ABSTRACT: Debris flows: some physical characteristics and behaviour

CGRG Bibliography of Canadian Geomorphology
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Abstract

The identification of steep ground that will be subject to debris flows is important to resource management in western Canada and the Pacific Northwestern United States. Such slope movements are initiated by natural processes, construction activities, or forest harvesting operations. Field observations of 449 debris flows in the Queen Charlotte Islands, British Columbia were made, and debris movement was classified according to seven characteristic site drainage types, depending on slope gradient, slope morphology (open slope, gully, or both) and frequency of the event (single or multiple). Initiation of events on open slopes is predicted by an infinite slope analysis, taking into account groundwater seepage. The location of the water table within the shallow soil profile is critical. The site drainage classes used are a good, indirect measure of this parameter. One-third of the events studies originate and end on an open, planar slope; the remaining two-thirds initiate on a gully headwall, a gully sidewall, or on an open slope and enter a gully. Slightly more than 50% of those flows within a gully reach the end and deposit on a fan or valley floor. Transportation and deposition of material in confined channels are governed primarily by water content of debris, channel gradient, and channel width. A width to gradient ratio summarized the influence of channel confinement on deposition. Examination of event profiles indicates that reaches with a width:gradient ratio <1 will likely experience erosion, and reaches with a ratio >1, are likely to experience deposition. The mean length of the depositional area is relatively short for events stopping on an open slope or in the upper reaches of a gully, being <50 m. Events stopping on the lower reaches of a gully are up to twice as long, and those stopping on a fan or valley floor are up to four times as long.

Bibliography of Canadian Geomorphology