Avenues Avalanche!

Let's have some fun with a post based on what I'll call "sidewalk science."  

The Hall of Fame Baseball catcher Yogi Berra once said "you can observe a lot just by watching."  This applies for sure to both meteorology and snow science.

This afternoon, right near my house, I encountered an avalanche in the front yard of a house on 16th Avenue in the upper Avenues.

Oh, I didn't actually see the avalanche happen, but I did see the aftermath, which I've summarized in the below.  It contains many of the hallmark characteristics of an avalanche.  The first is the starting zone, which in this case I measured to be 35°.  Most avalanches start on slopes between 30 and 45 degrees, so the slope here was right in that range.

There is also the crown or crown line, which is the upper fracture line of the slab that broke away to create the avalanche, as well as the flank or fracture line along the side of the avalanche.  There is one on the other side, although I have not labeled it.   

Most avalanches that kill people are slab avalanches in which a cohesive layer of snow slides down the slope.  Indeed that was the case here with the slab sliding about 20 inches or 50 cm down the slope as a cohesive layer.  In this case, the length and speed of the avalanche was such that the slab remained intact.  However, eventually it encountered the lower angle sidewalk where the slab decelerated, debris built up, and the toe of the avalanche was found.  

The bed surface for this avalanche was the grass rather than a weak layer in the snow.  The trigger was likely solar heating, which led to a lubrication of the grass by liquid water as the snow melted, weakening the bond between the snow and the grass.  Such an avalanche is referred to as a glide avalanche, which is defined as a release of snowcover as a result of gliding over the ground.  

So much snow science in such a small space!  However, there are real-world applications.  Glide avalanches can be deadly and destructive.  In the Wasatch Range, they are probably most common in Broads Fork, a subdrainage of Big Cottonwood Canyon where glide avalanches commonly occur on steeply sloped rock slabs.  Here's a link to a report about one in Broads Fork: https://utahavalanchecenter.org/avalanche/69596. Note the fully exposed rocks in the starting zone of the avalanche.  

Glide avalanches are hard to predict, but can have deadly consequences.  They are also common in the Alps where the mountains are not only steep, but there are a lot of grassy, high altitude meadows that are grazed in the summer.  The resulting steep, grass slopes can make the perfect bed for an avalanche.  I saw the aftermath of many glide avalanches when I lived in Innsbruck, Austria.  Below is a photo looking down at one during a ski tour on the Gammerspitz, a 2500 meter peak in the Alps south of Innsbruck.  It was on a south facing slope and likely was triggered by solar heating like the avalanche I saw today in the Avenues.  We avoided slopes like this on our tour! 

I found the number of glide avalanches and the expansive coverage of glide cracks in the Alps to be very disconcerting and often made conservative terrain choices because of them.  Fortunately it was easy to avoid avalanche terrain today in the Avenues.