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Show Combination avalanches are made up of two or more types described above. Most avalanches of any size contain more than one kind of snow. A natural slab avalanche release, for instance, may be due to the weight of new dry snow deposited on top. This is a dry snow and slab combination. A slab weakened by heat or radiation generally has a damp layer at the surface which produces a damp snow and slab combination avalanche ( Figure 18). A slide may even change from one type to another during its run. The Argenta avalanche in Big Cottonwood Canyon in 1948, started at a high elevation as a dry snow and slab combination. After traveling a mile and a half, picking up heavier snow on the way and developing much internal friction as it squeezed itself through a series of gullies, it terminated as wet snow. The climax avalanche is a special combination type ( Figure 19). Because of the amount of snow involved, it is always of major size. It may be touched off by a direct action release, but delayed action elements are involved. The distinguishing characteristics of this type of avalanche are that it contains a large proportion of old snow and is due to conditions which have developed over a considerable period of time at least a month and possibly an entire season. The peculiar danger of climax avalanches is that they occur typically on slidepaths which are inactive in such volume for long periods. In an area under observation they may go unrecognized, although there are usually signs which the trained observer can detect. Climax avalanches occur infrequently because they require an unusual combination of favorable factors: The combination may be simple, such as a great accumulation of snow in an unusual place; at low elevations the Coal Pit slide in Little Cottonwood Canyon, Utah, for example; or because of freak winds the Gad Valley slide in Little Cottonwood. It may be complex, such as the build- up of numerous slab layers which fail to stabilize either by avalanching singly or fracturing and settling in place. An example is the Cardiff slide area near Alta in 1951 which had not run in climax fashion for at least 25 years. A climax situation was known to exist and was prevented nearby in the area of heavy use by frequent artificial stabilization. The great slide cycles of the northwest in 1910 illustrate a slightly different combination: abnormal accumulations of snow over an ice layer. Penetration of a climax fracture is always in great depth, usually to the ground. They travel farther and spread out wider than ordinary avalanches on the same slide path. Fortunately they usually occur under conditions which would be forecast as hazardous in any case. Because climax avalanches seldom occur, we do not have much background data for them. In an area under continuous observation, the possibilities can be recognized. In areas proposed for development or under only intermittent observation, the problem is much more difficult. To summarize: Slides which run annually are expected and a large amount of information is available on which to base recognition of the hazard - 39 - |