Turtle Mountain is moving but the chances of another catastrophic rockslide are extremely low said Todd Shipman, a geologist with the Alberta Geological Survey, at a council meeting on Sept. 9.
In presentation on the Turtle Mountain Early Warning System, Shipman said the mountain has been moving between 1 mm and 2.3 mm per year since monitoring started in 2004 with the most movement coming during the spring/fall freeze thaw cycle. Anything less than 16 mm is considered low risk, he said.
Many organizations have studied Turtle Mountain since the 1903 Frank Slide but the Alberta Geological Survey’s current project is probably the most technically advanced.
The early warning system incorporates three categories of monitoring equipment including primary sensors that measure movement on the sub-millmetric level, secondary sensors that have a coarse resolution and tertiary sensors that incorporate data from primary and secondary sensors.
Shipman said they’ve experimented with a wide range of techniques with varying degrees of success. The survey data is being shared internationally to help better understand the physics of landslides.
The sensors range from extensometers, tiltmeters and crackmeters to laser ranging prisms, GPS and ground-based satellite.
“This mountain is the most monitored mountain in Canada when it comes to landslide threat,” said Shipman.
The survey has had excellent results using Ground Based InSar sensors, which use radar to measure movement. The sensor is built onto a three-metre long rail that takes an image of the mountain face every six minutes, said Shipman.
Previous studies have identified two areas of risk, though none of the possible slides would come close to replicating the 1903 slide. One area of risk is on the south peak; the other is on the north peak, with both areas facing Frank and Hillcrest.
A slide from the south peak could contain between 240,000 and 3.5 million cubic metres of rock; a drop in the bucket compared to the Frank Slide where an estimated 35 million cubic metres of rock detached in one go, said Shipman.
Models show that a south peak slide could threaten Highway 3, the railway, the river and the Hillcrest Ball Diamonds, though Shipman was quick to point out that the models should not be taken too literally.
“All models are wrong, but some are helpful,” he said. “They are used as a tool to understand things.”
Councillor Bill Kovach commented that a rockslide could potentially threaten Blairmore with flooding if the Crowsnest River is blocked.
The Alberta Geological Survey has not attached sensors to the backside of the mountain. Shipman said previous studies by structural engineers and geologists have concluded that the mountain would fail in a similar direction as it did in 1903.
“Much of the focus has been on the south peak and this information and precautionary stuff we did on the north peak. We haven’t seen much movement on that,” said Shipman. “I would say the south peak is of a higher risk. But that is just based on the fact that we don’t have enough data for the north peak moving.”
Shipman waxed poetic when asked to estimate the chances of another Turtle Mountain rockslide.
“I’m a geologist, that’s my background so it’s a matter of scale. As a geologist, all mountains become flat. All holes fill up, but I’m talking millions of years,” he said. “Things don’t ever stay up forever. All mountains erode. Some don’t do it as catastrophically as Turtle Mountain did; half the mountain was gone in a single event. Normally they go much more quietly.”
“Yes. Everything will eventually slide someday. But in our lifetime? Unlikely. Very low risk.”
The early warning system employs a colour coded alert system to determine threat level where green represents normal/seasonal fluctuations, yellow means an increase in velocity, orange means acceleration and red indicates an event is in progress.
In the event of a slide, Shipman says their control centre would immediately contact the Alberta Emergency Management Agency (AEMA) who would then assume responsibility for a response.
In 2008, after input from Corey Froese, spokesperson for the Alberta Energy and Utilities Board, council created the Turtle Mountain Restricted Development Area.
According to former councillor Garry Taje, Froese said that Turtle Mountain would fall again, though he could not give a timeline. This prompted council to change the Land Use Bylaw, restricting development near foot of the mountain.
Shipman announced that a further report on Turtle Mountain is being released in the spring.