Abstract

Persistent slab avalanches remain a challenge for backcountry avalanche risk management. Of the 222 fatal avalanche accidents in Canada with known weak layer grain type information since the winter of 2000, 56% involved FC and 29% SH layers. The median weak layer ages of these avalanches were 33 days for FC and 14 days for SH layers. Building on existing research on typical slope angles of avalanche release, our analysis of several long-term datasets provides a new perspective into the evolution of persistent slab avalanche problems and where accidental avalanches are triggered. The identified patterns offer practical insight into the characteristics of hazardous terrain in the evolution of persistent slab avalanche problems. A better understanding of incident patterns related to persistent slab avalanche problems can help practitioners better understand how to avoid these challenges. The nature of very low angle avalanches and lingering deep persistent avalanches on moderate terrain as statistical outliers coupled with the possibility to have long years of experience without encountering them causes them to be disregarded in education and snow safety programs alike. In spite of their infrequent occurrence, these avalanches are responsible for some very high profile accidents that disproportionately impact the guiding profession and the guides involved. This study shows that lingering deep persistent avalanches tend to persist on large moderate angled slopes; they do not occur only in steep terrain. These avalanches will always pose a dilemma for risk management, but recognizing the conditions when they are more likely to occur and that they can occur on lower
than expected slope angles allows us to make decisions on the basis of their possibility, not their probability.

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