Avalanche

An avalanche is a mass of snow, ice, and debris flowing and sliding rapidly down a steep slope (Colorado Natural Hazards Mitigation Plan, 2013, p. 3-138). An avalanche is defined in Colorado state statutes as a “geologic hazard.”

Snow avalanches occur in the high mountains of Colorado seasonally as the result of heavy snow accumulations on steep slopes. When the snow pack becomes unstable, it suddenly releases and rapidly descends downslope either over a wide area or concentrated in an avalanche track. Only part of an avalanche may release at once. Avalanches may reach speeds of up to 200 miles per hour and exert forces great enough to destroy structures and uproot or snap off large trees. They may be preceded by an "air blast," which is a strong rush of air that can measure over 100 mph and is capable of damaging buildings. They are more common with powder avalanches (where snow grains are largely suspended by fluid turbulence) that occur in parts of Colorado.

Avalanche paths consist of a starting zone, a track, and a runout zone. Generally, the runout zone is the critical area for land use decisions because of its otherwise attractive setting for development. Avalanche-prone lands may pass many winters or even decades without a serious avalanche. Lack of vegetation or a predominance of quick-growing aspen and low shrubs often characterize active portions of an avalanche track and the runout zone, readily identifying the area of seasonal peril.

Avalanches can be deadly. Over the past ten years, an average of 28 people per year have died from avalanches in the U.S. (Statistics and Reporting, n.d.b) Since records began to be collected in the 1970s, over 990 deaths have been reported as a result of avalanches.