Debris flows: some physical characteristics and behaviour

Abstract

Field observations on 449 debris flows in the Queen Charlotte Islands, British Columbia, are summarized. Movement of debris is classified according to seven characteristic types designated for the purposes of the study. Data on the physical characteristics of the events are presented. An analysis of the data is made with reference to event initiation, yield, and deposition using both mechanistic and morphological criteria. For those events which initiate on an open slope, the infinite slope model is used to establish a relationship between field drainage class and slope angle, for assumed frictional and cohesive soil strength parameters. The field drainage classes are directly related to the postulated location of the groundwater surface within the depth of the rupture plane. Deposition of channelized events is found to be influenced by the ratio of channel width to channel gradient, with the onset of deposition expected when the ratio exceeds unity. Five classes of depositional area are proposed. The magnitude of debris material deposited by an event is described by a channel debris yield rate which shows considerable variation with each event type. The conclusions of this study are based on postevent field measurements of deposited and eroded debris material. A limitation of the study arises from the difficulty of accurately describing an event some years after it has taken place. Key words : slope stability, debris flow, infinite slope analysis, debris yield.