Dynamics of the 1984 rock avalanche and associated distal debris flow on Mount Cayley, British Columbia, Canada; implications for landslide hazard assessment on dissected volcanoes

In 1984 a mass of Quaternary pyroclastic rock (est. vol. 0.74×106m3) slid from the western flank of Mount Cayley volcano in southwest British Columbia. The disintegrating rock mass entrained a further 0.20×106 m3 and formed a rock avalanche that travelled a horizontal distance of 3.46 km from its source over a vertical elevation difference of 1.18 km, equivalent to a fahrbِschung of 19°. From the superelevation of the debris trimline in the mid-path, it is estimated that velocities reached at least 42 m/s; in the upper part of its path velocities may have approached 70 m/s. The rock avalanche was partially transformed into a distal debris flow that travelled a further 2.6 km down Turbid Creek in a narrow channelised path to the Squamish River, temporarily blocking it. The motion of the rock avalanche, including the production of a distal debris flow, was successfully simulated using a dynamic analytical model. Both the results of this analysis and field evidence indicate that the rock avalanche did not come to a halt in the upper part of its path as suggested by Cruden and Lu (1992), but travelled to its distal limit in one uninterrupted movement. This finding has important implications for landslide hazard assessment at Mount Cayley and similar sites. The landslide is typical of those which occur on the steep slopes of dissected volcanoes and is one of seven high-velocity rock avalanches that have occurred in the Garibaldi Volcanic Belt of southwest British Columbia since 1855.