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A SNOW AVALANCHE is a mass of snow, Ice, and debris, flowing and sliding rapidly down a steep slope. Characteristics Snow avalanches occur in the high mountains of Colorado during the winter 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 ava I anche track. Avalanches reach speeds of up to 200 miles an hour and can exert forces great enough to destroy structures and uproot or snap off large trees. It may be preceded by an Ilalr blastll which also is capable of damaging buildings. Avalanche paths consist of a starting zone, a track, and a runout zone. In general 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. Only part of an avalanche starting zone may run, or several parts or all of an avalanche may release at once. 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, readi Iy identifying the seasonal peri I. Hundreds of snow avalanches happen each winter, most of them In remote places. Consequences Avalanches are extremely destructive due to the great impact forces of the rapidly moving snow and debris and the burial of areas in the runout zone. Structures not speCifically designed to withstand the Impacts are generally totally destroyed. Where avalanches cross highways, passing vehicles can be swept away, demol ished and their occupants ki lied. Cross country skiers, downhill skiers, and snowmobilers also are imper i I ed by snow ava I anches and severa I of the back country visitors perish each winter. Residences planned or erected in avalanche runout zones may not qual ify for 34 Snow Avalanches financing or insurance. i ~ , Aggravating Circumstances Man's actIvities frequently trigger avalanches and certainly man's activities create the hazard. The process only becomes a hazard when man interacts adverse I y with it. Where no structures exist or no recreational activity occurs, avalanches occur with no damage to structures or 1 ives being lost. Bui Iding construction in an avalanche path eventually may resu I tin the destr uct I on of property and the loss of I ife. Although most snow 51 ides are initiated by natural causes, sk I ers frequent I y tr I gger the sma II er ava I anches wh i ch take the i r lives by break I ng the snow sur face wh II e cross i n9 an area prone to "run". Ava I anches can a I so be triggered by sounds from shouts, machine noises, and sonic booms. Mitigation The cheapest and safest way to prevent property damage and save I ives is to stay out of avalanche paths and runout zones in winter. Methods of avalanche control include directional control of blowing and drifting snow by erecting snow fences to keep it away from the starting zone; planned release of small snowsl ides with explosives before the snow accumulation increases their destructive potential to unmanageable proportions; building snow sheds over particularly dangerous sections of rai lroad and highways. Sometimes diversion structures can divide an avalanche and minimize Its impact. Avalanche warnings are common in Colorado, but they do not remove the perl I, only alert one to it. 1 Land Use In general, land use within an avalanche area should not include buildings Intended for winter and early spring occupancy. Ordinari Iy, use of avalanche areas in the summer and fall constitute no hazard. In some cases, other hazards,