...THE WHITE DRAGON'S WAKE...

TYPES OF          AVALANCHE 

THE MECHANISM OF AVALANCHE RELEASE 

FORECASTING SNOW AVALANCHES 

AVALANCHE CONTROL TECHNIQUES 

AVALANCHE HAZARD RATINGS 

AVALANCHE TRAINING FOR DOGS 

INFO ON THE PAGE OWNERS 

Most avalanches of dangerous size originate on slope angles between 30 degrees and 45 degrees. They seldom occur below 30 degrees and hardly ever below 25 degrees. Above 45 degrees to 50 degrees sluffs and small avalanches are common, but snow seldom accumulates to sufficient depths to generate large slides. Though internal metamorphism or stress development may sometimes initiate snow rupture, avalanches are often dislodged by external triggers. An overload of new snow may dislodge an existing slab. Falling cornices or chunks of snow from trees are common natural triggers, similar in action to the sunballs or snow wheels which frequently initiate wet slides. In the absence of external triggers; unstable snow may revert to stability with passage of time and no avalanche occurs. Artificial triggers in the form of mechanical disturbance may be intentionally introduced for control purposes. Unintentional triggers are a major cause of accidents; most skiers who fall victim to an avalanche trigger the slide which traps them.  

Slab avalanches fall when a well defined snow later breaks free and slides away. The sliding surface is usually the interface between distinguishable layers of snow which has been formed by variations in weather or snow deposition. In some cases the sliding surface may be the ground (entire snow cover avalanches). There often exists a lubricating layer of low shear strength which allows the slab to break free from the sliding surface. This lubricating layer may be generated by deposition of fragile crystal forms (e.g., surface hoar), internal metamorphism, or the intrusion of melt water. In some cases the lubricating layer is absent, instability being provided simply by a poor bond between snow slab and the sliding surface.  

  The primary instability develops when the component of force parallel to the slope due to the weight of the slab exceeds the shear strength of the bond to the underlayer (sliding surface). The situation is mechanically complex due to irregular attachment of the slab to stable snow or other anchorages at the head, toe,, and sides. In general., slab avalanche release occurs when one of these attachments is broken by a trigger; redistributed stresses then exceed the strength of the other bonds and an avalanche falls. Only part of the slab attachments may be weak, while others are strong. In this case a trigger will initiate fracturing in the snow but the slab remains in place and no avalanche falls. Snow settles as destructive metamorphism proceeds, and on an inclined surface it also creeps downhill under the influence of gravity by internal plastic deformation and slip on the ground. Creep velocity varies with temperature., snow type, snow depth, slope inclination and profile., and ground cover. These variations from one zone of the snow cover to another develop creep stresses. The zones of creep tension are favorable locations for slab avalanche fracture lines; these commonly occur on convex profiles or at the head of open slopes where the snow cover first finds anchorage (trees, ridge top, etc.). A snow slab under tension may not only break free when triggered but shatter into blocks as well when the stress is relieved. In hard snow of high tensile strength this release may achieve almost explosive violence. Creep stresses are in large measure responsible for the dangerous and unpredictable character of slab avalanches.