Table I gives results from a recent study (Reference Perla,Perla, 1977) of 205 cases of snow-slab failure observed in Switzerland, U.S.A., Canada, and Japan. With reference to Figure 1 , the parameters of Table I are explained as follows:
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θ, inclination of the bed-surface, typically measured immediately below the crown,
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h, slab thickness, measured at the crown centre,
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ρ, mean slab density over the thickness h,
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Bxz, the approximate shear stress at the bed-surface prior to failure, computed from ρgh sin θ,
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ρB, density at the bed surface, typically measured by weighing a 50 mm diameter sample,
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T B, temperature at the bed surface, typically measured in a pit excavated back into the crown.
Slab avalanche nomenclature and coordinate system.
Summary of Slab Measurements
The statistics of Table I are probably biased by two factors: First, the samples were taken on accessible slopes; and second, on a given sample day, the observer tended to sample the thicker slabs as opposed to smaller, more innocuous or less spectacular slabs. As a consequence of these factors, it is likely that the sample mean of θ is shifted to a lower value than the population mean, and it is almost certain that the sample mean of h is shifted toward a higher value. A third bias is that wet slabs were not included in proportion to their occurrence since the observations were mostly taken during winter and early spring; only 7 of the 205 observations were taken after 31 March. Thus, the sample mean of T B is probably shifted to a colder value than the population mean. Lastly, it is possible that 50 mm density samples sometimes over-estimate ρB, especially when failure occurs in a thin, weak layer.
Assuming that the slabs fail at stress Bxz it is possible to find an approximate relationship for the shear strength of bed-surface layers versus bed-surface density ρ B. Figure 2 is a scatter plot of 72 observations of Bxz versus ρB. A power-law fit (r= 0.81) to the data of Figure 2 is
Equation (1) is subject to the possible bias in ρB. Perhaps a curve-fit based on data obtained with thin, oval density tubes would show an increase in the exponent (larger than 2.06).
Shear stress at bed surface Bxz versus bed surface density P B, 72 cases.
Taking into account the above biases and limitations of the study, it is nevertheless possible to conclude :
Less than 1% of all slab avalanches initiate where the slope angle is less than 25°.
Less than 5% of all slab avalanches initiate where the slope angle is less than 30°.
The average shear stress at the bed surface prior to failure is in the range of 102N m-2 to 104N m-2.
The most prevalent bed-surface temperature is in the band -5°C to 0°C.
Over 95% of all slab avalanches have a bed-surface temperature of -10°C or warmer.
The shear strength at the bed surface has a large variance as a function of density, but on the average varies approximately as the square of the density.
