Quantitative analysis of debris torrent hazards for design of remedial measures

Debris torrents, which are rapid flows of soil and organic debris down steep mountain channels, are a major natural hazard in many parts of British Columbia.A series of recent occurrences along the Squamish Highway, north of Vancouver, led the provincial government to initiate a systematic study of debris torrents in this area. This 2 year study involved hazard risk assessment and resulted in the design of a comprehensive system of remedial measures comprising debris retention barriers and basins, channel improvements and diversions, and the reconstruction of bridges. It was necessary in the course of this effort to formulate new design procedures covering the dynamic behaviour of debris torrents, as there was no accepted practical methodology suitable for Western Canadian conditions.This paper provides an approach to determining magnitude (volume of debris material involved), frequency, peak discharge, velocity, conditions for deposition, runout distance, behaviour in bends and run-up against barriers, and dynamic thrust and impact loadings. The procedures are based on some of the more practical concepts available in the specialized literature, supplemented by simple original theories and calibrated against several recent debris torrent events from British Columbia for which sufficiently detailed observational data exists. To be generally applicable, the procedures require a wider and more thorough calibration. They are presented here as working hypotheses that can be used as a guide to the assembly of additional data and provide a rational design tool to supplement the application of experience and judgement. Key words: debris torrents, debris flows, slope hazards, landslide dynamics, remedial measures, engineering design.