FACTORS CONTROLLING DEBRIS AVALANCHE INITIATION

Heavy rainfall during a January 1983 storm triggered numerous debris avalanches and associated debris torrents in the Smith Creek basin, western Whatcom County, Washington, US. Four classes of debris avalanches are recognized: wedges, drainage depressions, logging roads, and discontinuity surfaces. Nine different debris avalanche headscarps that are representative of these 4 classes are studied in detail. The geometric configuration of each headscarp, the properties of soils comprising the headscarps, and water tables based on numerical models of hillslope hydrology are used in limit equilibrium slope stability analyses to back-calculate root cohesion (Cr) values at failure. The calculated values are consistent with those reported by others. The conclusions of this study are based on modeling results, which are only verified by post-failure observations at the avalanche headscarps. Direct field measurement of the hydrologic and shear strength parameters is required before the modeling results and conclusions drawn can be confirmed. -from Authors