Snow Avalanche Activity, Upper Kahiltna Glacier
Alfred C. Pinchak, Case Institute of Technology
Introduction. The avalanche observations described in this work were obtained by members of the 1968 Smith-Nash Mount McKinley Expedition as they approached the south summit along the standard West Buttress route. By observing and recording snow avalanches on the surrounding slopes it was anticipated that with the help of statistical methods some definite statements could be made regarding the following:
1) correlation of the rate of avalanche frequency with time of day or
2) with air temperature, 3) independence of avalanches and 4) the distribution of waiting times between avalanches.
Because the observations of this report were limited both in time and location the conclusion developed here may not, in general, be applicable to other mountain regions.
Observational methods. Unfortunately, few avalanches were observed when the expedition was above 8600 feet on the Kahiltna Glacier. Thus the statistical study was limited to observations in the region from the Base Camp at 7300 feet on the southeast branch of the Kahiltna Glacier to 8600 feet on the main fork of the glacier. This region was observed from May 25 through May 29 as the expedition supplies were relayed up glacier. Observations were also taken during the return to Base Camp— June 13 and 14. Only two small snowstorms were encountered during the entire expedition, with most of the days being clear and bright.
Avalanches were observed on all the slopes bordering the aforementioned glaciers. In particular, Mount Hunter (14,570 feet) and Mount Crosson (12,800 feet) provided some avalanches of major proportions. The south-facing slope of a peak near Base Camp was extremely active with small cascades of ice, rock and snow. As these avalanches were much smaller than those on the surrounding peaks (and much more frequent) these data will be analyzed separately. The peak was informally named "Mount Ruth” by one of the expedition members. Data consist of time and approximate location of each avalanche. Air temperature was taken at frequent times during the day, but because of the high level of physical activity demanded by expedition tasks it was not feasible to set up 24 hour per day avalanche observations.
Observations and discussion. An Unusual Avalanche. At 1806 hours on the second day of the expedition (May 26) we observed a major avalanche on the north slopes of Mount Hunter. After dropping for 2000 feet this avalanche swept across the mile-wide valley and up the slopes on the other side (Figure 1). The avalanche crossed the valley in approximately 10 to 20 seconds and carried up the opposite valley wall. Because of the large distance covered and the speed of travel (roughly 300 feet/second) it is most likely that this avalanche was of the "airborne type.” A layer of air is trapped below such an avalanche and this greatly facilitates its movement (Shreve 1968). The avalanche cloud did not move across the valley with uniform velocity but cloud front velocity increased as it moved across the flat glacier surface. Because it required about five minutes to settle completely, it was concluded that the avalanche material consisted of finely pulverized snow particles. A simple trignometrical calculation indicated that the avalanche cloud was 500 to 600 feet high.
Distribution of Avalanche Frequency. All of the data, with the exception of that collected from "Mount Ruth,” were divided into even hour intervals and the number of avalanches in each hour were counted. A plot of relative frequency vs. number of avalanches/hour (Figure 2) shows nearly a Poisson distribution except for the few observations at 4, 5 and 6 avalanches/hour.
Waiting Time Distribution. If the waiting time distribution is calculated for a random process following Poisson statistics then an exponentially decreasing function should result (Maguire, et al., 1955) and these statistical techniques have been applied to icefall avalanches. (Pinchak 1968). Unfortunately a large number of avalanches are required to produce a "smooth” distribution curve of waiting times between avalanches and the number of avalanches observed in the general region was not sufficient to provide such a curve. However, the avalanche data from Mount Ruth gave a reasonable waiting time distribution as will be seen below.
Correlation of Avalanche Frequency with An Temperature. The air temperature range was divided into 5°F increments and the average number of avalanches observed for each temperature increment were computed (Figure 3)-Two items are of interest here: 1) the trend of increasing avalanche frequency with increasing air temperature and 2) the relative lack of avalanches in the 30°—35°F range. This second result is rather unexpected. As the air temperature passes through the freezing level, meltwater begins to form in the upper layer of the snow cover or meltwater present begins to freeze depending upon the direction of the air temperature change. Thus a "state of stress” is set up in the snow pack whenever the air temperature goes above or below the freezing level (32°F.) As a result, we would expect a relative increase in the avalanche rate for the 30-35°F temperature interval.
An alternative interpretation of Figure 3 would divide the air temperature range into just two regions: above and below freezing. Below freezing, the avalanche rate is nearly independent of air temperature. When the air temperature goes above freezing, the avalanche rate increases very markedly with temperature. The reasons for this behavior are not immediately clear. Yet it seems that the phenomenon of melting must be critically involved. But other factors also affect the rate of melting or the air temperature at which melting actually occurs. The air velocity and humidity and solar radiation are also of major importance in "setting the stage” for an avalanche. In view of the complexity and interaction of the various meterological parameters, it is surprising that Figure 3 is not just a random scatter of points.
Avalanche Activity on "Mount Ruth.” As mentioned previously a large number of small avalanches were observed on the south slopes of a small peak (10,450 feet) near Base Camp (7300 feet). As these avalanches were so numerous and relatively small they are being analyzed independently from the remainder of the data. These observations seem to indicate that the avalanche activity occurs in "flurries” with "resting periods” in between. This would support the contention that appropriate atmospheric conditions produce a morphological change in the slope stability. When critical conditions are reached the slopes avalanche and thus become stable again. If suitable weather conditions continue to persist, the slopes undergo additional change and the stage is set for another round of avalanches.
One interesting observation was made as a white-out moved up the valley to the Base Camp region and covered the slopes of Mount Ruth. The white-out cloud bank extended only to approximately 9500 feet. Thus the tall (12,000 feet +) peaks in the surrounding area were still in direct sunlight. Under these conditions the avalanche activity on Mount Ruth was very abruptly terminated while activity on the slopes of the large peaks increased. This effect supports the contention that solar radiation is a major factor in avalanche "priming.” Apparently the white-out shielded the slopes of Mount Ruth from solar radiation while radiation reflected from the upper surface of the white-out effectively increased the incident radiation flux on the slopes of the high peaks.