Baker BC - very reactive surface layer observation

Saw the wrinkling of the snow surface last week in the Cariboos (as in your picture), daveb. The set up was as follows: Low density snow with a thin layer of surface hoar deposited on top of it followed by 9" of very low density new snow (5% water content) that fell at a snowfall rate of 1-1/2" per hour. Entire slopes broke up into a wrinkled pattern.

At times energy would propagate out from the skiis (presumably through the surface hoar layer) for about 50-75' from a skier's position. The 9" of new snow would then begin to break up. Slopes over about 40 to 42 degrees would slide as a very soft slab, but those under that angle would merely wrinkle.

Here's what I think was happening: As the not cohesive snow began to fail on top of the surface hoar it would begin to move downslope, but because the underlying snow was so weak the moving snow would begin to move turbulently, and was only able to dive into the underlying low density snow which then stopped the momentum of the failing new snow. So the slope would wrinkle but not slide until a critical slope angle was reached.

This abnormal failure mechanism was certainly interesting to me, thanks for reviving this thread Gary. It appears the Baker snowpack I observed was less complex than the Cariboo snowpack you observed. Baker was low density layer on high density layer with no apparent weak layer separating the two. Cariboo had the surface hoar weak layer between the low & lower density.

Nonetheless, both snowpacks behaved similar. Compression fractures/wrinkles (CFW) on lower angles & 50’-75’ propagation (close to what I observed) of very soft slabs on steeper slopes 40-42 degrees (also close to what I observed).

What I found really interesting about the CFW is the partial failure (as opposed to full failure, ie an avalanche). It was intriguing the snowpack had enough cohesion to store the energy necessary to propagate, but once the energy was released there was no avalanche, just CFW.

Gary, your Cariboo snowpack analysis got me thinking nerdy about the Baker snowpack. At Baker, since there was high density snow below the CFW layer I observed, the only action was in the CFW layer. Seems like when the CFW appear, the layer must consolidate (the snow density increases and void air space decreases). With less void air space, more contact, and therefore more cohesion, must be occurring between snow flakes in the layer. There must have been enough consolidation to increase cohesion and bring the sliding snow to a halt.

Interesting. I think the Cariboo example was easier to understand. What's difficult to understand in your Baker case, daveb, is how the energy was able to propagate for 50-75', which is quite a ways, and infers a weak layer. So maybe, somehow the energy propagated through the very low density new snow. But that is hard to believe because it is difficult to understand how new snow that is not at all cohesive is able to transmit energy?

The guide in the Caiboos said that wrinkled snow occurs there about 3 times a season. I think that it doesn't slide because it has so little cohesiveness. It is easy to understand why it would break up (lack of cohesion) but in your Baker case was the underlying higher density surface still soft and porous or was it firmer as if it had a skin on top of the layer? i.e. Is my diving theory realistic?

The guide also suggested it (FCW as you call it) is something someone ought to study. But as it is the first time I've seen extensive wrinkling, I would think the research scientist might be in for quite a wait for data points.

I worked one winter at Mica Creek, which is due east of Blue River a short distance and that winter I never saw this phenomemna.

I had a small revelation/why didn't I think of that moment. Skinning from the Baker upper parking lot to Bagley Lakes, we observed a very thin frozen water layer on the surface. This layer was not apparent where the CFW (as you probably guessed I just made that up) were observed. The wind was howling, as it commonly does, on the way to Bagley Lakes so it probably kept any snow from accumulating on this layer. In the trees, however, the snow probably accumulated on the layer which then provided the sliding surface for the CFW. That makes the observation much less mysterious.

I had a small revelation/why didn't I think of that moment.  Skinning from the Baker upper parking lot to Bagley Lakes, we observed a very thin frozen water layer on the surface.  This layer was not apparent where the CFW (as you probably guessed I just made that up) were observed.   The wind was howling, as it commonly does, on the way to Bagley Lakes so it probably kept any snow from accumulating on this layer.  In the trees, however, the snow probably accumulated on the layer which then provided the sliding surface for the CFW.  That makes the observation much less mysterious.

In your Baker example, that would mean that the energy propagated through the low density new snow that was cohesionless. That may also have been the case in the Cariboos. Perhaps there is enough structure, as ephemeral as it is, to enable propagation. The surface hoar in the Cariboos was just a small amount, not big crystals or anything.

Today's avalanche at Mount Baker. Titled as "once in 100 years".





Unstable, indeed!