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UBC Theses and Dissertations

Debris supply to torrent-prone channels on the east side of Howe Sound, British Columbia Dagg, Bruce Ronald


Debris torrents (channelized debris flows) are a geomorphological process only relatively recently recognised in southwest British Columbia. They are of interest both because of the tremendous amount of geomorphic work they do, and because of the hazards they pose to engineering works and residential developments. Fourteen torrents on the east side of Howe Sound, near Vancouver, since October 1981, have claimed twelve lives. Debris torrents differ from water floods in that they involve large amounts of coarse organic and inorganic debris. Therefore, a major requirement for torrent occurrence in a given channel is a supply of mobilizable debris. This thesis examines debris supply mechanisms and rates of debris supply in four small watersheds along Howe Sound, near the village of Lions Bay. An inventory of major debris sources has been compiled, and selected typical sites are examined in detail. Study methods include airphoto interpretation, ground surveying and reconnaissance, field instrumentation and site monitoring, dendrochronology, and materials sampling and testing. Debris supply is controlled by natural factors such as the nature and distribution of the bedrock and surficial materials, topographic gradient, vegetation, weather, and surface and groundwater hydrology, and by human activities such as logging and road construction. A wide variety of debris supply mechanisms operate in the study area, including rockfall and rockslide, talus shift, debris slide, soil wedge failure, ravelling, and snow avalanche. In addition to delivering debris to channel systems, some of these processes are capable of triggering debris torrents. Debris redistribution in channels occurs through debris torrents which do not reach the fans, fluvial processes (bedload transport), and snow avalanches. Active debris removal from main supply points, and storage elsewhere in the channel system, can decrease the frequency but increase the magnitude of torrent events in the basin. The wide variety of debris supply, debris redistribution, and torrent triggering mechanisms acting in this relatively small area points to a need for careful study of individual basins if the torrent potential in an area is to be understood. Regionally-based climatological or hydrological models of torrent occurrence should be employed for preliminary hazard assessment only.

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