just seven days ago today, the Red and Assiniboine rivers crested simultaneously in Winnipeg and it was a ho-hum event.

At that day's provincial flood briefing, a government staffer had even taken the time to calculate that the river flow at James Avenue was great enough to fill an Olympic-size swimming pool every two seconds.

Sure, provincial flood fighters were concerned about the impact of the large April 30 rain/snowstorm in western Manitoba. But panic? Nah.

Fast-forward to the frenzy now gripping the province from Brandon all along the Assiniboine to Headingley.

The Armed Forces have been called in to fight a flood in Manitoba for the first time since 1997's Flood of the Century. The Portage Diversion channel is getting an emergency upgrade. And, most dramatically, the province is prepared to risk flooding 150 homes by cutting an outlet in the Assiniboine near Portage la Prairie to prevent a greater catastrophe.

So how did we get to this point?

The province has never taken flooding along the Assiniboine for granted. Just the opposite. It spent millions of dollars this spring propping up dikes between Portage and Headingley and increasing Brandon's flood fortifications.

But it clearly never anticipated a flood of this magnitude.

To illustrate, just a week ago, it released a "revised" crest for the Assiniboine River at Brandon that accounted for the late-April storm. The new prediction: Flows through Brandon would peak at about 26,000 cubic feet per second within a week.

Then the forecast quickly took a scary turn.

On Friday, provincial flood officials projected a crest of 32,000 cfs for Brandon for May 12-14. They blamed a faulty gauge operated by federal officials on the Qu'Appelle River at Welby, Sask., for the big volume jump.

But by late Sunday, Manitoba's second city was already experiencing flows of 38,000 cfs.

Our flood experts never anticipated anything of that magnitude, nor did they predict anywhere near the peak flows west of Portage (53,000 to 56,000 cfs.) anticipated for Saturday.

Why not?

Part of it may be a lack of experience. Though provincial officials have become adept, through long practice, of predicting Red River floods, they have far less experience in assessing flows along the Assiniboine.

Jay Doering, dean of graduate studies at the University of Manitoba and a civil engineering professor, points out that the last big Assiniboine flood was more than a generation ago, in 1976.

"I believe a lot of it is just the fact that we're in uncharted territory," he said Tuesday. "If we had had more floods that were similar to this, the warning bells would have gone off earlier."

Instead, 35 years after the river's last big flood, the Assiniboine is experiencing a one-in-300-year event.

Doering thinks that problems with gauges and forecasting both contributed to the gross underestimation.

The federal Water Survey of Canada took issue with Manitoba's suggestion that its gauge at Welby was malfunctioning. But Doering said flood data he's looked at suggest there were issues in interpreting the data from that gauge as well as one downstream along the Assiniboine River at Miniota. (A third gauge he assessed, near Brandon, appeared not to be functioning at all.)

The problem, he said, is that there is no model to calculate flow rates based on the unprecedented river depths the gauges were measuring in western Manitoba.

Given the unprecedented nature of this year's Assiniboine flood, it's difficult to assign blame for the miscalculations, Doering concludes.

"I don't know that these events are necessarily foreseeable. It always takes a close call like this to get the message that we need to be doing more," he said.

That should include a permanent expansion of the Portage Diversion, maybe more water gauges out west and improved gauge-monitoring, he said.

He also said Manitobans shouldn't expect that this year's Assiniboine River flood will be an isolated incident.

"What I think you're going to see post-2011 is the same amount of attention paid to the Assiniboine watershed (as to the Red)," he said.

Republished from the Winnipeg Free Press print edition May 11, 2011 A6

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