The Avalanche Review, VOL. 3, NO. 5, MARCH 1985
Copyright © All Rights Reserved; AAA
Wet-slab avalanches: Certainly the experts in this type of avalanche are Northwesterners, uh, oh, and New Zealanders. For example, during a rainy period everyone feels that the only way to control for avalanche hazard is to close off the affected area. Explosives do not respond well in wet snow. Too much of the energy is absorbed within the liquid water and not enough is available to perturb buried weak layers.Wet-slabs do seem to respond to added shearing motion, however. For instance, one common triggering technique is to induce a loose avalanche above a suspected slab region by firing into rocks or loose snow. This will then flow over the lower elevation starting zone and help trigger the slab avalanche. Small wet-slabs respond to the swooping motion of a broad turning skier also. This is a more dangerous triggering mechanism, but no less effective.
To control for the expected spring avalanche cycle, control teams often place an entire case of explosives at the base of the snowpack. Sometimes it doesn't work. sometimes it works all too well. Chris Stethem (former snow safety director at Whistler Mountain, B.C., Canada and now a private avalanche consultant) sent me a fracture-line profile of a unusual wet-slab avalanche that occured in-area one year. They had been skiing on the slope all year and it had large moguls. This spring day they threw a one kg hand charge onto the slope and it broke out to the ground. Maybe there was just enough cohesion within the slab from the previous cool night to propagate the shock wave through the snowcover and perturb the basal depth boar that had finally broken down from penetrating liquid water.Soft-slab avalanches: There seems to be quite a variety of opinions about the triggering mechanisms that work best for potential avalanches with slab snow less the 250 kg/m3 density. Some say ski checking works best, other say explosives work best. First of all we need to get a few things straight: 1. Soft snow is compressible. Sometimes a skier's motion is simply absorbed in compaction and no energy is transmitted through the slab; 2. The delicate fabric of new, soft snow crystals are easily deformed. Often the energy imparted by an explosive is used up in rearranging the grain fabric before it gets a chance to spread through the slabstructure; 3. Over-snow blasts help to distribute the energy much more efficiently through the air; and 4. Blasting rocks helps to distribute the energy more efficiently through the ground. Rather than generalize about the problem, let's itemize.
Soft-slab avalanches formed within the present storm snow: Timing is critical. The characteristics of both the slab and the weak-layer change rapidly. Often if you let the snowpack lie as it is it will stabilize by itself over a few hours But, if you're concerned about its building up to a damaging size, it's best to initiate the small ones first.Every time the wind shifts or the temperature changes or the amount of water vapor available within the precipitating cloud changes, a new layer of snow is formed. A typical unstable slab structure formed within new snow is one where a thin layer of platelike crystals is deposited underneath a thicker layer of needles or stellars, or slightly rimed crystals. The needles and rime-free stellars change shape much more quickly than the rimed particles and require more critical timing. For instance new snow exhibits much more cohesion (like the pile of pick-up sticks or jacks you used to play with) during deposition. Skiing is tough and avalanche initiation is easy. After a few hours the delicate crystals begin to round and are no longer interlocking. They loose some of their cohesion. It's more difficult to transfer energy to the weak-layer and initiate avalanching. After a few more hours bonding between grains begins to take place. Cohesion increases and energy is more easily spread to parts of the structure. However, by this time, the weak-layer has probably already formed some bonds of its own. The shear strength has increased and the unstable slab structure has all but disappeared. Moral ...do control work during or immediately after a storm. This makes sense, and it sounds familiar too.
Because of the more stable thermodynamic nature of the little globules of rime and because they offer much more area for contact surfaces, rimed snow crystals form good, cohesive slab material to begin with and don't change as quickly as other, more delicate crystals. Timing is not quite as critical. However, be aware that rimed snow can build up to significant depths without stuffing off.Soft-slab avalanches formed over a smooth crust: The trick here is to disturb the adhesion between the crust and the new snow. Sometimes this adhesion is fairly strong so it is necessary to upset it somehow. The distributed energy of a skier, or an oversnow explosion works well.
Soft-slab avalanches formed over a weaklayer of faceted grains (surface hoar or grains recrystallized under a temperature gradient): The shear strength of these types of weak-layers is very, very small. Virtually any kind of shear load will be enough to undermine the entire basal pinning of the overlying slab.The inhibiting factor in this case seems to be the tensile strength of the slab-group. This is why triggering mechanisms are aimed at the maximum tensile zones, or snow pillows. These zones can either be near the top of the slope or down lower within the starting zone, depending upon the wind's strength during deposition of the slab material.
Ski cutting across the top of a slope is a lot easier on the ol'ticker than ski checking through the middle of a ripe slab. Therefore, when the snow pillows are near the top, ski cutting is effective. When they are low down on the slope, ski cutting above that region is less effective, and ski cutting the correct spot is suicidal. This is when explosives are handy. Plopping a charge into a snow pillow is a well established control procedure that works.Hard-slab avalanches formed over a thin layer of weak snow: Again the tensile strength of a hard-slab is high. Often they can even bridge across a slope and sound nearly hollow underneath. The unstable hard-slab structure is extremely persistent and often exasperating. The fracture area virtually needs to be sculpted out of the slab material. Two well placed bombs, one at the top, and one at the bottom can work. Claire Israelson (avalanche forecaster and park ranger in the Lake Louise area of Alberta, Canada) explained some of the hari-kari techniques they resort to on particularly stubborn hard slabs. When two bombs don't work, they find that a quick and watchful ski traverse across the toe region provides just enough of an added stress concentration for the slab to fracture and break out of the slope.
Hard-slab avalanches formed over thick, faceted weak-layers: Again these are persistent unstable structures. In these cases people often speak of "flexure waves" formed in the slab material by the collapsing weak-layer. Bob Brown (professor of engineering and engineering mechanics at Montana State University in Bozeman, Montana) has measured the increased tensile stress built up by these waves. Hal Hartman (snow safety director at Snowmass, Colorado) claims you can even see the flexing slab. Certainly the whooshing sounds of collapsing basal grains can be heard. The trick here is to perturb the weak layer low on the slope so that the collapsing process will continue upslope and build up greater flexural amplitudes. Reed Bahnson (former snow ranger and snow safety director of Mt. Alyeska, Alaksa and now a private avalanche consultant) often times his explosive charges. The first one is detonated at the down-slope edge of the starting zone to coincide with the arrival of the flexure wave. Hmmm, kind of funky, but it seems to work.
This is not the ultimate collection of triggering techniques. I am sure that there are many of you who have tried a number of different ideas based upon your own, unique snow conditions. I do believe that a collection of these methods is an excellent way to get just a little better idea of snow and avalanche mechnics.
The Avalanche Review, VOL. 3, NO. 5, MARCH 1985
Copyright © All Rights Reserved; AAA