The Avalanche Review, VOL. 10, NO. 5, MARCH 1992
Copyright © All Rights Reserved; AAA


by Andre Roch

The first thing to say is that, when an avalanche occurs, it means that the situation is dangerous. The second thing to say is that, after the avalanche has occurred, it is easy to say that of course there was a danger.

Simplifying the conditions, the situation at Alpine Meadows in 1982 was as follows: There was a thickness of three meters of wet winter snow. On top two meters of snow accumulated during four days of storm, giving a total thickness of 5m (or about 15 ft) of snow cover. The runs were closed throughout the storm, and so was the parking lot.

The Forest Service set off explosions on the mountain on the first and second days. On the second day, the explosions brought down a surface avalanche on the slope dominating the parking lot. I thought that it was a good sign because it meant that the bottom layers were stable.

On the third day, there was no blasting in spite of the fact that the storm was still going on. During the fourth day, the blasting took place in the morning until nine o'clock. The slope above the parking lot was bombarded across its whole width and at different levels (at least two levels) without effect. Then the parking lot was no longer restricted.

At five o'clock in the afternoon, the whole slope started avalanching. A great mass of snow covered the parking lot, killing two people. It slid still farther, through the flat clearings in the forest.

When I saw the slope for the first time, I laughed and could not believe that an avalanche there could slide so far and cause so much damage. The slope is in fact not very steep; it is broad but not very long. I talked to Nick Kindschi from Davos [Switzerland], who had been chief of security for the Parsen area. He had the same impression as I had had, that this situation could not have become dangerous in the Alps. But! This was of course in California, near the Pacific, where the snow accumulation can be enormous. When snow 15 feet thick gets moving, it is such a great mass that nothing can stop it. This is the first point.

The second point is: How can an avalanche start suddenly on the fourth day when it was blasted for the three previous days and the blasting was not only at one point but all over the slope? The best explanation I can think of is the one I tried to tell the jury, with the reservation that I could not imagine any other cause which could have started the slide after the continuous blasting: snow creep. Snow creeps continually on a slope. On a steeper part of the slope it creeps faster, then some tension occurs. If the bottom layers are anchored on some rocks, there is a compression zone above and a tension zone below. If the whole width of a slope is thus under traction, when a snow layer breaks at one place, the rupture will propagate itself across the whole slope and the avalanche starts.

The next question is: Why had the blasting not started the avalanche? I suppose that the explosions were more or less swallowed inside the wet snow layers. There is still another possibility that I did not mention: when between layers a differential creeping occurs (i.e., when some layers slide faster than others), it may rub the crystals against each other, diminishing their friction force and destroying slowly the force of bond between them. Even the wetness of the snow may diminish the resistance of the layers, which may suddenly start to slide.

It is not a mystery but it is complex. My life experience tells me that if one wants to avoid avalanche danger, one should stay in bed-and make sure that the house is not in an avalanche path.

Andre Roch is an avalanche consultant in Geneva, Switzerland. In 1985 he was an expert witness for the defense in the Alpine Meadows trial.

The Avalanche Review, VOL. 10, NO. 5, MARCH 1992
Copyright © All Rights Reserved; AAA