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Canada's top ten weather stories of 2013

1. Alberta's Flood of Floods

Figure 1a. Map of Canada, highlighting Southern Alberta. Click to see large map.

Alberta’s super flood of 2013 washed across one-quarter of the province and through the heart of Calgary – the fourth largest city in Canada.The disruptive flood cut off dozens of communities throughout the province and prompted the largest evacuation across Canada in more than 60 years with up to 100,000 Albertans told to leave their homes.

It was also Canada’s costliest natural disaster – more expensive than eastern Canada’s 1998 ice storm. Economists project damage losses and recovery costs from the flood to exceed $6 billion, including a record $2 billion in insured losses. In its wake, the flood caused unbelievable infrastructure losses from 1,000 km of destroyed roads and hundreds of washed-away bridges and culverts. Among insured losses were thousands of cars and homes demolished and damaged by backed-up sewers and small rivulets that exploded into raging torrents.

Image 1a. Alberta flood waters several feet high.

Southern Alberta is no stranger to flooding, especially in June – typically the wettest month of the year and a time when mountain snowmelt begins to appear on the Prairies. This year’s super flood, which extended from Canmore to Calgary and beyond, was exacerbated by several antecedent hydrometeorological events in the headwaters of the Bow River watershed. To begin with, it began snowing in southern Alberta before Thanksgiving 2012 and didn’t stop until a month after Easter. The mountain snowpack in May was immense, over one metre in places. Further, the spring was wet leaving the ground saturated and streams and rivers bloated. Calgary and some foothill weather stations had greater rainfall amounts between May 23 and 24 than those experienced during the flood a month later. At Livingstone, 96 mm of rain fell on May 25. And a brief warm-up that month started melting the nearly one-metre deep snowpack at the treeline. Weeks before, satellite imagery had revealed basin groundwater to be higher than average leaving the land with little extra capacity to take up additional water from rain and melting snow.

Image 1b. Rescue workers helping child from flooded home.

So it was no surprise that the water bomb that hit on June 19 wreaked havoc. The storm featured an intense and slow-moving moist upper low that parked itself over southern Alberta, delivering three days of torrential rains. What was not typical was that it stalled and sat over the mountains for days due to a massive high-pressure ridge to the north that blocked it from moving east and pinched it up against the Rocky Mountains. The stationary, wide-ranging low drew in warm air and cargoes of moisture from the Pacific Ocean, the Gulf of Mexico and beyond before drenching the Rockies watershed in southeastern British Columbia and southern Alberta. Interestingly, the same high-pressure system had earlier contributed to the devastating forest fires in Colorado and record-high temperatures in Yukon and Alaska. Beginning late on June 19, the skies opened and poured for 15 to 18 hours − a fire hose aimed directly at southwestern Alberta. The trapped low studded with thunderstorms just kept drenching the mountains, melting the snowpack but not thawing the partially frozen ground. The already saturated soil on thinly covered steep slopes couldn’t take any more water.

Image 1c. Rescue workers helping people by boat.

Calgary received 68 mm over 48 hours, but the rainfall west of the city in the elevated headwaters of the Bow and Elbow rivers was exceptionally heavy and torrential – more typical of a tropical storm in quantity and intensity. Rainfall rates of 3 to 5 mm/h are considered high; rates from this storm were 10 to 20 mm/h in the higher elevations, with several stations reporting 50 to 70 per cent of their storm rainfall in the first 12 hours. Totals averaged 75 to 150 mm over two and a half days, with Burns Creek (west of High River at 1,800 m elevation) recording a phenomenal 345 mm. At Canmore, over 200 mm of rain fell – ten times that of a typical summer rainfall. Also contributing to the flood, the warm air and rain melted up to 60 cm of snowpack, which was about 25 per cent above normal for that time of year, instantly engorging streams and rivulets.

Image 1d. Flood waters were half way up telephone poles.

Rampaging floods and mudslides forced the closure of the Trans-Canada Highway, isolating Banff and Canmore at the epicentre of the mountain flooding. Raging creeks ate away at riverbanks and backyards, leaving behind crumbling decks and twisted fences. Trees were literally skinned of their bark 10 metres above the ground by gravel and boulders barrelling along in rushing waters. In Canmore, the swirling Cougar Creek left entire homes teetering along its widening banks and sent residents in waist-deep water scrambling to safety. Emergency crews used helicopters, boats, combines, front-end loaders and manure spreaders to rescue stranded residents. More than two dozen towns declared states of emergency. Entire communities, including High River and Bragg Creek, were under mandatory evacuation orders. The rate at which the river sped through High River, a town of nearly 13,000, was faster than that over Niagara Falls, submerging over half the town. Several First Nations communities were particularly hard hit by the floods, with many residents still not back in their homes six months later.

Image 1e. Flood devastation overturned railway tracks.

In Calgary’s downtown, 4,000 businesses were impacted and 3,000 buildings were flooded. Water rose at the Saddledome up to the 10th row. In Stampede Park, stables and barns were under more than two metres of water. And at the partially submerged Calgary Zoo, officials moved several exotic animals to its ranch facility south of the city. The debris flood of the Bow and Elbow rivers washed away roads, rail lines and transit systems as well as several pedestrian bridges, and inundated dozens of city parks and more than 100 km of riverside pathways with water, mud, downed trees and other debris. The tragedy associated with the flooding went beyond the cost of replaceable property and belongings. Four people died after being swept away in the fast-moving waters, and the lives of thousands of Albertans and their families were changed. The sheer volume and force of raging waters caused visible and permanent changes to the landscape and beauty of southern Alberta forever, including natural carving of the landscape and river channels that would normally take centuries to evolve being destroyed in less than two days.

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