The rockslide and debris flow from Mount Cayley, B.C., in June 1984

Abstract

A major rockslide and debris flow occurred on Mount Cayley, British Columbia, in June 1984. Approximately 3.2 million cubic metres of volcanics travelled 2.0 km down Avalanche Creek at velocities up to 35 m/s to dam the confluence of Avalanche and Turbid creeks. The breaking of the landslide dam caused an extremely fast debris flow. The velocity of the debris flow and associated wind gusts, up to 34 m/s, caused superelevations, hurled rocks and wood through the air, uprooted trees, and spattered mud 16 m up trees. The debris flow removed the logging road bridge and road approaches at the mouth of Turbid Creek, blocked the Squamish River during surges, and introduced huge quantities of sediment to the Squamish River. The uniaxial compressive strength of wet tuff collected from the head scarp of the rock slide is 1.0–1.5 MPa, about two thirds of the strength of dry specimens, the friction angle on wet tuff bedding surfaces is 30°, and the slake durability index is 26%. Weak tuffs form the rupture surface of the 1984 rockslide on Mount Cayley, and their slaking contributed fines to the debris flow. The tuff, steep, narrow creeks intersecting at high angles, high precipitation, and snow and ice accumulation in creeks combine to make the western slopes of Mount Cayley extremely hazardous. Key words : rockslide, debris flow, volcano, landslide dam, Coast Mountains.