Rock slide–debris avalanches: flow transformation and hummock formation, examples from British Columbia

Abstract

Rock slides quite commonly transform into flow-like landslides along their runout paths. At three initial rock slides in northern British Columbia, which occurred between 2002 and 2005, around 50–70% of the entire runout distance is composed of debris avalanche and debris flow deposits, which is comparable to other composite landslides around the world. Saturated ground conditions at the time of rock sliding make entrainment or undrained loading as agents of flow transformation from initially dry rock slides to partially saturated debris avalanches, respectively, fully saturated debris flows likely. Hummocks occur as clusters within the rock slide parts, whereas the flow-like depositions have more subdued morphologies. We show that late- or post-emplacement motion of individual hummocks is possible and can even divert in direction from the dominant landslide trajectory by responding to the underlying topographic gradient. Analyses of the well-preserved deposits suggest hummock formation above a basal sliding plane (low-angle normal fault) and along subordinate shear zones (high-angle normal faults) within a largely translational mass movement, thereby supporting the hypothesis of hummock formation proposed by Paguican et al. (2014). Pebbles on top of rock slide-debris avalanche boulders and on top of snapped-off trees record much higher dynamic debris flow heights. These and other features are not recorded on ancient landslides due to rapid erosion.