Measurements and weather observations at persistent deep slab avalanches

Persistent deep slab avalanches are generally hard to forecast and can release under diverse conditions ranging from storms to clear days to locally induced stress on the snowpack. For the formation of many natural avalanches, a point is reached where the mass loading of overlying snow exceeds the mechanical properties of the weak layer. This can occur from additional loading above the weak layer, such as from precipitation or wind loading. Furthermore, natural failure can occur from solar warming and temperature variations. External stresses applied to the snowpack from skiers, snowmobilers, and other forces can also trigger deep slab avalanches. We accessed and made field measurements at 41 persistent deep slab avalanches that released after the avalanche cycle of the respective persistent weak layer. These measurements included the properties of the failure layers, slab load, and preceding weather to determine trends and correlations between such variables. The failure planes were analyzed using the Deep Tap test (DT), Propagation Saw Test (PST), shear frame tests, and hand hardness. The overlying loads were calculated using density measurements. Spatial variability across the crowns was also assessed by the use of multiple profiles and tests. Precipitation, temperature, and wind data were obtained from the nearest automatic weather station to the respective avalanche start zone. DTs often yielded sudden (Q1) fractures and the cut length in PSTs was usually less than 60% of the column length when the fracture propagated to the end. Tests were repeated at multiple sites for a few deep slab avalanches and they indicated that DT taps and shear strength of the failure layer tended to increase with crown height. Crown height typically varied substantially at each deep slab avalanche. Preceding weather prior to the accessed deep slab avalanches showed an increase in precipitation and change in temperature for some releases and little change in wind speed and direction for many releases.