A Dawson City landslide is moving an average of six centimetres a year. That’s one of the updates residents heard during a public meeting held in the community on Aug. 18.
The Sunnydale slide is located on the west bank of the Yukon River, across the water from the south end of Dawson. The Yukon Geological Survey (YGS) began looking at it in 2020. At that time, a resident pointed out deformations on the Top of the World Highway, near Sunnydale Road, and linked them with shallow slides on the front of the river-facing slope.
Derek Cronmiller is a permafrost geologist with the Yukon government’s department of Energy, Mines and Resources. He says YGS was previously doing manual monitoring and looking at historical data. That can leave larger gaps in data sets.
“We’ve moved to more active monitoring,” Cronmiller says. “So we have a monitoring network with five stations on the Sunnydale slide, which gives us information on movement every hour in the office.”
Cronmiller says ongoing monitoring has shown that the six-centimetre average varies from almost zero movement in some portions of the slide to 26 centimetres in others. He says the 26-centimetre measurement is a lot for a slope of this size.
At this point, he says continued monitoring is looking for signs that the slide could be entering “a phase of rapid movement” where rock could come down the slope and into the Yukon River.
Cronmiller says YGS has modelled six different scenarios of this event, though larger and smaller slides are possible.
A smaller slide would be one that occurs when the river’s water level is low. In this scenario, 700,000 cubic metres could cause damage that would be limited to erosion along the waterfront in Dawson. There would be little to no chance of the resulting waves overlapping the dike at Dawson’s south end.
A larger slide could see 2.3-million cubic metres of rock fall into the river. In this event, waves of several metres in height would overlap the dike and impact more than 40 per cent of Dawson, mostly at the south end.
Cronmiller says YGS doesn’t have a good sense at the moment of how likely it is either scenario will occur. If it does, he says the major concern would be for infrastructure rather than for the personal safety of residents. That’s because of the movement of slides like this one.
“This type of slide, it doesn’t occur suddenly. It’s not like we should be sleeping with our ears open […] we don’t have to be concerned about it sneaking up on people. We expect days, at a minimum, of warning. It’s more like we would be at a raised level of alertness for months to weeks [before a slide occurred].”
Cronmiller points to the village of Birenz, Switzerland. A massive rockfall occurred there in June 2023. However, residents had evacuated the village a month earlier because monitoring suggested a slide was impending. Cronmiller says it was nice to see an example of an emergency response working out for the best.
In the Yukon, he says a response plan will be decided upon by those in the Dawson landslide working group, including the City of Dawson, the Yukon government’s departments of Highways and Public Works and Community Services and the Trʼondëk Hwëchʼin First Nation.
The News contacted the territorial departments, but no one was available to speak by press time.
Contact Amy Kenny at email@example.com