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Calgary Floods: It could happen again


Rob Davis
Meteorologist

Tuesday, June 25, 2013, 12:40 PM -

The week began like any other for the population of Alberta. But by Friday, Jun. 21, a day we usually equate with the beginning of summer, what happened with the weather became a footnote on provincial history.

It’s the worst flood to hit Calgary, AB, in any Calgarian’s lifetime. Hundreds of thousands of people are displaced, homes are damaged (or completely washed away) and total damages throughout the province are expected to climb in the billions of dollars.

At its peak, The Bow River in Calgary was estimated to be flowing at 1,740 cubic metres per second. That’s about 100 million litres of water rushing past every minute; more than twice as much water as the 2005 floods. It is a devastating flood that will not be forgotten by many Calgarians.

But it’s left many living in the city’s floodplains wondering: Could this happen again? And how often do floods of this magnitude happen in the area? Could it have been worse?

THE STATS

To answer those questions, you have to look at the past. Continuous water flow records of the Bow River in Calgary began in 1911. Before this year’s flood, the highest recorded instantaneous flow level was 1,520 m3/s, set in 1932. In other words, this year’s peak water flow of 1,740 m3/s is the highest in the city’s recorded history.

So this was rare, right? Only happens once a century, if that? Well, maybe not.

Although continuous records began in 1911, Calgary’s earlier history unearths some of the city’s worst floods. Three major floods occurred in the decades prior to 1911. And in fact, until this year, Calgary’s eight worst floods in history had all occurred before 1933.

YEAR     PEAK FLOW    
1         1879 2265 m³/s (estimate)
2 1897 2265 m³/s (estimate)
3 2013 1740 m³/s (estimate)
4 1902 1550 m³/s (estimate)
5 1932 1520 m³/s
6 1929 1320 m³/s
7 1915 1130 m³/s
8 1923 841 m³/s
9 1916 810 m³/s
10 2005 791 m³/s

Two floods in particular – in 1879 & 1897 – were unprecedented, with river flows 50 percent higher than the 1932 flood event. Best estimates put each of those two floods at about 35 per cent worse than this year’s devastating flood.

COULD IT HAPPEN AGAIN?

Studies have tried to answer that exact question, but there is still some debate. One of the key reasons does seem to be fairly agreed on though.

Take a look at The Weather Network's chief meteorologist, Chris Scott, who recently described the meteorological pattern that lead to this historic flood. The 1929 & 1932 floods were caused by similar weather systems, drawing in moisture from the Gulf of Mexico. But the frequency of these types of storms has inexplainably diminished. Also, since the 1932 flood, no rainstorm has resulted in anything even close to what we just witnessed. This could be due to a shift in weather patterns, or purely due to chance.

But consider that floods of similar magnitude to this year’s flood are approximately one in 70 year events, and the odds of any given year seeing a flood of at least this level is about one per cent. Considerably low, but not impossible.

To put it another way, it is more likely than not that a flood of this magnitude – or worse – will occur again in the 21st century. In fact, there’s almost a 30 per cent chance it will happen again by 2050.

Which means the information gathered and lessons learned from 2013 will go a long way to educate and help in any preparation efforts all future Calgarians, and Canadians.

SOURCES:

1) Report on Six Case Studies of Flood Frequency Analyses by C.R. Neill, Northwest Hydraulic Consultants, Edmonton, Alberta and W.E. Watt, Queen’s University, Kingston, Ontario April 2001 prepard for Alberta Transportation -- Transportation and Civil Engineering Division -- Civil Projects Branch http://www.transportation.alberta.ca/Content/doctype30/Production/ffasixcase.pdf

2) http://www.environment.alberta.ca

3) http://www.wsc.ec.gc.ca.

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