Calgary faces 3.9% chance of '1-in-100-year' flood before major mitigation is built

This story was originally published on June 14

It's been five years since Calgary's rivers spilled their banks and flowed into the heart of the city, taking one person's life and causing billions of dollars in damage. Four more people were killed by the flooding across southern Alberta.

The 2013 disaster remains the costliest flood in Canadian history and etched in our collective memory. Recall the Stampede grounds underwater, the C-Train tracks bent so badly they resembled a roller-coaster, the tens of thousands of people ordered to evacuate their homes, the countless lives upended.

Recall, too, the outpouring of support, the armies of volunteers helping perfect strangers, the legacy of the now-annual Neighbour Day celebration.

The flood brought out the best in Calgary, but it also brought out a "never again" attitude. Civic leaders and ordinary citizens called for major mitigation to protect the city. Various proposals for dams, reservoirs and even an underground channel were debated. Most were rejected.

Half a decade later, nothing big has been built.

Mayor 'wary' every spring

Smaller-scale projects within city limits have made things safer, to a degree, but those defences would be largely overwhelmed if something on the scale of what happened in 2013 were to happen again. It's not until major mitigation is built upstream that Calgary will be truly protected.

On the Elbow River, which is the focus here (another article in this series will look at the Bow River), the provincial government aims do that by building a dry reservoir in Springbank. But it estimates the project won't be complete until 2023, at the earliest.

In the meantime, many Calgarians are left anxious around this time each year, wondering how fast the snow in the mountains will melt and how much rain the storm clouds on the horizon will bring.

"I, like many residents in Calgary, every spring am still a little bit wary," Mayor Naheed Nenshi said.

Nenshi says he finds some reassurance, at least, in the community-level mitigation work that's been completed to date — projects like berms and flood walls and hardened riverbanks.

"They're very good work and they're important work. That said, they'll never do the whole job," he said.

"Ultimately, we'll need those upstream solutions."

So, where do things stand with those upstream projects?

Let's review.

Settling on Springbank

As you may recall, there's been plenty of controversy surrounding how — and where — to do major mitigation work.

Shortly after the flood, the city seriously considered a proposal to build a massive diversion tunnel from the Glenmore reservoir. The province also looked closely at the option of a dry dam at McLean Creek in Kananaskis Country, but ultimately rejected the plan.

Alberta's previous PC government settled on the Springbank reservoir in 2014, and the newly elected NDP government concurred, once it took over the province's reins in 2015.

Even though the NDP had supported McLean Creek prior to the 2015 election, Transportation Minister Brian Mason said it became clear once the new government looked at the plans in detail that Springbank was the best choice.

The McLean Creek dam, he said, would have had a far greater environmental impact and come with an unacceptably high risk of failure, if there happened to be a high-water event during the construction phase.

"An earthen dam could then just let loose and it could just destroy the whole ecosystem downstream from there," Mason said. "So that's a risk of a catastrophic failure that was flagged for us when we were looking at McLean Creek."

Still, not everyone agrees.

Opposition and delays

Property owners in the area where the province plans to build the Springbank reservoir continue to push back against the plan.

Among them is Lee Drewry, who doesn't want to see the land that's been in his family since the 1870s altered forever.

"The whole area behind me would be covered in silt, about two metres deep, and it would be essentially useless," he said, gesturing toward the picturesque valley that would become the reservoir.

Drewry wants the province to take another look at the McLean Creek option and says many landowners in Springbank feel the same way.

But Mason says that's a non-starter, at this point, as any further delays in getting upstream mitigation built would bring added risk to Calgary.

The timeframes for the Springbank project are already uncertain, as it stands, as is the price tag.

The province still needs to acquire land for the reservoir from private owners in the area, either through direct transactions or, if they refuse to sell, through expropriation, which could take more time.

Construction costs are estimated at $292 million, while land acquisition could run from $80 million to $140 million, depending on how much land is bought directly versus expropriated.

But Mason said the province won't even begin the first steps in the expropriation process until it gets environmental approval for the project from the federal government.

A federal regulator sent the Alberta government's initial environmental impact assessment back to the province last year, saying it lacked detail. The province resubmitted the documents in the spring and a response is due back from the feds by March 2019.

That has put the project 10 months behind schedule, and Mason said the province is now looking at revised timeframes for when Calgary can expect at least some level of upstream mitigation.

"We will hope that the reservoir will be functionally operational to a 100-year flood level by the flood season of 2022 and fully operational for a 2013-level of flood the next year, 2023," he said.

In the meantime, Calgary is left exposed to a small but significant risk every flood season, which typically runs from May 15 to July 15. Each year that passes means another roll of the dice.

And the longer we keep rolling, the greater that risk becomes.

'Simply a question of chance'

Do a bit of fancy math, and you'll discover there's a 3.9-per-cent chance of a "1-in-100-year" flood happening sometime in the next four years.

To put that in perspective, it's slightly more likely than rolling snake eyes in craps (2.8 per cent) and slightly less likely than being dealt a blackjack (4.8 per cent) on your first hand at a casino table.

Calgarians may feel comfortable with those odds, especially as more time passes from the 2013 flood. For many of us, it's no longer top of mind like it was in the immediate aftermath of the disaster. But human beings are prone to something known as "recency bias." It's a common mistake our brains make, overemphasizing recent events while downplaying things that happened in past.

So it's only natural for the river to seem less threatening now than it previously did, especially since high-water events have been relatively rare in Calgary for most of our lifetimes.

But this is where the "1-in-100 year" flood risk that we so often hear about gets tricky — and often misunderstood.

"That really means a flood that is of such magnitude that it has a chance of occurring of one per cent ... in any given year," said Frank Frigo, the city's leader of watershed analysis. "So, having a 100-year flood does not mean that, if you have one, then you'll have 99 years where you do not."

And, given the nature of probability, something that has a one per cent annual chance of happening, in theory, can end up happening quite irregularly, in actuality.

In Calgary, Frigo noted, there were two major flood events in 1879 and 1897, both bigger than the 2013 flood. That was followed by several smaller cases of flooding in the early 1900s. But then, for the better part of a century, the rivers were relatively calm and the city went on an extended run without any significant floods.

"And that isn't because the natural system can't produce them," Frigo said. "It appears, based on all hydrologic analysis, to simply have been a question of chance."

'We are at risk every year'

Paul Battistella, for one, isn't comfortable leaving things up to chance.

He's with Flood Free Calgary, a group of businesses, organizations and citizens that has formed to advocate for "timely flood mitigation" upstream of the city.

They want to see the Springbank reservoir built as quickly as possible in order to reduce the risk Calgarians face each spring and summer.

"I think what people need to understand is there's a probability of an event occurring like 2013 every single year — and potentially something that's worse," he said.

"So, unless we have upstream mitigation ... we are at risk every year. And it's an unacceptable risk."

Acceptable or not, it's our reality.

For the next four years, at least, we'll continue to throw the dice.

This is the first in a series of stories from CBC Calgary on the five-year anniversary of the 2013 flood. The next will look at mitigation plans on the Bow River.

Calgary: The Road Ahead is CBC Calgary's special focus on our city as it passes through the crucible of the downturn: the challenges we face, and the possible solutions as we explore what kind of Calgary we want to create. Have an idea? Email us at:  calgarytheroadahead@cbc.ca