Is Montana at Risk?
Identified Hazards for the State of Montana
Basic Disaster Information:
Avalanche
An avalanche is defined as a rapid, downslope movement of a mass of snow. An avalanche hazard develops whenever humans or their constructions are exposed to moving snow. Avalanches can be the cause of human deaths and injuries, damage to property, interruption of communications, and blockage of transportation routes. Avalanches occur throughout the mountains of Montana and, to a limited extent, elsewhere in the state. The avalanche hazard most directly threatens winter recreationists, homes and businesses in mountainous regions, and communication and transportation networks throughout the highlands.
Montana’s borders bracket a region in which many avalanches occur. Two of Montana’s ski areas, Bridger Bowl and Big Sky, are respectively the second and fourth most avalanche-prone ski resorts in the entire United States.
The two dominant factors causing an avalanche are an accumulation of snow and an inclined surface. Avalanches occur when the force of gravity pulling on the snowpack overcomes the frictional and cohesive forces holding the snow in place. Steeper slopes and greater masses of snow cause an increase in the effectiveness of gravitational force. Various physical characteristics within the snowpack affect the frictional and cohesive forces. The factors that contribute to this complex interaction of forces can be grouped in two categories: terrain and weather.
Terrain: If it is assumed that an accumulation of snow is possible anywhere in Montana, then we can evaluate the potential for hazard solely on the basis on terrain characteristics. The most important factor by far si terrain steepness. Wet snow avalanches can start on slopes of 20 degrees or less, but the optimum slope angle for avalanche starting zones is 25-45 degrees. Slopes steeper than 45 degrees will not normally retain enough snow to generate large avalanches, but they may produce small sluffs that trigger major avalanches on the slopes below. Therefore, all slopes of 20 degrees and greater should be considered as potential avalanche sites.
Other terrain factors affecting the magnitude and frequency of avalanche activity include slope length, shape, roughness, aspect and elevation. Slope length contributes to the force an avalanche can generate. Avalanche fatalities due to suffocation have, however, been reported on very short slopes. Slope shape and roughness contribute to the frictional forces that hold the snowpack in place. Concave and irregular slopes tend to be more stable than smooth, convex slopes. Boulders, shrubs, and trees add to slope roughness and therefore provide more stability than grass or smooth rock. Avalanches do, however, occur occasionally in heavily timbered areas. Slope elevation and aspect dictate the depth, temperature, an moisture characteristics of the snowpack that develop due to the interaction of the contributing weather factors.
Weather: Weather variables, especially snowfall, affect the timing and duration of hazardous snow avalanche conditions. Over 80% of the avalanche accidents reported in the United States have occurred during or within 24 hours after a storm. Snowfall amount, rate of accumulation, moisture content, and snow crystal types all contribute to the magnitude of unstable conditions.
Wind is another critical weather variable capable of depositing large amount of dense snow into starting zones. Wind direction during and after a storm will determine which slopes will receive the most snow and will therefore be most susceptible to avalanche. Strong winds are capable of creating an unstable load rapidly even when little or no new snow has fallen. Because the prevailing storm track in Montana is west to east, east facing leeward slopes often receive the most snow from wind loading.
Temperature is a weather factor that affects the physical nature of the snowpack. Low air temperatures over a thin, permeable snowpack cause a temperature gradient change weakening the snowpack and leading to an ongoing avalanche hazard as subsequent storms add to the snow load above this weak layer. This condition is most prevalent on north-facing slopes during winters with sever or prolonged low temperature events. High temperatures can also contribute to avalanche hazards by melting snow and reducing friction by increasing pore pressure in the snowpack. This condition develops best on slopes with southern exposure and the hazard is highest during the warmest part of the day.
The complex interaction of all these weather and terrain factors contributes to the location, size and timing of avalanches. In the absence of detailed scientific observation, any accumulation of snow on a slope steeper than 20 degrees should be considered a potential avalanche hazard.
Of the four major avalanche hazards, the interruption of communications lines probably occurs most frequently. Places of highest hazard included ski areas, mountain passes, and other areas where transmission lines cross avalanche paths. In regions where important highways or railroads cross areas subject to frequent snowslides, losses resulting from blocked roads, buried railroad tracks, and destroyed bridges can reach into the millions of dollars.
Many avalanche paths show recognizable signs of previous activity. The most obvious sign is a treeless swath down a forested mountian side ending in an irregular fan shaped clearing where the slope flattens out. Within the clearing there may be trees and other debris carried down by the avalanches. The trees that are still standing are battered and may have no branches on the uphill side. A new generation of trees in the slide path may help to estimate the time since the last destructive avalanche. The removal of trees by one avalanche clears the way for successive avalanches to extend the slide path further downslope.
Many potential avalanche paths do not show these signs fo previous avalanche activity. These paths may be in treeless or sparsely forested areas. Some of them may rarely avalanche because they are located where snow loading is uncommon due to elevation or aspect. Remember: if a slope exceeds 20 degrees, the potential exists for avalanches whenever an unstable snowpack accumulates. Since most avalanches are directly linked to storms, the occurrence of a winter storm is a good sign of the onset of an avalanche hazard. Larger storms will create a more severe hazard. High winds associated with snowfall are especially effective at loading snow in avalanche starting zones. Winds can often create an avalanche hazard without any associated snowfall by transporting and redepositing older snow onto avalanche paths. Masses of snow overhanging ridges or gullies indicate that wind loading has taken place on the slopes below, and these cornices may grow large enough to fall off and trigger avalanches.
The most certain sign of avalanche hazard is avalanche activity. Usually when one slope in hazardous, many of the nearby slopes are also expected nearby. The historical record or avalanche accidents cites numerous cases where rescue parties searchinf for avalanche victims themselves become victims of the same avalanche cycle.
Besides storms, wind and current avalanche activity, there are many more subtle signs fo developing snowpack instability which the trained observer can use to assess the current avalanche hazard. These signs include visible cracks in the snow surface and audible settling sounds that include visible cracks in the snow surface and audible settling sounds that indicate failure of a weak layer within the snowpack. By digging snow pits, the trained observer can make periodic close inspections fo the snowpack near starting zones to monitor the amount of snow accumulated and to estimate the cohesive strength of the various snow layers. By observing changes in the snowpack throughout the winter, the observer can often recognize the onset of instability. This improves hazard prediction beyond the simple techniques of assuming high avalanche hazard whenever there is a major storm or wind event.
