Dynamic analysis of an extraordinarily mobile rock avalanche in the Northwest Territories, Canada

The pre-historic rock avalanche at Avalanche Lake was a spectacularly mobile rock avalanche that resulted in the largest documented runup of any landslide on earth. The runout of the 200 Mm(3) event was a complex and three-dimensional process that created three distinct depositional lobes. There is some controversy as to whether the presence of glacial ice played an important role in the dynamics of this event. To investigate this hypothesis an advanced, three-dimensional numerical landslide runout model was used to reconstruct the dynamics of this event. It was found that a conventional runout model is able to reproduce the bulk characteristics of this event, including its spectacular runup, without accounting for glacial ice. A sensitivity analysis was performed to determine the factors that control the mobility of this event. It was found that low strength in the source zone, as well as the presence of significant internal strength, is required to reproduce the 600 m runup. This has important implications for the hazard analysis of rock avalanches. It appears as though large-volume rock avalanches can move with a friction angle lower than that expected for dry fragmented rock, and the runout process can be strongly influenced by internal strength. These important factors must be accounted for when performing forward analyses of this type of natural disaster.