Disturbance assessment of recent large snow avalanche events in Colorado

Background/Question/Methods

During the winter of 2012-2013, several large destructive avalanches resulted in accidents with tragic outcomes for backcountry travelers in mountain locations across Colorado. Historic avalanches created fresh trimlines, widening existing avalanche paths by uprooting, stripping, and breaking trees. The snow slides left behind many downed trees and extensive vegetation damage that offer a unique opportunity to improve our local knowledge of avalanche frequency and magnitude. We have worked with the Colorado Avalanche Information Center staff to map vegetation characteristics in areas of recent large avalanche events. For each location, we gathered observations of past avalanche incidents, and used repeat photography to track changes in the avalanche path vegetation over time. Next, we used field measurements to survey vegetation disturbance, assess relative tree ages, and estimate maximum runout distances.  We also collected disc samples from uprooted, broken, and downed trees to determine the dates of past avalanche impact scars recorded in the woody plant tissue. 

Results/Conclusions

Avalanche signals detected in the discs of downed trees included impact scars from the moving snow and wind blast, development of reaction wood in response to tilting, approximate dates of tree establishment, and relative growth rates as recorded in the annual tree rings.  At each location we collected 30-40 disc samples from downed trees that ranged from 18 to 312 years in age.  For several paths the estimated number of large avalanche years doubled, based on analysis of tree samples. We found evidence of both synchrony and asynchrony in path histories. Large destructive avalanches were not unprecedented at these locations, but we also found evidence that historic events on individual paths did not always coincide with notable avalanche years in the surrounding landscape. For example, one avalanche destroyed trees that had survived on a slope for more than 300 years, widening the path by 50-70m, and depositing deep piles of debris. We used local snowpack, vegetation, and terrain characteristics as inputs in a 1-dimensional model, to estimate that the flowing snow was as high as 4.8m, and moving at up to 31.2m/s. The slide continued for 60 seconds, resulting in a mass of debris of 14,000 metric tons, with an estimated volume of 45,000m³. The estimated maximum impact pressure was 310Kpa, which is about three times the pressure needed to uproot a mature spruce tree. For Colorado, and perhaps elsewhere, we conclude that vegetation ecology methods can be used to map avalanche frequency and magnitude.