Depositional processes in large-scale debris-flow experiments

This study examines the depositional process and characteristics of deposits of large-scale experimental debris flows(to 15 m3) composed of mixtures of gravel (to 32 mm), sand, and mud. The experiments were performed using a 95-m-long, 2-m-wide debris-flow flume that slopes 31°. Following release, experimental debris flows invariably developednumerous shallow (-10 cm deep) surges. Sediment transported by surges accumulated abruptly on a 3° runout slopeat the mouth of the flume. Deposits developed in a complex manner through a combination of shoving forward andshouldering aside previously deposited debris and through progressive vertical accretion. Progressive accretion by theexperimental flows is contrary to commonly assumed en masse sedimentation by debris flows. Despite progressivesediment emplacement, deposits were composed of unstratified accumulations of generally unsorted debris; hencemassively textured, poorly sorted debris-flow deposits are not emplaced uniquely en masse. The depositional processwas recorded mainly by deposit morphology and surface texture and was not faithfully registered by interior sedimen-tary texture; homogeneous internal textures could be misinterpreted as the result of en masse emplacement by asingle surge. Deposition of sediment by similar, yet separate, debris flows produced a homogenous, massively texturedcomposite deposit having little stratigraphic distinction. Similar deposit characteristics and textures are observed innatural debris-flow deposits. Experimental production of massively textured deposits by progressive sediment accre-tion limits interpretations that can be drawn from deposit characteristics and casts doubt on methods of estimatingflow properties from deposit thickness or from relations between particle size and bed thickness.