EDMONTON – Everything lined up perfectly in southern Alberta this week for rampaging floods that not only astonished with their speed but also broke records expected to stand for a century.
“Large floods really occur because there’s a number of factors that have to line up,” said University of Alberta hydrologist Uldis Silins.
The table was set early this week when a slow-moving, wide-ranging weather system carrying lots of water moved over the eastern slopes of the Rocky Mountains west of Calgary. That system at first released a steady, soaking rain that took up much of the thin mountain soil’s capacity to absorb moisture.
About 40 millimetres fell between Tuesday and early Wednesday.
“The ground is already now starting to be quite wet at that point,” Silins said.
Then the heavens opened and dumped as much as 180 millimetres over 16 hours on already-sodden meadows. That’s about twice the rainfall much of the region usually gets during the entire month of June.
Almost all the water in rivers such as the Elbow normally comes from its source in the alpine peaks.
By Wednesday, normal didn’t apply.
“What happens is broader areas of the landscape that normally would not be contributing a lot of flow start to contribute flow,” said Silins. “Almost the entire area of the watershed starts to add to the flow of the river because the ground is completely soaked and all of the precipitation is effectively running off over the landscape and entering the streams.”
The impact was increased by the shape of the land.
“It’s a long, narrow watershed,” Silins said. “It is a fairly steep watershed. That means the water is going to be carried down valley fairly quickly.”
The result was stream flows that simply washed away previous records.
The record for the Elbow River near Bragg Creek — one of the communities left awash by the flood — had been 377 cubic metres per second. The peak flow during the 2005 flood, considered a once-in-a-century event in many parts of the watershed, was 308 cubic metres per second.
On Thursday, the Elbow at that same point peaked at at least 513 cubic metres per second.
“The monitoring station quit recording flows for about a four-hour period,” said Silins. “The river was still rising, so the peak flow was certainly quite a bit above.”
Other area rivers, such as the Crowsnest and the Sheep, almost certainly either matched their records or set new ones, he said.
The current flood overall will be “at least as big or a fair bit bigger (than 2005), depending on location.”
With water levels from 1995 and 2005 now washed away, Silins said scientists are starting to ask if heavy floods are becoming more frequent.
“It’s one that water scientists and resource managers are very concerned about. The broader issues of climate change and the projected intensification of weather patterns … is certainly a cause for concern.”
Scientists can’t definitively link extreme weather events to a changing climate. But many, said Silins, are trying to find out.
“It’s certainly an issue that’s of concern, particularly in the Canadian West, where we’re seeing what we think are some changes in weather patterns.”