Skip to main content

Publications - Occasional Papers

Development of Methodology for Evaluation and Prediction of Avalanche Hazard in the San Juan Mountain Area of Southwestern Colorado

INSTAAR Occasional Paper 13

1975, 141 pp. (cost: $5)

This report covers research conducted by the San Juan Avalanche Project, Institute of Arctic and Alpine Research, University of Colorado for the period August 1973 to August 1974. The research is supported by a contract with the Bureau of Reclamation, U.S. Department of Interior, and has as its purpose the study of the nature and causes of snow avalanches in the San Juan Mountains of southwestern Colorado, and specifically within the area of Red•Mountain Pass, Molas Divide, and Coal Bank Hill. The ultimate objective of the project is to develop a methodology to evaluate and predict avalanche hazard within the study area in order to be able to accurately forecast avalanche occurrences. The project has undertaken the study of the problem of avalanche initiation by analysis of the complex relationship which exists among terrain, climate, and snow stratigraphy. When the project was initiated, only a limited amount of climatological data was available for the study area. It was recognized that an avalanche prediction model relies heavily upon data gathered from highly accurate, reliable instruments installed on carefully selected sites. Therefore, a network of fixed instrumentation is utilized to measure meteorological parameters, determine certain physical properties within the snowpack, and detect avalanche events. Data regarding certain meteorological and snowpack parameters, as well as that related to the accurate accounting of avalanche events, including the numerous descriptive parameters which catagorize each event are dependent on highly skilled and well trained field observers.

The primary snow study site is located at Red Mountain Pass (3400 m)and includes instrumentation providing such basic information as air temperature, temperatures within the snowpack, wind speed and direction, precipitation rate and amount, snow settlement rate, and net all-wave radiation at the snow surface. In addition, an isotopic profiling snow gauge provides snow density and water equivalent values throughout the snowpack at 1.0 cm intervals. Trip wires have been installed in the paths of frequently occurring avalanches in order to acquire accurate event times.

The initial objective of establishing a research procedure capable of adequately observing and recording the various phenomena associated with avalanche initiation was accomplished during the first winter’s research. The next step was to attempt to determine the relative contribution of each factor and to isolate those processes which contribute most directly to avalanche formation. During the next two winters’ research, considerable emphasis was given to the study of snow stratigraphy.

Detailed investigations into the physical properties of the snow within the study area were prompted by the fact that the San Juan Mountains exhibit climatic extremes not found in more northerly latitudes where most practical and scientific knowledge of snow avalanche formation has been accumulated. The combination of high altitude, low latitude, and predominately continental climate produces a specific radiation snow climate.

Generally, this condition is the result of two factors. First, the extreme nocturnal radiational cooling occurring on all exposures produces snowpack temperature gradients of a magnitude .sufficient to cause significant recrystallization or temperature-gradient metamorphism. The second factor is the substantial amount of solar energy available to south- and west-facing slopes. This daytime condition causes melt just below the surface and subsequent freeze-thaw crusts. These two situations continue to influence the snowcover throughout the winter and the resulting stratigraphy is highly complex.

Therefore, during the second winter’s research considerable emphasis was directed towards a better understanding of the snow stratigraphy within the study area through the acquisition of abundant snow pit data. These snow pits are of three types. One type is located at standard level snow study sites, a second type is located on a test slope or avalanche release zone, and the third is associated with the actual fracture line of an avalanche. During the third winter, this emphasis was sustained with particular attention directed towards the tempera ture-gradient process. Snow temperatures were measured throughout the depth of the snowcover on a daily basis at sites at three different elevations. Periodic snow pits at these sites provided data showing the relationship between the magnitude of the temperature gradient and type of subsequent metamorphism determining crystal morphology.

As part of the daily operational procedure, this project produces an “in-house” stability evaluation and avalanche hazard forecast for the study area. Such forecasts are made for each 24 hour period and at more frequent intervals during storms. Each avalanche occurrence forecast is evaluated the following day in terms of actual conditions and events subsequent to the initial forecast. During the third winter the avalanche forecast procedure was further refined and the trend towards forecasts for specific groups of paths, as well as a general area forecast continued. Methods employed by the field observers to evaluate numerous meteorological and snowcover parameters in order to produce an avalanche forecast were isolated and described. On the completion of the third winter’s data collection, work began on the development of a statistical model for the purpose of avalanche prediction.

Finally, the overall objective of the San Juan Avalanche Project continues to be to identify and analyse those processes which contribute most directly to the initiation of avalanches within the study area, to establish empirical relationships among these processes, and to expose these relationships to a detailed statistical analysis.

PDF (9 MB)