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July 12, 2024: Creep vs. Glide
At some point during the last season a couple of misconceptions arose in the media. One was creep in the snowpack, which was confused with glide in an article which found it's way into Yahoo news as well as at least one ski oriented news site. An amusing aspect of this was that the author, whoever it was, took the opportunity to tell readers that they should take a "Level 1" course to learn about these things. Whether they took such a class is not known of course, but if they did they were probably misinformed on at least this point.
Creep and Glide are two different mechanical phenomena within the snowpack. They are covered in the Avalanche Institute Level 1 course in the modules on Snowpack, but are also simply defined in our glossary.
Creep is a differential motion or "flow" within an inclined snowpack. The base is anchored to the ground while the surface is dragged downhill. This is somewhat analogous to laminar flow in a fluid. This difference in motion with depth introduces stresses, particularly shear and tension, within the snowpack. A favorite statement among course instructors and others is that convexities are often trigger points for avalanches. Creep is a major reason for this, although some people parroting the convex bit can explain this and others cannot.
Creep can cause stresses on trees, lift towers, and other structures as well as deformation within the snow. Some vegetation which is bent over in spring such as Slide Alder and bent trees results from creep and not necessarily by an avalanche. Although some of this does also occur from avalanches, perhaps more of it.
Avalanche Center -> Education Center -> Glossary -> Creep
Glide is motion of the entire snowpack down a slope. Unlike creep the base is not fixed but slides along the ground. Glide occurs on smooth surfaces such as bare rock or grass, without any anchors. Because the entire snowpack is pulled downslope cracks, called "Glide Cracks" (not surpisingly), can open through the entire snowpack to the ground. This is most common when the snowpack is warm and wet, such as in spring.
Our course module has some photos of spring glide cracks, and the buckling which can go along with those. We also have some photos in the gallery of a test installation for the "Snowgripper", on a grassy slope in Austria.
Avalanche Center -> Education Center -> Glossary -> Glide
The issue arose this winter when cracks appeared on a ski slope in the Alps (and the same thing happened in at least one other location later). The news story attributed these to creep. However, creep does not cause these cracks. (We never say never, but any such occurrence would be extremely rare.) These were almost certainly from glide, as indicated by several factors.
The first is that they were characteristic cracks through to the bare ground. The second is that the underlying ground was clearly smooth with no anchoring. A third factor was that these were not on a convexity but on a section of uniform slope angle. And a final factor was that rain had fallen around that time or shortly before. The first three factors were evident in the photo, the last follows from recent weather.
As far as applications to recreation, creep can help understand why convexities are areas with more stress. And glide is usually mentioned in the context of avoiding areas with glide cracks in spring. The last glide avalanche fatality I can recall was in Big Cottonwood Canyon in Utah quite some time ago, and it was claimed that was the second one ever in the US. So apparently it is not a common problem, although it would be foolish to tempt fate by being on such slopes. (These avalanches occur when they're ready to, it's not clear that human presence is a very big factor.)
Beyond recreation creep needs to be considered when installing lift towers and similar objects, and glide avalanches can be large enough to present a threat to communities when they do decide to release. The Snowgripper installation in the photos linked above was above such a community which had a history of glide avalanche problems.
