A detailed hazard and risk assessment of Cougar Creek has revealed the June 2013 flood event was more rare than originally suspected.
It has also shown this year’s flood is not the worst-case scenario for the mountain creek, with the possibility of more significant debris flow events occurring in future.
Those were a few of the details from a hazard and risk assessment being conducted by BGC Engineering presented at a torrent mitigation symposium held by the province earlier this month in Canmore.
BGC’s Matthias Jakob provided the symposium with a preview of the findings so far and residents of Canmore can expect the final assessment to be complete and presented at a public forum in the new year.
Part of the work involved developing a magnitude and frequency model for flood events, then modeling that work to create hazard and risk maps. Through assessing the 2013 event, Jakob said it was between a 200 and 400 year event, which has a 27 per cent chance of occurring during somebody’s lifetime.
“The frequency magnitude relationship is one of the most challenging bits and pieces in geomorphology or engineering geology,” Jakob said. “To really figure out how often and how big these events can get you need to know that in order to plan your mitigation.
“Without the frequency magnitude you’re purely guessing.”
The work done so far has included looking at failure mechanisms that can occur in the watershed. Jakob said he found the main Cougar Creek channel has been dammed several times in the past by sedimentary landslides from adjacent slopes.
“Eventually, landslide dams breach catastrophically because landslide dams are not engineered,” he said.
Technically called landslide dam outbreak floods, they can be catastrophic and much larger in size than what Cougar Creek experienced in 2013.
Jakob said the June 2013 event deposited 90,000 cubic metres of additional sediment on the fan reaches of Cougar Creek. While reconstructing events from the past, he said the analysis found clusters of repeat discharges of 200,000 to 300,000 cubic metres of sediment on the fan.
“This is twice the discharge of the Bow River during the flood event of 2013,” he said. “So Cougar Creek can produce events that well exceed the discharge of the Bow River.
“The only way they can do that is through landslide damning and indeed we found evidence of landslide dams when we hiked the channel.”
Manager of engineering Andy Esarte said the frequency and magnitude curve of the risk assessment is necessary to understand what long-term mitigations are necessary.
“The best analysis and infrastructure will eliminate risk, but there is always a possibility events will be larger than expected and that the infrastructure won’t perform as planned,” he said. “It is essential then that we are not complacent and we make sure our residents are prepared.”
Jakob used multiple methods to collect information to develop the frequency and magnitude relationship, including looking at historical accounts, examining core samples from trees, and analyzing geological layers of the earth in trenches.
The return period classes included in the assessment look at one to 10 years, 10 to 30 years, 100 to 300 years, 100 to 1,000 years and 1,000 to 3,000 years.
He said by knowing return period classes, approximate water volume and sediment concentration and approximate peak discharge rates from various empirical equations, you can define certain specific hazard scenarios.
One hazard scenario identified is that of the Elk Run Boulevard culvert being unable to handle the capacity of water and sediment coming down the creek.
Jakob said in the case of the 2013 flood, the culvert would have failed had it not been for the heroic efforts of the excavators on either side of the channel.
“It would have been a completely different disaster,” he said.
The consultants prepared models of an event where the culvert fails by clogging with woody debris in the beginning. Jakob said in the model a much larger amount of water went straight through the east side of Cougar Creek residential area with significant energy and into the industrial area.
“From the hazard assessment, we are moving over to the risk assessment. Keep in mind this is the combination of hazard and consequences, so using the hazard assessment alone does not suffice,” he said. “So we are looking at risk, we are looking at buildings and building damage, we are looking at the vulnerability of roads, utilities and critical facilities and using a multiple criteria system to look at overall risk levels and that feeds into a risk management framework.”
Within the Cougar Creek fan there are 4,100 permanent and temporary residents, 1,600 workers, 322 students, 74 children in daycare and 1,400 hotel rooms. There is $310 million in residential buildings, $17.1 million in commercial development, $16.2 million of institutional buildings, $13.7 million of industrial and $19.6 million in other values, including below surface utilities. The area is home to 330 companies and represents annual revenue of $169 million. In comparison, the rest of Canmore has 867 companies and $403 million in annual revenues.
“It all plays into the risk-based decision making,” Jakob said. “If an event of a certain magnitude occurs, how much business will be interrupted? What is the short-term interruption and what is the long-term interruption?”
“We are in the process of figuring out what the different consequences will be for all these different events and that will ultimately allow the Town and the stakeholders … to find out what is the optimum mitigation strategy for the long term.”
