Avalanche
It’s frightening to realize that you could be far below snow line on a summer day and end up getting bulldozed by an powerful wet snow avalanche. It’s a reality that we all need to learn about and this is the exact right time of year to focus on it.
This video shows the power of a wet avalanche running from a high alpine zone, into a lower elevation green zone. Wet avalanches move slower than powder avalanches, but they have power. They move with snow that has a very high density and is very heavy. Once they get going, they like to keep going. Just watch as this avalanche runs for thousands of feet down into the vegetation below.
watch this wet avalanche in France destroy a chairlift in 2012
WET AVALANCHE INFORMATION from UTAH AVALANCHE CENTER:
When it comes to wet snow avalanches, a lot of good, but general, simplifications are thrown around, such as watch out for the first warm day, rain on cold snow, 3 non-freezing nights lead to wet slab avalanches, etc. It’s certainly a lot more complicated, but less studied than dry snow avalanches.
Every other year, at the National Avalanche School, one of my favorite talks is Larry Heywood’s “Wet Snow Forecasting and Control”. Looking back through my NAS 2011 notes, here are a few interesting points from his talk.
Wet Snow Fatalities
- Wet snow avalanches – comprise about 9% of US avalanche fatalities
- 35% were human triggered
- 47% were natural triggers
Wet snow is at 0 deg C, and composed of ice, air and water. While not field practical, studies show the percentage of water in the matrix is important.
- At less than 7% liquid water, ice to ice bonds still exist, and air is continuous in snowpack.
- At greater than 7% liquid water, the snowpack has continuous water, meaning no inter- grain contact, and this is when the snow layer has very low strength.
Stratigraphy is important . It affects –
- Rate of infiltration
- Pattern of infiltration
- Concentration of water
- Defines capillary boundaries
In addition, the age of snow and history of prior melt-freeze events influence the chance of wet snow avalanches. Many practitioners feel having a faceted layer in the snow pack is key for wet slab avalanches.
Larry Haywood’s Wet Snow Triangle –
Thanks to the Utah Avalanche Center for this quality information on Wet Snow Avalanches.
I witnessed a *huge* wet avalanche one hot day in May many years ago on Mt. St. Helens… in fact the first year it was open post-eruption. I was on Monitor Ridge, having a lunch break on an unseasonably warm day, when an enormous “boom” startled us from our reverie. A crown fracture several hundred yards wide opened just below the summit ridge directly above us and the *entire* slope we were preparing to ascend broke away and s l o w l y started sliding towards us. Thankfully we were sitting on the ridge line several hundred feet below where it faded into the summit snowfield, and the ridge acted as a ship’s prow and parted the falling ocean of snow. We then sat and watched for at least twenty minutes as it slowly made its way down either side of the ridge and plowed its way thousands of feet down the south side of the volcanic cone – through and below tree line – destroying everything in its path. Truly awe-inspiring sight and sounds.
Wow. Great story, Chuck, glad you were in a safe zone. Wet Avys are ground shaking. I almost got caught in one near Mammoth in 2006. Ripped out to the ground and sprayed 40 feet high once it hit its own staunch wall. Scary stuff.
Never seen snow do this. Scary.
That avalanche killing the chairlift is hard to believe.