Alberta under water: The 4 factors that led to massive flooding

A flooded downtown Calgary is seen from the air on Saturday, June 22, 2013. (Jonathan Hayward / THE CANADIAN PRESS)

A combination of several factors created “perfect” conditions for the massive flooding that continues to wreak havoc throughout southern Alberta, as hundreds wait to return to homes they were forced to flee. Here are some factors that contributed to one of the biggest disasters in Alberta’s history:

#1. Heavy rainfall

Extremely heavy rain began falling Wednesday. Some areas were pummeled by 200 millimetres of rain – more rainfall in a few hours than those places average in months.

Situated in the foothills west of Calgary, Kananaskis received more than three months’ worth of rain in a single day.

#2. Late snow melt

Adding to the mix are poor ground conditions due to a late snow melt, Canmore, Alberta hydrologist John Pomeroy told CTV News.

He said warm rain and moist air melted substantial snow packs located on high mountains, accelerating the flow of water in the rivers.

The melting is known as a “rain on snow” event, and is rarely seen in Alberta.

“They are much more typical down in the B.C. coast or down in the U.S. Pacific Northwest,” Pomeroy said.

Several days of thunderstorms further drenched the snow-covered soil, so the ground was completely saturated and could not soak up any more water to help stem the flow.

Once the water reached the rivers, levels rose dramatically, as did the flow. The Bow River spiked to 10 times its normal flow rate.

#3. Geography and infrastructure

Most major citiesare historically located along watersheds, which were once trade routes.  But these older locations – like New York and New Jersey during last year’s Superstorm Sandy -- are now at risk.

Dr. Brent Ward, a geology professor at Simon Fraser University, said the infrastructure in cities in Alberta are inadequate to handle increased construction in the floodplain.

“We can try to protect these structures with a series of levees or some sort of floodways, but eventually these things will fail.”

Ward acknowledged the solution isn’t simple.

“I think what you have to do is to start to restrict development in the floodplain,” he said. “I think cities have to think about the long-term cost of allowing increased development.”

Pomeroy said the infrastructure was built for a bygone era.

“The infrastructure we have was built for the 20^th century, and the more benign climate that we had then,” Pomeroy said.

#4. So did climate change play a role?

In the past 100 years, says Pomeroy, there has been “immeasurable change” in the type of rainfall that reaches the Prairies, with larger volumes of rain and less snow.

Pomeroy suggests climate change could have played a factor in the deluge and resulting floods.

“The rain themselves could not have been prevented, though I suspect they’re a manifestation of our changing climate,” he said.

With a report from CTV’s Daniele Hamamdjian