Wet Snow Avalanches And Rescue, Part 1

In the spring I have counted one hundred and thirty-six different kinds of weather inside of four and twenty hours. ~Mark Twain

Spring also means a whole lot of changes for the snow and avalanche conditions, and spring can be a most dangerous time rescuers. Last Friday there was an avalanche accident off-piste at Val d’Isère, France, which resulted in a serious and risky rescue. Fortunately the knowledge, skill, and professionalism (and probably some luck, too) of the ski patrollers saved the skier’s life. Part 1 of this two-part series focuses on the differences of wet and dry avalanches and what this means to rescuers. Part 2 will suggest how rescuers should adapt their strategies to deal with wet avalanches.

Avalanche accident on the Face du Charvet, Val d'Isère, France. Photo by Henry Schniewind. http://www.henrysavalanchetalk.com

Accident: Face du Charvet E Face, 11:45 Val d’Isère April 1

The pisteurs (ski patrollers) at Val d’Isère faced a challenging and dangerous lunchtime rescue after an avalanche swept a skier down the east-facing Face du Chavert. During the rescue several additional avalanches threatened the patrollers, but they were able to reach the victim and short-haul him away in a helicopter.

American avalanche professional and ex-pat Henry Schniewind, who runs Henry’s Avalanche Talks, happened to be in the area and watched the rescue unfold. Take a look at his website to get more details including pictures and video. (If you’re going to France, especially the Val d’Isère, Tignes, or Maribel areas, be sure to go to his local avalanche talks. Even if you’re not headed to France, but you do want to learn about avalanches and if you’re willing to spend a little money, his website has some excellent on-line avalanche education through his Ride Hard Ride Safe program.)

According to Henry some new snow had fallen the night before (and three days earlier) on the 600m-tall east face of the Face du Chavert. Just before noon a small point release [I am sure it was not the first] turned into a large and wet loose snow avalanche as it cascaded down the rocky faces and chutes. The slide swept one skier from a group of five over a cliff. Ski patrollers responded and “at least” 3 or 4 additional “big” avalanches spilled down the face. During one of these avalanches the rescuers had to race to the side to escape. Henry said the roar of the avalanches was impressive. Fortunately, the patrollers dodged the avalanches and were able to reach and evacuate the injured skier but at great risk.

A Short Editorial Comment
The group’s decision, which included a “guide”, to be where they were was wrong. Guide or no guide, it was the wrong time to ski the face. It should have been an easy decision not to go. Fresh snow, rocky and east-facing terrain, light freeze overnight and intense morning sun should have triggered alarm bells. By the way, the avalanche danger was rated “marque,” which is the same as “considerable” in the North American ratings.

Wet Snow – A Different Type of Avalanche Dragon
A wet, springtime avalanche is a different beast compared to its mid-winter, dry-snow cousin. Avalanche savant Bruce Tremper summarizes these differences in his excellent book, Staying Alive In Avalanche Terrain. He writes, “Wet avalanches are triggered differently, they move differently, they’re formed by different conditions, you forecast for them differently, their deposits are different, and the scars they leave on the vegetation are different.”

Bruce goes on to point out that when it comes to how avalanches start the main difference between dry and wet snow avalanches is that dry snow avalanches are caused by overloading the strength of buried weak layers, and wet avalanches are caused by decreasing the strength of buried weak layers. In general, the primary culprit for dry avalanches is the additional stress, or load, from the weight of more new snow, wind-drifted snow, or even the weight of a skier or snowmobiler. In wet snow often it’s thaw conditions that melts bonds between snow grains, which reducing the snow’s strength that’s the culprit.

Snakes and Avalanches
For rescuers this difference is profound. In dry snow, avalanches tend to act more like a snake. Both are secretive, stealthy, and many people might say both are unpredictable. Snakes and dry avalanches are solitary and are often silent. You can travel all day and not see or hear either, but to get bit, generally you have to provoke the snake or avalanche.

Bees and Avalanches
Wet avalanches, on the other hand, are more like a swarm of angry African bees out looking for trouble. Both bees and wet avalanches are overt, conspicuous, noisy, and often come in swarms. When you see one, you’re certain to see more. It’s this fact about wet avalanches that poses a serious threat to rescuers.

Is It Safe?
In dry snow you have to find the trouble; most avalanche accidents are caused because the victim (or a friend) triggered the slide by venturing into the avalanche starting zone. As long as conditions (winds, temperature, precipitation, etc.) stay the same and other triggers (people) can be kept away, rescuers can work on avalanche debris with little or no threat from additional avalanches. In many cases, rescues can be performed with remarkably little risk.

In wet snow more accidents are caused by naturally occurring avalanches where the victim was in the wrong place at the wrong time. (Certainly, education, experience, timing and safety margins can help keep one out of harm’s way.) More often the avalanche crashes down on the traveler who might not even be in the steep slopes of the starting zone. Unlike dry snow, when conditions (sunshine, temperatures, etc) stay the same with wet snow you can count on additional avalanches. When a natural wet avalanche has resulted in an accident, rescuers – either companions or professionals – will face the immediate threat of additional avalanches. Rescues in wet avalanches can be very dangerous affairs.

Next week: Part 2. What rescuers should do when dealing with wet snow avalanches.

*************

Any thoughts so far, or experiences to share?

Thanks for reading.
Dale Atkins
RECCO AB

This entry was posted in Accidents, Search and Rescue. Bookmark the permalink.

3 Responses to Wet Snow Avalanches And Rescue, Part 1

  1. Dan Moroz says:

    Dale ,very timely subject and timing. I would like to share a few thoughts on wet snow from observations I made over the years. First, I think differential creep between the warm sun influenced surface layers and mid pack layers that are still
    cold enough to have structure , plays a large roll in a cause and timing of wet snow avalanches. As the surface layers warm they become more plastic and “tug” on the interface between the warming layers and the colder more solid mid pack due to gravitational stresses pulling downhill. This stress can overload deep slab instabilities that may exist causing deep slab failure. This could be the reason behind finding dry and wet avalanche debris in the same run out zone. That is possible failure mechanism one.
    Mechanism two : As the surface snow transitions from “mash potatoes” that has great slab like characteristics to “7-11 slurpee slush” (perhaps less slab characteristics) the purcolation of water downward thru the slush maybe lubricate and destroy the crystal bonds between the warm deforming surface layer and the somewhat colder mid pack layers. This could lead to a surface wet slide that could trigger a deeper dry snow slide that has been previously stress from differential creep. Point release slides starting at rock bands then causing slab releases lower down the slope is a great example and commonly seen result. Depending on night time temperatures this process doesn’t really end at night fall. The energy absorbed and held by the surface snowpack continues to heat lower layers even though it has a melt freeze “cap layer” or new snow “cap layer” if a storm moves in.
    I have found that explosives on wet snow seemed to be most effective not when the snow’s surface was wet, but when the sun angle became less on the snow’s surface and it started to refreeze forming a layer of stastrugi or suncrust. This layer may be more effective and transmitting the acoustical shock wave over a larger surface area to cause failure then having all the explosives energy absorbed in one isolated spot in wet slush. There was a finite timeframe before the crust became too strong and held things together like a fiber glass “cap layer”

    My last obsevation (I have way too many so I’ll keep it to three!) Is that snow workers may not realize that the process of spring time warm up of the snowpack occurs not only from the sun surface downward, but also upward from the ground due to geologic heat. Often we find wet facets at the bottom of the snowpack while digging snowpits. I don’t think enough importance is given to this phenomena. I think a very strange thing happens stress wise within the snow pack around the time that heating from the ground surface meets up with heating from the top surface. The snowpack goes isothermal from two directions which may cause a region of instability mid pack as the two temperature regimes get closer together and/or meet. Vegatation and rocks also plays a key roll in this process as heat is introduced also horizontally further complicating the stress factors and weaknesses within the spring time snowpack. From obsevations of many wet slides and also photographs it seems that in the vast majority of fx lines of wet snow there is vegatation or rocks linking areas of the fx line. Also in the area of the entire slab that released sometimes it is noted that the release had a lot of vegatation within the lower layers of the slab failure or rock bands signifying a greater faceted bed surface indicating greater weakness. Once again I think vegatation (willows, sage, krumholtz, etc.) has a much greater influence on deep slab instabity especially in the springtime with wet snow avalanches and should have greater importance in stability evaluations.
    In closing there are many factors in determining spring time stability and the above thoughts are only a few that goes into the evaluationbut I feel are sometime overlooked.

  2. Dale Atkins says:

    Great points!! There’s a lot about wet avalanches that we don’t know. I am intrigued about your comment regarding explosives being more effective when the surface snow starts to cool down and crust up. Though I haven’t heard this in recent years, but years ago I remember some pretty savvy old-timers saying, “Watch out for the afternoon shadows!” Their experience told them that on the warm, sunny, and spring-type days that end-of-the-day shadows caused problems. We know thermal affects to snow take some time (it’s a great insulator); however, mechanical affects — when the snow surface quickly turns crusty can be immediate and significant. It’s very interesting to read your comments about your experience with explosives and this crucial time period.

    And, I would like to add “Thank you!” for the reminder that the snowcover warms in both directions: top down and bottom up. You may be on to something about the zone of instability that may develop as the warming happens, and I think two-direction warming may have another consequence for avalanche workers.

    It seems that when spring-time (thaw conditions) avalanche accidents happen, especially at ski areas or along highways, we (myself included) are often surprised by how fast conditions — snow become wet and lost strength — changed. I suspect that we are surprised because we often think of snow warming from the surface down. However, in reality and just as you say, the snow is already warm at the bottom with heat moving upwards at the same time heat energy is penetrating downwards. It’s sort of a head-on isothermal crash in the making. And head-on crashes always happen faster than anticipated.

    You’ve added some great points and please keep ’em coming. Thank you for sharing your knowledge and experience. I always learn something — and have for many years from our discussions. As I just mentioned, please keep ’em coming.
    Dale

  3. Dan Moroz says:

    Reread what I wrote and I meant to say firnspeigel not sastrugi when the snowpack refroze and the explosive mitigation window opened. 2 different items for sure!

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s