Debris flows in the southern Coast Mountains, British Columbia : dynamic behaviour and physical properties

Debris flows in the southern Coast Mountains exhibit different dynamic and sedimentologic characteristics, depending on the lithology of their source areas. Fine-textured debris flows originating in the Quaternary volcanic complexes are much more mobile than those originating in the coarse-textured plutonic rocks which form most of this mountain range. Mobility can be described as the velocity of flow, the distance of travel of debris flows, and the slope required to sustain flow. The objectives of this study are to examine the effect of different sediment composition on the mobility of debris flows, and to determine which rheologic models are most applicable for modeling debris flows in these geologic environments. About 25 debris flow events in or adjacent to the southern Coast Mountains were examined, ranging in volume from 10² m³ to over 10⁷ m³. Field methods included sampling of grain-size distribution, measurement of the deposit and channel dimensions, and observation of the stratigraphy of debris flow fans. Shear strength, permeability, and consolidation tests were performed on samples of reconstituted debris, representative of typical fine-textured and coarsetextured debris flows. These samples were also used to model debris flows in a flume. The coarse-textured, plutonic-source, debris flows typically had a distinct, inverselygraded, clast-supported, surface layer of cobbles and boulders. Their deposits tended to be irregular in thickness, with lobes and levees of coarse material. The fine-textured, volcanicsource, debris flows had no such surface layer, and their deposits were generally uniform in thickness and surface morphology. These observations, and corroborating evidence from the flume results, suggest that fine-textured debris flows behave according to the Bingham flow model, while coarse-textured debris flows can be better described by a granular, or dilatant, flow model. A clay content of about 4% in the matrix (sub-4 mm material) is a useful measure to distinguish the two populations. Several debris flow events of intermediate behaviour and sediment composition were also examined. The permeability of the debris, and hence its rate of consolidation, is an important factor controlling mobility. The volume of debris flow events was found to be the most significant factor controlling runout distance.