Feasibility Study and Technical Report - Pretivm

AVALANCHE FREQUENCYAvalanche frequency is the reciprocal of avalanche return period and is typically referredto as an order of magnitude ranging from 1:1 (annual) up to 1:300 (1 in 300) years.Each winter, the probability of an avalanche with a specified return period is constant.Avalanche frequency is dependent upon snow supply and terrain. Frequency decreaseswith distance downslope in the track and runout zone. Snow supply is determined by:• the frequency of snowfalls and amount of snow• the wind transport of snow into the starting zone.Snow and weather conditions vary from year to year; therefore, the frequency ofavalanches is not uniform.The primary terrain factors in avalanche formation are incline, slope orientation (aspect)with respect to wind and sun, slope configuration and size, and ground surfaceroughness. Slope configuration is important because features such as gullies will oftenhave more frequent and larger avalanches than open slopes. Ground roughnessdetermines the threshold snow depth for avalanches to occur, which is particularlyimportant in light snow climates where snow may not exceed threshold depths duringsome winters.AVALANCHE MAGNITUDEAvalanche magnitude relates to the destructive potential of an avalanche and is definedaccording to the Canadian avalanche size classification system. This classificationsystem is summarized in Table 18.3, which provides a general description of destructivepotential, magnitude, and typical path length.Table 18.3Canadian Classification System for Avalanche SizeSizeDestructive PotentialTypicalMass(t)TypicalPathLength(m)TypicalImpactPressures(kPa)1 Relatively harmless to people