Snow Avalanche Hazards and Mitigation in the United States (1990) / Chapter Skim
Currently Skimming:
2. The Avalanche Phnomenon
2. The Avalanche Phnomenon
Pages 9-19
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From page 9... ...
Falls and sometimes topples characterize the failure of snow cornices and the release of snow from building roofs (Paine and Bruch, 1986; Taylor, 1985) , but typically the initial failure mechanism of the snow cover is translational sliding, utilizing a sloping surface of weakness within the snow cover or at the ground-snow interface.
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From page 10... ...
l (soil reep) Combination of two or more principal types of movement Snow Snow fall Snow topple Snow slump Snow block slide Many Units Snow slide Snow spread Snow flow (snow creep)
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From page 11... ...
Point releases usually involve small volumes of snow and can be predicted without much difficulty, so they generally present only a small degree of hazard. In contrast, slab avalanches initiated within cohesive snow cover on slopes steeper than 25 degrees provide most of the avalanche hazards and are the primary focus of defense and control measures.
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From page 12... ...
Slab avalanches present a significant hazard due to this difficulty of prediction, in addition to their potential for release over large areas. Escape from these avalanches can be difficult or impossible (see frontispiece)
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From page 13... ...
In contrast, slab release occurs when a cohesive cover of snow rests above a layer of lesser strength along which the eventual sliding failure occurs, when shear stress exceeds shear resistance. Slab release typically results from a complex series of events, often originating within a snow cover creeping downsIope (McClung, 1987~.
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From page 14... ...
Between 1980 and 1985, Alaska recorded 441 events affecting people; in those events 27S persons are known to have been trapped, injured, or killed If. ~ Fredston, Alaska Mountain Safety Center, written communication, 1986~.
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From page 15... ...
~ Fredston, Alaska Mountain Safety Center, personal communication, 1986~. Frequent avalanche activity threatens transportation corridors along numerous yearround highways and railroads in such areas as Washington state; the Alaskan coastal region; California's Sierra Nevada; and canyons in Utah, near Jackson Hole, Wyoming, central and western Colorado, and western Montana.
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From page 16... ...
State highway personnel want to restrict logging, which is carried out under government permit, but the decisions involve the State Department of Natural Resources, the land-owning railroad company, and the contract logging firm. Communications and decision making are hampered by the lack of established guidelines for timber management in avalanche hazard zones and lack of guidance from state or federal agencies with competence in avalanche mitigation.
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From page 17... ...
Private property owners have mainly incurred ex-post costs with a major avalanche potentially resulting in financial ruin for affected individuals because of the unavailability of avalanche insurance or other means of spreading the costs of damage. With the recent implementation of avalanche hazard zoning ordinances by some county and municipal governments, ex-anti costs in the form of mapping and additional planning and construction, as well as indirect economic losses from property devaluation, are also beginning to be incurred by private home and property owners.
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From page 18... ...
However, this figure does not include salary costs for personnel employed directly in avalanche control nor the costs of plowing, snow removal, or avalanche control on Cayuse Pass, Chinook Pass, and Washington Pass, which are normally closed during the winter season but require considerable labor and equipment to clear avalanche debris prior to closing and upon reopening (Wilbour, 1986; cf. Sherretz and Loehr, 1983~.
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From page 19... ...
This estimate reflects the actual costs of known damage resulting from known avalanche events affecting bridges, buildings, vehicles, power lines, railroads, highways, and miscellaneous structures. Again, however, it does not account for snow removal from highways and railroads or for the cost of avalanche rescues or commercial losses due to delays.
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