The City of Calgary and many surrounding communities experienced severe flooding in June 2013, and amidst the fears, doubts and relief experienced post-flood there are several questions that decision-makers and engineers need to ask as the clean up is underway:

  1. exactly how big was the flood,
  2. how frequently can we expect a flood that big,
  3. is this because of climate change,
  4. how likely is it that we would receive an even larger flood, and what would the impact be, and
  5. how can we design for this type of flooding?

Using some charts from reports prepared for the City of Calgary and raw data from the Governmental of Alberta, I’ll try to explain questions one and two, and in answer to question, I’ll provide some of the solutions that have been proposed in similar flood prone situations in other communities ((Note that every effort has been made to verify the accuracy of the data presented, however, there may be more accurate flood levels or the flood frequency data may have been modified since that data found in reports available on the internet.)).

Calgary Flood of 2013

Bow River Flood Data - June 2013
Bow River Flood Data – June 2013

1. Exactly how big was the flood?

The raw data off the Alberta Government website produced the following flow data for the Bow River at Calgary, (at right), which shows a peak river discharge of 1683 cubic meters per second – more than five times the flow just one day before. Some of the comments from long time residents are that they had never seen flooding like this, and they would be right. Aside from the flooding of the Elbow River in 2005, nothing of this magnitude had been seen since the flood of 1932.

So, how does this flood compare with other historical floods?

The Bow River at Calgary (7860 km2) has a continuous record from 1911. The three largest known floods all occurred before 1911 – in 1879, 1897 and 1902. Reasonably reliable estimates are available for the floods of 1897 and 1902…

The fourth-highest known flood at Calgary occurred in 1932. Since then ((this document was written prior to the 2005 flood)), there have been no floods of much significance. The nine highest events all occurred before 1934…

The 1929 and 1932 events, the fifth- and fourth-highest known, both occurred on June 3. Both resulted mainly from heavy rain in the foothills west of the city. When there is heavy rain in the foothills, the headwaters are usually receiving snow. The largest rainfall events are associated with cyclonic storms drawing moisture from the Gulf of Mexico. Snowmelt floods do not occur at Calgary.

via: Alberta Flood Case Studies

Since 1932, there had been no storms of any significance causing flooding until 2005, and now again in 2013.

The authors of the above document indicate that the reasons for this are unknown:

More important than the storage effect is a large statistical discrepancy between the earlier and later records, apparently caused by a major shift in rainstorm  patterns after 1932. The cause of this shift and its possible periodicity are unknown.

Statistically, scientists have split the data into two separate storm data sets:

It is suggested that the lower set represents ordinary within-bank annual maxima, associated  with strato-nimbus cloud conditions over the eastern slopes, and the upper series represents major floods associated with cumulo-nimbus cloud conditions.

All of this to say that while no storms of this size have been seen in recent years, the chance of them happening in any given year has existed, as seen by record storms from before 1932. The table below shows an estimate of the return period or likelihood of a particular flood condition occurring on the Bow River at Calgary.

Flood Frequency - Bow River at Calgary

From the table above, it would appear that the flood peak was equivalent to between a 1 in 22 to a 1 in 50 year flood. Various comments around the web have thrown out numbers like 200 year flood, 100 year flood, etc, however, from the data linked to above it appears that this flood is definitely less than a 1 in 70 year event on the Bow River at Calgary.

2. How frequently can we expect a flood like that to occur?

The truth is, it could happen again next year, but based on historical data, that would be very unlikely. A 1 in 100 year flood has a one percent chance of happening in any given year, while a one in 10 year event has a 10% chance of happening in any given year. And just because you have a 1 in 100 year even in the spring, it doesn’t mean that you couldn’t get another one in the fall of the same year ((We’ve discussed storm frequency on UrbanWorkbench before: [Stormwater Design]( “Stormwater Design”) and [Rain Storms and Return Periods]( “Rain Storms and Return Periods”) )). The term “one-in” is just representing the probability of a certain size of event happening, and a larger event is much rarer ((for an excellent page on storm frequency, visit the USGS homepage –  

Calgary Alberta, Flood of 2013

3. Was this storm because of climate change?

While I do believe that climate change poses a real risk to the stability of weather patterns, and that we are more likely to start seeing more extreme weather in the future, specifically blaming the Calgary flood of June 2013 on climate change is a long shot, as is seen by the historical records of even greater floods in the past 150 years. Here are some of the statements being made about the link between climate change and the flooding in Calgary:

While the triggers for each of these flooding events have been unique and complex, involving everything from the location and operation of dams and other water management infrastructure to weather patterns that seemed to get stuck in place, new scientific research shows that during the course of the next several decades, global warming is likely to increase the risk of river flooding events around the world, putting millions more people at risk, especially in rapidly growing developing countries in Southeast Asia, South Asia, eastern Africa, and the northern half of the Andes.

A study published in the journalNature Climate Change on June 9 found that flood frequency as well as the number of people at risk of inundation from flood events are both likely to increase as the world continues to warm.

The physical science behind the findings is relatively simple: as the air and oceans warm, more moisture is added to the atmosphere, giving storms more water vapor to work with and wring out as rain or snow. Studies have already shown an observed increase in extreme precipitation events in parts of the Northern Hemisphere, including a large uptick in heavy precipitation events in the Midwest and Northeastern U.S. However, until the Nature Climate Change study was published, researchers had not yet been able to shed much insight on the changing contours of global river flood risk.

via: Climate Central – As Calgary Floods, Scientists Warn of Rising Risks

From David Suzuki:

Can we say the recent flooding and extreme weather in Southern Alberta and B.C. were caused by global warming? Maybe not, but we can say we should expect more of the same – and worse if we don’t do something to get our emissions under control. As many scientists warn, climate change isn’t coming; it’s here. We may be able to adapt to and cope with some of its current effects, but it will become increasingly difficult if we continue to ignore the need to wean ourselves off fossil fuels, through conservation and switching to cleaner energy.

via: Huffington Post – Is Alberta Flooding a Sign of Climate Change?

and lastly a video explaining some of the atmospheric processes that may be contributing to an increased likelihood of these events occurring in the future:

The second part of the series will be up later this week, answering the questions;

  • how likely is it that we would receive an even larger flood, and what would the impact be, and
  • how can we design for this type of flooding?

In the meantime, here’s a sneak-peek of the flood [100-year flood mapping for the City of Calgary]( “100 Year Flood Map of Calgary”). Please leave a comment and let me know what you think about the Calgary flood situation, how the City of Calgary and the Province are handling it, and what they should do in the future to prevent or manage this type of disaster.

Published by Mike Thomas

Mike Thomas P.Eng. ENV SP, is the author of and Director of Engineering at the City of Revelstoke in the Interior of British Columbia, Canada.

6 replies on “How Large was the Flood in Calgary?”

  1. Interesting and informative post. I like that you’ve gone way back in to the records to find out that, contrary to what everyone is saying, this current flood is not the worst (it’s merely the worst that they can remember).

    I note that Suzuki has stuck his oar in again. If, as he says, this is all caused by “man-made” climate change, then it seems pointless to cut back on fossil fuel use because the change is already in place. Logically, pollution will decrease as large populations are displaced and driven back to the Stone Age by rising water levels.

  2. Even Suzuki knows that Calgary was built on a floodplain. A floodplain. Its’ called a floodplain for a good reason…it floods…Duh.

    A flood on a floodplain is about as surprising as a drought in Texas, a tornado in Oklahoma or snow in the arctic.

  3. I started looking into this because of comments made yesterday by our Alberta government that the flooding of 2013 was a 1 in 1000 year event. Your data says 1 in 22 to 1 in 50 year flood. My thought is that having folks who enjoy an element of credibility spout off about the potential for flood events (and go unchallenged) when the data shows their statements to be widely misleading adds nothing more than additional risk to the situation. How much society should invest in flood mitigation is directly proportional to the risk at hand. A 1 in 22 year event is a lot more risky in real dollar terms than a 1 in 1000 year event. Canada as a country is not likely to be around in 1000 years so why would you invest in flood control if that was the flood frequency. A 1 in 22 year event means that an investment in a house for example is likely to be wiped out four times in flood prone areas over the lifetime of the families living there. That is a real threat that most families can ill afford. Even more so when the wheel of chance is spun immediately after each event. What our politicians appear to be doing is playing Russian roulette with peoples lives and property. They should be called out on this.

  4. Have just been reading George Monbiot (Muddying the waters) on how farm and land use policy in UK contributes to the flood proneness there. As an ex-resident of Calgary I wonder what land use/drainage basin policies are in place in the Bow system to reduce or exacerbate the effects of heavy rainfall?

  5. I’m frustrated with how the province and the City of Calgary are “selling” and responding to the flood of 2013.

    The engineering report you reference has 2 different maximum flow rates in it. The first was 2265 m^3/sec with a return period of 70 years. The second was 1980 m^3/sec with a return period of 100 years.

    The 2013 maximum flow rate was about 1700 m^3/sec, significanly lower than either of the flow rates estimated in the report. At best the 2013 flood was a 1 in 50 year event, not the 1 in 200 yr or 1 in 500 yr flood that some politicians seem to be labelling it as.

    Furthermore, if you read the engineering report, the majority of the big historical floods came prior to 1934, with a suspicous absense of major flood from 1934 to 2013. What if the conditions of pre 1934 have returned ? We could be seeing flood flows of 2,000 m^3/sec every 20 years.

    Given that the 2013 flood flow has a real frequency of about 1 in 40 years, I think that many of the recovery efforts are wasted and we need to start planning for what a real 1 in 100 year flood would look like.

    First, whenever someone refers to a 1 in 100 year flood map, they need to state what flow they are expecting it to be. Is their 1 in a 100 year map based on a flow of 1500 m^3/sec ? 1800 m^3/sec or 2000 m^3/sec ?

    Second, what does the flood map for 2,00 m^3/sec actually look like ? Is downtown Calgary wiped out worse than it was ? Hillhurst ? Sunnyside ? Mission ? If so, we need to STOP SPENDING MONEY / DEVELOPMENT IN THESE AREAS.

    Third, what is going to happen to memorial drive in a 2,000 m^3/sec flood ? Sections of it were almost lost in the 2013 flood. Is it time to rebuild it as a raised roadway ? Are sections of Deerfoot at risk as well ? These need to be addressed now, before there is another serious flood.

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