The Rubble Creek landslide Garibaldi, British Columbia

Abstract

During the late winter of 1855-56 or early spring of 1856 about 33,000,000 cubic yards of volcanic rock slid from the high cliff known as The Barrier, near Garibaldi, B.C. This debris travelled down a rather sinuous path along Rubble Creek valley to its confluence with Cheakamus River about A miles from the Barrier and about 3400 feet lower. The initial material appears to have travelled as a high velocity tongue of debris which swept from one side of the valley to the other as the debris stream rounded curves eventually to be deposited on Rubble Creek fan. Velocities calculated from the superelevation of the debris as it rounded three different curves indicate that the debris was moving between 88 and 110 feet per second. A minimum velocity of 80 feet per second was calculated using the principle of conservation of energy where the debris overtopped a small hill at the apex of the fan. All of the trees in the path of this slide were uprooted and carried away. The trees adjacent to the slide were scarred and bruised by moving debris. The initial high velocity tongue was apparently followed by mud flows which deposited large rounded boulders and poorly sorted, volcanic debris on an area of the fan which was not covered by the initial slide. This material was apparently slow moving, as it piled up high on the uphill side of some trees which later died and fell across the top of the debris. Some xenolithologic debris cones similar to those found at Sherman Slide in Alaska and elsewhere also occur in the area of mudflow material. The slide deposit is formed of angular poorly sorted volcanic clasts weighing up to about 250 tons. The slide debris can be distinguished from underlying fan deposits by the lack of fine gravel and silt sized particles in the fan material. Deposits of debris similar to the debris of the 1856 slide, beneath some of the fan deposits, show that an earlier slide may have occurred. The mechanism which triggered the landslide is not known, but blockage of a subsurface drainage system, which drains the area behind The Barrier and escapes as springs at its toe, could have raised groundwater pressures enough to trigger the slide. In addition, as the area is one of recent volcanic activity a local earthquake may have been the immediate cause. In any event the underlying cause for the landslide was that the excessively steep and high cliff face of lava was apparently deposited against glacial ice, and subsequently, lost support when the ice melted. Studies using a scale-model of the topography of the area and bentonite slurries were carried out to find out if the movement of the 1856 slide could be modelled and if so, could the movement of possible future slides be predicted. Although no mathematical basis was developed for the modelling it is thought that if a material could be found which modelled the complex movement of the 1856 slide,future slides could also be modelled. Although modelling of the 1856 slide was not entirely successful several insights were given into the movement and deformation of prototype slides of the same type as Rubble Creek Slide. There has been at least one destructive slide in the area of Rubble Creek fan in the recent past and because it cannot be demonstrated that conditions have changed substantially since the 1856 slide it is only prudent to accept the possibility of the occurrence of another slide in the near future.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat