This is the story of two relatively recent flood events in Canada.
Flooding in southern Alberta (Canada) in June 2013 resulted in four fatalities and unprecedented damage to property. More than 250 mm of rain fell over a 36 hour period in the foothills west and southwest of Calgary and began rapidly flowing east through the province’s river valleys bringing destruction across southern Alberta. Bow River experienced flows eight times higher than normal, and Elbor River experienced flows twelve times higher than normal. City of Calgary (4th largest city in Canada with population of 1.1 M) was hit very hard. Over 100,000 people were displaced throughout the region. More than 35,000 people were without power for weeks. Downtown Calgary was out of reach for two weeks. Twenty bridges were closed and many roads without access for days. Some 2,200 Canadian Armed Forces (CAF) troops were deployed to help in flooded areas. Total damage estimates exceeded C$6 billion and in terms of insurable damages, is the costliest disaster in Canadian history.
Two weeks later in July 2013 the City of Toronto (the largest city in Canada with population of 4.7 M) experienced a heavy rainfall event, exceeding 126 mm over two hours (74.4 mm July average), that overwhelmed stormwater and sanitary sewer systems. At least 500,000 people were affected, 1,400 train passengers stranded for hours, all major traffic arteries flooded and over 300,000 people left without the power for a significant period of time. Urban flood losses, including damage from water and sewage that entered homes and businesses through the backup of municipal sewers, were extensive, approaching C$1 billion and making this storm the most expensive storm in the Province of Ontario.
These two events prompted the Minister of Public Safety Canada to include in his ‘Report on Plans and Priorities’ to the Prime minister the following statement: …”The rising cost of natural disasters and the financial burden on Ottawa is the country’s biggest public safety risk”…
Actions to prevent or reduce the risk of flood damage in Canada should include actions to address both riverine and urban flooding. Riverine floods are the most common natural hazard experienced by Canadians. In the 1960s and 1970s few Canadians experienced damage from urban flooding. However, over the past few decades there has been an alarming increase in urban flood losses. Indeed, water damage from sewers backing up into basements and other losses due to extreme rainfall in urban areas likely resulted in urban flood losses more than ten times greater than riverine flood damage.
Best practices to prevent and reduce the risk of loss from riverine flooding are well known, and have been tested around the world for several decades. Prohibition of development in zones of flood risk, investments in structural flood defence and a variety of other tools are available to eliminate or reduce the expected loss from riverine flooding. The foundation for riverine flood management involves a clear determination of acceptable risk of flood damage.
Best practices for reducing the risk of urban flooding have emerged over the past 25 or 30 years and are distinct from actions to reduce the risk of loss from riverine flooding. The frequency and severity of urban flood damage is determined by factors that include rainfall patterns, lot level actions by property owners and the state of the local sewer infrastructure. Every household connected to the storm or sanitary sewer system is at some risk of loss. Best practices to reduce the risk of urban flood damage include lot level actions by property owners and public investments in sewer systems.
The tragic losses in southern Alberta and Toronto have opened a window of opportunity over the next 12 to 24 months for the Government of Canada and other stakeholders to take action to reduce the risk of loss from flooding. Some of the recommendations include:
Most of the damage from flooding and other natural perils is preventable through the application of existing and emerging knowledge.