DAWSON CITY – Helicopter activity around the Moosehide Slide in August and early September was in aid of the work of the Moosehide Slide Working group.

It was formed under the last Dawson council as a result of a report from BCG Engineering.

The company had been tasked to determine the potential for danger posed by the slide’s movement.

The hazard assessment was studied in 2020, and in 2021 there followed a “quantitative assessment of life-loss risk should a rock avalanche occur.”

The working group is chaired by Carolyn Relf.

It consists of representatives from the City of Dawson, the Tr’ondëk Hwëch’in, the Yukon Geological Survey, Yukon Government Community Development and Community Services and the Emergency Measures Organization.

Relf presented the interim findings to municipal council on Aug. 10.

In a cover letter, she wrote that “the Slide poses an avalanche risk which Yukon Government and Dawson City’s previous mayor and council committed to address through the provision of an array of monitoring instruments and the development of an associated emergency response plan.”

The slide does move a few centimetres annually, but the study implies that there is possible concern for more substantial movement.

This might be seen as more urgent after last April’s slide event beside the Robert Service Way in Whitehorse.

“Of particular concern,” says Relf’s letter, “are two blocks of rock that are perched in the head scarp (the uppermost part of the slope).

“The upper edges of the blocks are delineated by fractures that are subject to expansion and contraction in response to annual freeze-thaw cycles. BCG assessed four different hazard scenarios as part of their study, identifying homes that would be impacted in each scenario.”

One dramatic photo in the slide presentation showed a tree which had been split in two from its base up several metres by the movement of the ground, though it had not died at the time the photo was taken.

It has been proposed and agreed upon to install a variety of types of monitoring equipment at significant places along the slide, in line with the four major blocks of concern that could be affected by tension cracks.

“The planned monitoring system consists of three mutually independent sets of instruments:

“- 6 Global Navigation Satellite System (GNSS) instruments and a base station (to be installed on the roof of the Municipal Building). These instruments will record ground movement in the head scarp;

“-   6 tiltmeters, which will record any rotation of blocks;

“-   4 extensiometers, which will straddle the upper fractures that delineate Blocks 1 and 2 and sense whether they are opening.

“The system will also include a weather station to enable correlation between changes in ground creep rate and local variations in temperature and precipitation.”

The installation by BCG has been taking place since August, and should be in place by the end of this month.

The study will continue until the spring of 2025 to determine what kind of movement is normal for this site.

The federal government provided funding for this project as part of a $25-million commitment to address climate change in the Yukon.