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\L Snow Avalanches A SNOW AVALANCHE Is a mass of snow, ice, and debris, flowing and 51 idlng rapidly down a steep slope. Characteristics Snow ava I anches occur in the high mounta i os of Co I crado 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 avalanche track. Avalanches reach speeds of up to 200 ml las an hour and can exert forces great enough to destroy structures and uproot or snap off large trees. It may be preceded by an Ilair blast" which also 15 capable of damaging bui Idings. 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 sett I n9 for dave I opment. Ava I anche-prone I ands may pass many winters or even decades wrthout a serious avalanche. Only part of an avalanche starting zone may run, or several parts or a I I of an ava I anche may re I ease 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, readily identifying the seasonal per it. Hundreds of snow ava I anches happen each'll inter, 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 specl fically designed to withstand the impacts are generally totally destroyed. Where avalanches cross highways, passing vehicles can be swept away, demolished and their occupants ki lied. Cross country skiers, downhill skiers, and snowmobilers also are Imperl led by snow avalanches and several of the back country visitors perish each winter. Residences planned or erected In ava I anche runout zones may not qua I' fy for 34 financing or insurance. 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 adversely with it. Where no structures exist or no recreational activity occurs, avalanches occur with no damage to structures or I ives being lost. Building construction in an avalanche path eventually may result In the destruction of property and the loss of life. Although most snow slides are initiated by natural causes, skiers frequently trigger the smaller avalanches which take their lives by breaking the snow surface while crossing an area prone to "runll. Avalanches can also be triggered by sounds from shouts, machine noises, and sonic booms. Mitigation The cheapest and safest way to prevent property damage and save I !ves 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 sma II snows I ides with exp I os i ves before the snow accumu I at i on increases their destructive potential to unmanageable proport ions; bu i I ding snow sheds over part i cu I ar I y dangerous sections of railroad 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 per ii, on I y a I art one to it. Land Use In general, land use within an avalanche area should not include buildings Intended for winter and early spring occupancy. Ordlnari Iy, use of avalanche areas in the summer and fall constitute no hazard. In some cases, other hazards,