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Three years later: What caused the 2013 Alberta floods?

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Scott Sutherland
Meteorologist/Science Writer

Tuesday, June 21, 2016, 5:46 PM - Southern Alberta made headlines in June 2013, as all of Canada watched communities there deal first with torrential rains, then rising floodwaters and mass evacuations. Looking back, three years later, what made the region so vulnerable to flooding?

From June 19 to 22, 2013, a deluge of rainfall across the eastern slopes of the Rocky Mountains coursed through streams and rivers, resulting in extensive flooding across southern Alberta. 

Four people drowned in the floods. 100,000 residents were driven from their homes by rising waters. Up until the 2016 Fort McMurray wildfire, this was considered to be the costliest natural disaster Canada had ever seen, with an estimated total cost of property damage around $5 billion.

Geography, Hydrology and Meteorology

What caused this devastating flooding? It was a combination of the geography of the region, the hydrology of the local soils, and the extreme weather conditions that set up over the region over those three days.

As the region is part of the Great Plains, just like the Prairie provinces to the east, much of southern and southeastern Alberta is quite flat terrain. However, unlike the Prairie provinces, the relatively flat terrain of Alberta has a very gradual upward slope towards the west, before rising more sharply into the foothills and the Rocky Mountains. As we'll see below, this slope played a very important role in the event.

When it comes to a flooding event, it's not just about how much rain falls on a region, but how much water the local soils are capable of absorbing, and how much the local waterways can effectively channel. If heavy rains follow a reasonable dry spell, the soils will have a large capacity to absorb that incoming water and streams and rivers will have plenty of capacity as well. This will tend to limit the potential for flooding. If the soil is already saturated or nearly saturated, however, and waterways already swollen, the fresh influx of rainwater has nowhere to go except to flow over land and further swell streams and rivers until they overflow their banks. For southern Alberta, in June of 2013, the region was very much in the latter situation. Heavy snowfall throughout the fall, winter and much of spring had left a deep snowpack on the mountains, and wet weather through the spring had kept the ground fairly saturated leading up to this event. This left the area vulnerable for what was to come.

Adding to the problem was the meandering course that many of Alberta's rivers follow, as time and water flow have carved many sharp turns and curves into these rivers and streams. While normal flow will simply continue to shift the position of these meanders, as the flow rate increases, it becomes more likely that the waters will crest the banks and overflow the land in between.

It was the weather that played the most important role, though, as the jet stream - that meandering ribbon of strong winds, high above the ground - set up in pattern over western Canada that produced a worst-case scenario.

According to Weather Network Chief Meteorologist Chris Scott:

"Sometimes, the jet stream pattern gets stuck or ‘blocked’ as we say in meteorology. This is the setup that took shape across western North America [at the time]. The large 'up' or ridge in the jet stream, which caused record warmth in Alaska and hot weather across parts of Canada’s north, blocked a strong dip in the jet stream from moving quickly from west to east. In the image below, the 'H' represents High pressure, or the up in the jet stream, and the 'L' represents Low pressure, or the down in the jet stream."

Courtesy: College of DuPage Weather Lab, with edits by C. Scott

With the winds blowing from east to west, driving up that fairly gentle slope and then encountering the foothills and Rockies, it produced a conveyor-belt of gradually-cooling air that only needed a moisture source to result in a significant rainfall event.

As Chris Scott points out, this setup tapped into one of the most potent moisture sources there is for North American weather:

"With the counter-clockwise winds around this low pressure system, a channel of very moist air was tapped from the Gulf of Mexico and pulled into Alberta. Like a fire hose, this atmospheric river of water vapour blasted up against the foothills and Rocky Mountains, rising, cooling, condensing and releasing tremendous amounts of precipitation."

Courtesy: College of DuPage Weather Lab, with edits by C. Scott

According to Environment Canada, 68 mm of rain fell in Calgary over 48 hours, but it was the rainfall up in the foothills and across the eastern slopes of the Rockies that really set off the intense flooding that followed. The rainfall rates there were up to four times higher than what is considered "high" in EC's official records - 10 to 20 mm/hr at some stations, and averaging out at from 75 to 150 mm total rainfall for the duration of the storm. Two particularly strong focal points of the storm were in the region of Canmore, which saw around 200 mm of rain, and Burns Creek, where an incredible 345 mm fell.

Total rainfall from June 19-22 across southern Alberta. Credit: Alberta Ministry of Environment and Parks

In Canmore, the intense rainfall fell not only over the town, but onto the slopes of a large granite "bowl" just to the east of the town, which is ringed by five mountains - Mt. Lady McDonald, Mount Charles Stewart SE2, Mount Townsend, Mount Fable and Grotto Mountain. With the rainwater flowing down into the basin between these peaks, it had only one exit point, Cougar Creek, which runs right through the southern part of the town. As all of that concentrated water flowed straight down the creek, it tore apart the creek's banks, destroyed homes and flooded a significant portion of the town, before pouring into the Bow River, on its way to Calgary and other townships to the southeast.

A perspective, terrain view of the region around Canmore, AB. Credit: Google Earth. Labels added by S. Sutherland

The rainfall that fell over Burns Creek fed directly into several different waterways that flowed down to the east and south, following the down-slope terrain out of the mountains. This added to the swollen flow of the Bow River northwest of Calgary, and the Elbow River to the west of the city, as well as the Sheep and Highwood Rivers, which join up to the Bow River south of Calgary.

High River, a town along the banks of the Highwood River, about 40 kilometres south of Calgary, suffered some of the worst flooding of the event, with water levels rising above car roofs and trapping several residents atop their houses. The entire town was evacuated during the crisis.

Although the flooding in downtown Calgary and in the town of High River captured most of the media attention in 2013, the Old Man River, running through southern Alberta, drew from that Burns Creek rainfall as well, which impacted on several communities along its banks and further along, after the river joins up with the Bow River to form the South Saskatchewan River.

Two specific examples, Lethbridge, on the Old Man River, and Medicine Hat, on the South Saskatchewan River, were inundated by flood waters, as shown in the videos below:

The flooding in 2013 was considered to be a "1 in 100 year" event. These 100-year floods have roughly a 1 per cent chance to happen any particular year, showing just how rare they are. Comparisons for the event only had to reach back to 1995, however, and in some cases the 1995 event was worse.

In Lethbridge, in 2013, the Oldman River peaked at a flow rate of over 2,600 cubic metres per second. For some scale, in just one minute, that amount of water could fill over 150 million 1-litre bottles. However, in early June 1995, the river peaked at a much higher rate - over 4,600 m3/s. According to researchers at the University of Lethbridge, those 1995 rates were the highest flood levels reported since 1908.

For Medicine Hat, 2013 was slightly worse than what they saw in 1995 (flow of over 5,400 m3/s in 2013 vs 5,100 m3/s in 1995), since the city straddles the South Saskatchewan River, and thus had the combined flow from both the Oldman River and the Bow River. Both years work out to be fairly comparable.

Given the risks associated with global warming and climate change, these 1 in 100 year events are expected to become more frequent - perhaps 1 in 50 years or worse - and with these comparisons between 1995 and 2013, we may already be seeing this trend in action.

Sources: College of DuPage Weather Lab | Alberta Ministry of Environment and Parks | Environment Canada | Google | With files from The Weather Network

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