Snowmobile Triggered Avalanches in the Renshaw area near McBride, BC

Following are pictures posted to the CAC Discussion Forum. They are from two snowmobile triggered avalanches in the Renshaw area near McBride, BC.

The CAC has a Discussion Forum to make it easy for professionals and recreationalists to post photos and comments related to each of the Bulletin regions in Canada.

THIS IS ANOTHER GREAT FREE RESOURCE FOR SLEDDERS TO USE BEFORE EACH TRIP!

CLICK HERE to enter the CAC Discussion Forum for the North Columbia

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CLICK HERE to view photos – Renshaw Incident #1

Comments posted along with this second incident “While traveling to a different riding area we traveled a much traveled path at the bottom of a valley between 2 mountains when the avalanche broke right near….” CLICK HERE to continue reading

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CLICK HERE to view photos – Renshaw Incident #2

Comments posted along with this first incident “We were playing on a west facing slope when one snowmobiler triggered slide above him. He had time to ….” CLICK HERE to continue reading

Notice the similarities in the avalanches and the slopes in these two separate events that were reported on Jan 29, 2011.
Both avalanches have areas that have released to the rocks uncovering jagged rock bands. These slopes tend to be easier to trigger for two reasons. One: the snowpack is often thinner where the rocks are sticking up in through the snowpack, two: the change in slope angle leaves the snow above relatively unsupported.

What is an unsupported slope and what does it matter?
Consider a concave slope, a hill where the slope angle gradually gets less steep until it reaches the valley. Basically, the snow on the lower angle terrain is quite comfortable right where it is (gravity is not stressing it) and it tends to help support the snow above it. Slopes without that concave area of compression or support are missing an important factor for stability. Slopes with no support are relying on the tensile strength of the snow (hoping that the snow layers do not rip to allow a fracture to propagate across the slope) and they also rely on the bond or shear strength BETWEEN the layers in the snowpack and the ground.

Avoiding thin, rocky patches is highly recommended in almost any snowpack, but in this one it is a must!

We would like to recommend that you take a moment to read the report that the Canadian Avalanche Centre released February 18, 2011. It speaks to this problem of unstable basal layers (unstable layers of snow at the base of the snowpack next to the ground).
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The report includes a description of what is going on and some recommended ways to travel during this time period to reduce our chances of being in the wrong place at the wrong time.

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