Soil, weather factors align to make 2013 flood costly

Last year’s flooding in southern Alberta may have been the most costly weather disaster in Canadian history, but it was not unprecedented.

For scientists observing the rainy days around June 20, it was an opportunity to watch what happens when rain, snow melt and frozen ground meet.

“It was not only a disaster but a scientific learning experience for us,” John Pomeroy told a water conference held in Calgary March 24-25.

Pomeroy, who is the Canada research chair in water resources and climate change at the University of Saskatchewan, happened to be doing research in Alberta’s Kananaskis Valley when the flood waters arrived.

He said conditions were colder and wetter than normal so alpine snow packs still held hundreds of milli-metres of water equivalent, and much of the soil was frozen or near saturation. Rainfall intensities of at least 250 mm fell throughout a large area over the course of several days.

The result was a flood that engulfed urban communities and caused $5 billion worth of damage.

Roger Drury, an engineer with TransAlta, said the eastern slopes of the Rocky Mountains have experienced 57 major floods in the last 100 years.

“We can definitely have larger floods than we did in 2013,” he said.

“2013 was definitely not the biggest flood on the Bow River.”

Back to back disasters have occurred in some years, such as 1915 and 1916 when both the Bow and Red Deer river basins were hit.

The flood of 1995 in the Oldman River basin was more intense with higher stream flows, but it did not cause the same amount of damage as last year.

Last year’s flooding was caused by a series of incidents in which warm weather in the Yukon and Gulf of Mexico collided over a colder southern Alberta.

Stuart Rood of the University of Lethbridge said these weather systems seem to happen every few years.

The situation is exacerbated because the Rockies in southern Alberta form a weather pocket that acts like a baseball glove capable of holding heavy rainstorms. The results are floods of varying intensity, he said.

“Most years the flood peaks are pretty modest, but these are occasionally interrupted by massive flows many times higher than the average,” he said.

A statistical decline in flood intensity is occurring because upstream dams are able to control what could be more serious events.

“This conclusion that floods are not intensifying is in contrast to a climate intensification theory, but probably doesn’t apply to our region,” he said.

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