ABSTRACT: Debris flows and debris avalanches triggered by rock slope failure.

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Abstract

Debris flows and debris avalanches triggered by rock slope failure can be very destructive. In these types of landslides the initial failure volume may increase up to ten times by entrainment of material along the landslide path. This has important effects with respect to velocity and run-out distance. The best known cases of these types of mass movement are the 1962 and 1970 events in the Rio Santa valley, Cordillera Blanca, Peru. In these events falls, involving rock and glacier ice, from the North Peak of Huascaran were transformed into high-velocity debris avalanches/debris flows which swept over the towns of Ranrahirca (1962) and Yungay (1970) with a total loss of life in both events in excess of 30,000 people. Our analysis of these events shows them to be quite similar to the extent that the 1962 event presaged the 1970 event to a remarkable degree. Both events entrained a considerable volume of snow in their paths and also entrained significant volumes of morainic material from the valley-side slopes below Huascaran. In doing so, both events were transformed into high velocity debris flows, parts of which reached the Pacific Ocean at a distance of 160 km from their source. We also analyze transformational behaviour in broadly similar examples from the Coast Mountains of British Columbia. Examples include events at Jane Camp (1915), Pandemonium Creek (1959), Nomash River (1999), Glacier Bay (1999) and Legate Creek (2007). In all these cases, a landslide involving a relatively small initial volume of bedrock was transformed into a high velocity debris avalanche/debris flow of increased volume, by the entrainment of colluvial material from slopes beneath the source rock slope. We note that the hazard assessment of these types of failures is complicated by the difficulty of estimating the probability of occurrence and the difficulty of predicting the transformational behaviour involved in the movements. In the first instance, magnitude/frequency relations based on the occurrence of initial rockslope failure mask the magnitude and frequency of the resulting enlarged flow. In the second instance, transformational behaviour is dependent on the susceptibility of material to entrainment and its subsequent ability to decrease the resistance of the moving mass through some form of fluidization. Debris flows and debris avalanches triggered by rock slope failure are a major geohazard in mountain regions of the world and pose a major challenge in landslide hazard assessment.

Bibliography of Canadian Geomorphology