Top ten weather stories for 2012: story eight

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Table of Contents

8. The Year of the Urban Flood

Map of Canada highlighting the cities that were impacted by floods. These cities include: Edmonton, Calgary, Thunderbay, Toronto, Hamilton, and Montreal.

While flooding typically hits rural areas hardest, 2012 brought equal opportunity flooding to many urban Canadians. Late on May 26, a low-pressure system that moved from North Dakota sat south of Thunder Bay, Ontario. Over the next two days, waves of thunderstorms – seven hours in total – pounded the city with between 35 and 120 mm of rain (including 71 mm in less than six hours). The ensuing flash flood caused washouts on numerous roads and trails, cut power to homes and businesses, and filled thousands of basements with up to two metres of dirty sewage water. Angry torrents also tore up chunks of asphalt and stalled vehicles on roadways, in parking lots and on bridges. A potential crisis developed when pumps at the city’s water treatment plant failed leaving the facility with a massive back-up of brown water. Authorities in Thunder Bay and surrounding areas immediately declared states of emergency and the province later termed it a disaster. Damage to both public infrastructure and private homes and businesses was extensive with initial costs exceeding $100 million. Rainfall in May totalled 206 mm (310 per cent of normal), smashing the previous monthly record set 41 years ago. Half of that amount fell during the May 26 storm on a super-saturated ground that couldn’t take any more.

Flooded city streets.

Just three days later, on May 29, a double-header rainstorm pounded the Montreal area. Torrents of water overwhelmed the city’s beleaguered sewer system and caused widespread flooding on streets and in public buildings. The storms were part of a sharp cold front that triggered thunderstorms and high winds, and prompted tornado warnings in southwestern Quebec. The rush-hour hits started around 5 a.m. with up to 40 mm of rain during a lengthy and noisy thunderstorm. Twelve hours later a brief but more powerful storm dumped a further 50 to 80 mm of rain on the city. Together, the two events dropped up to 120 mm of rain in the downtown core at a rate approaching a 100-year recurrence. The intense rains turned sloped city streets into waterfalls. Mud and water flowed through windows filling basements to the rafters. Major thoroughfares became canals and intersections formed lakes with water lapping up the door handles of cars. The water rose nearly a metre deep on some streets, blowing manhole covers in geysers two metres high. Afternoon commuters sloshed through massive puddles and waded ankle-deep in smelly sewage at subway entrances only to be faced with a quasi-shutdown of the city’s transit system and commuter train service. Water flowed into thousands of buildings across the city – including several schools and colleges, the National Library and the Archives of Quebec – with heavy rains damaging hundreds of works of rare art at one Montreal museum. The storms also caused power failures affecting 28,000 people.

Toronto joined the urban flood club on July 15 when short-lived afternoon thunderstorms left some with nothing more than a wetting while others bailed out basements filled with sewage water to the ceiling. Scarborough received the brunt of the storm with about 88 mm of rain falling over a couple of hours. And on July 22, an intense downpour in Hamilton dumped 140 mm on the city in less than four hours leaving puddles deep enough to row a boat on. Yet amidst reports of hundreds of flooded basements and thousands of fallen trees, some neighbourhoods recorded barely a sprinkle of rain. Later in July, as residents of Steinbach, Manitoba slept, a powerful stationary summer storm set up over the city. Streets, parking lots and yards were turned into rivers and small lakes when the July 25 storm dumped between 80 and 110 mm of rain, most of it in less than hour. Understandably, the city’s storm sewer system was overwhelmed by the gully washer, resulting in major flooding at the city’s pumping station, along major roads and in numerous basements.

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