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 Quarternary pyroclastic rock test. vol. 0.74 x 10(6) m(3) ) slid from the western flank of Mount Cayley volcano in southwest British Columbia. The disintegrating rock mass entrained a further 0.20 x 10(6) m(3 ) 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, equival ent to a fahrboschung of 19 degrees. 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 th at travelled a further 2.6 km down Turbid Creek in a narrow channelised pat h to the Squamish River, temporarily blocking it. The motion of the rock av alanche, including the production of a distal debris flow, was successfully simulated using a dynamic analytical model. Both the results of this analy sis and field evidence indicate that the rock avalanche did not come to a h alt 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 ste ep slopes of dissected volcanoes and is one of seven high-velocity rock ava lanches that have occurred in the Garibaldi Volcanic Belt of southwest Brit ish Columbia since 1855. (C) 2001 Elsevier Science B.V. All rights reserved .