OCR Text |
Show of several minor depressions of gullies. A deeper gully with the same exposure but protected by scattered trees is not slabbed by wind from this quarter. Evidence from some observers that slab always forms under conditions of l#w temperature and high relative humidity has not thus far been confirmed by experience at Alta. The behaviour of windslab is better understood than the manner in which it forms. It is a brittle, unstable structure. Typically it is very hard, but it may be soft, an intermediate stage of development perhaps, and still retain its brittle quality. It has a very poor bond to the undersurface. Under an old slab there is generally an air gap since the slab cannot settle at the same rate as the general snow mass. If there is an air gap under the windslab, it is unquestionably in a condition of stress. Under the skis it has a hollow, drumlike sound. On gentle slopes it will fracture with long cracks ranking outward, and settle with a dull thud. On steep slopes it seems almost to explode, as if it were under tension. Our motion picture sequences of slab avalanches do not, however, confirm this explosive effect. They show the entire shield breaking loose at once and then fracturing instantly into blocks. Because of the erratic wind conditions at Alta, slabs do not form consistently on any particular slope or exposure. Neither are they so likely to be extensive or to be piled one on top of the other. Two of the permanent closed areas at Alta are favored slab locations. One, which overhangs the lift^served area, is now under control by explosives and has been removed from the danger list. Any one of the three types of avalanche already described can and frequently does occur in combination with either one or both of the others. The typical natural release slab avalanche, for example, is nearly always the result of new snow falling on a slab already in place. Although a great weight of snow may be involved, the false stability of slab may keep it in position indefinitely, one slab piling on another until finally a very minor increase in snow depth or any other disturbance sets off its chain Reaction. In one case at Alta, two inches of new snow started a slab avalanche cycle. A slide may change from one type to another during its descent. The Argenta avalanche of March 19^8, began as a slab-plus~dry snow combination. It traveled a mile and a half, picking up heavier snow on the way and developing much internal friction and weight as it squeezed itself through a series gullies. When it finally came to rest on the canyon floor it was damp. - 3 2 - |