Snow Avalanche Hazards and Mitigation in the United States (1990)

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SNOW AVALANCHE PROBLEMS 5 1 Snow Avalanche Problems This report by the Committee on Ground Failure Hazards Mitigation Research addresses the problems and mitigation issues concerning snow avalanche hazards in the United States. Other reports by the committee have considered problems due to landslides (National Research Council, 1985) and ground subsidence (National Research Council, in press). The present report is the first publication on snow avalanches by any National Research Council committee; therefore, it is essential to include both a general and historical perspective in order to provide sufficient background for discussion of current problems. This information is not available elsewhere through any single published source. The purpose of the report is to provide national, regional, and local governments; government agencies; and private decision makers with an overview of the snow avalanche situation in the United States and to outline steps that can be taken to minimize domestic avalanche problems. Four major points are emphasized: 1. Support for avalanche programs has diminished alarmingly at a time when increasing numbers of people are using mountain areas for recreation and commercial and other types of development are increasing in formerly remote areas. 2. The incidence of avalanche accidents is increasing and is expected to continue to increase in the future. 3. There is a lack of nationwide coordination, accepted standards, and effective information flow among those involved in avalanche mitigation. 4. There are no standardized procedures for avalanche control and equipment testing. Control techniques and equipment that use explosives have specific hazards and problems that must be addressed. Snow avalanches have caused natural disasters as long as mountainous areas have been inhabited. They are a common occurrence in mountainous terrain throughout the world, wherever snow is deposited on slopes steeper than about 20 to 30 degrees. In the United States, where avalanches are the most frequent form of lethal mass movement, avalanche hazard exists from the lower-elevation coastal mountain ranges to the higher mountains of the continental interior. 

SNOW AVALANCHE PROBLEMS 6 By definition a snow avalanche is simply snow moving rapidly down sloping terrain. A moving avalanche may also contain soil, rock, vegetation, or water, but by definition the initial failure that triggers an avalanche occurs within the snowpack or at the interface between snow and subjacent terrain. Avalanches range from a harmless trickle of loose snow descending to a new angle of repose to a huge and devastating mass of snow moving at high speed down a long steep slope, with enough energy to destroy everything in its path. It is important to know that, unlike other ground-failure hazards such as rockslides, which once released are spent, snow avalanches automatically “reload” with each snowfall and can “fire” several times in a given year. Small avalanches or sluffs run in uncounted numbers each winter, while larger avalanches, which may encompass slopes several kilometers wide and include millions of tons of snow, release infrequently but have the potential to inflict the greatest destruction. Avalanches of moderate size can damage structures and have the ability to bury, injure, and kill people. In the United States approximately 10,000 avalanches are reported each winter, with an estimated 10 to 100 times that number occurring unobserved or unreported (Armstrong and Williams, 1986). Terrain and weather patterns combine to determine the frequency of avalanche events. Large frequent snowstorms in combination with steep slopes will produce a high number of avalanches during a given winter season. Under ordinary circumstances, avalanches tend to run in the same location and down the same paths year after year, with the danger zones often becoming well known. However, exceptional weather conditions can produce avalanches that overrun their normal path boundaries or even create new paths where none existed for centuries (Fitzharris, 1981), as illustrated by the destruction in Switzerland of a 573-year-old stone building in 1957 (Friedl, 1974). Unusually high snowfall can provide short-lived but great hazard, in which even historically stable slopes may become dangerous (Figure 1). A factor in most avalanche releases is the presence of structural weaknesses, often induced by internal changes in snow cover. Hence, a large overburden of snow alone may not result in avalanching if it is internally strong and anchored to the layer below, but a shallow snow layer can slide from a mountainside if the snow is poorly bonded to the underlying material. Snow avalanches represent a complex problem in mechanical stability; thus, attempts to provide a better understanding of the phenomenon have focused primarily on the physical processes taking place within the constantly changing winter snow cover and the dependence of those processes on temperature and other meteorological factors. A hazard arises whenever property or human activity lies in the path of a potential avalanche. Snow avalanche hazard has been familiar to inhabitants of the European Alps and Scandinavia for many centuries, but it is a more recent problem in the United States. During the active period of gold and silver mining from 1880 to 1920, approximately 400 people were killed by avalanches in Colorado, many trapped within structures. More recently the primary hazard has been to individuals engaged in recreation activities, with deaths and injuries frequently occurring at some distance from developed facilities. Such events have the potential to affect the local economy of many mountain regions and to exert a significant effect on federally managed lands. U.S. citizens may also be endangered by avalanche hazards abroad. Those exposed to risk include not only Alpine recreationists (Vila, 1987) but also military personnel, as illustrated by the 1986 NATO exercise in Norway, during which 31 men were struck by a naturally released avalanche; 16 were killed and 11 injured (Kristensen, 1986). In a distinct 

SNOW AVALANCHE PROBLEMS 7 Figure 1 (a) An “attractive” potential development site in a century-old lodgepole pine forest on a fan beneath Deadman Gulch, Colorado Front Range, 1976. Small avalanches over previous several decades had been contained by adjacent gullies. (b) The same area in May 1984, showing the effect of a “100-year” dry snow avalanche. This avalanche far exceeded the boundaries of previously recorded events and destroyed many acres of the pine forest that had colonized in the runout zone for over a century. These photographs provide valuable before-and-after documentation of the “design avalanche,” the event magnitude that should be considered in land-use planning and design of exposed facilities. Because most avalanche paths have not recently produced an event of design magnitude, many planners and others tend to ignore or underestimate the potential avalanche threat. (Courtesy of A. Mears and Paula J. Lehr) 

SNOW AVALANCHE PROBLEMS 8 category are the military catastrophes of the Tyrol in World War I, where estimates of avalanche-caused fatalities ranged from 40,000 to 80,000 (Fraser, 1966). Avalanche danger is alleviated in three fundamental ways: by modifying the terrain, by modifying the snow cover, and by modifying human behavior. A number of engineering techniques have been used to divert or deflect moving snow from facilities; other techniques are used to prevent destructive avalanches from releasing. Reforestation provides a natural form of protection, but avalanche risk may substantially increase in the near future due to forests dying or deteriorating as a result of air pollution. The most common technique for reducing avalanche hazard is to artificially release potential avalanches at a selected safe time. This practice inhibits the formation of large avalanches by producing more frequent smaller ones. While this method may not provide as high a degree of protection as some terrain-modification techniques, it is less expensive in the short term; the technique is commonly used at ski areas and along highways and railroads. Avalanches are usually released by explosive charges, detonated on or near the snow surface close to the expected fracture point. Such charges are placed by hand or delivered to the slope using some form of artillery or mechanical conveyance. Because avalanches can affect winter vacationers, widespread public education about avalanches is of particular importance. Instruction on how to evaluate and avoid avalancheprone terrain and on rescue techniques is important for reducing hazards to downhill and cross-country skiers and snowmobilers. The highly mobile nature of these activities makes control with structures and explosives difficult. Centralized avalanche information and forecast centers such as those located in Colorado, Utah, Washington, and some other areas are an essential ingredient in avalanche education. In some cases land-use management and zoning can be used to protect the public in avalanche-threatened areas. Yet despite the increasing hazards posed by snow avalanches to mountain residents and tourists in the United States, there is no coordinated national program for avalanche mitigation. There is no recognized national leadership, no systematic means to improve understanding of avalanche processes or to improve mitigation procedures, and no adequate and comprehensive mechanism for information transfer and exchange.