DEPOSITIONAL PROCESSES IN LARGE-SCALE DEBRIS-FLOW EXPERIMENTS

This study examines the depositional process and characteristics of de posits of large-scale experimental debris flows (to 15 m(3)) 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 degrees. Following release, experimental debris flows invariably deve loped numerous shallow (similar to 10 cm deep) surges. Sediment transp orted by surges accumulated abruptly on a 3 degrees runout slope at th e mouth of the flume. Deposits developed in a complex manner through a combination of shoving forward and shouldering aside previously depos ited debris and through progressive vertical accretion. Progressive ac cretion by the experimental flows is contrary to commonly assumed en m asse sedimentation by debris flows. Despite progressive sediment empla cement, deposits were composed of unstratified accumulations of genera lly unsorted debris; hence massively textured, poorly sorted debris-fl ow deposits are not emplaced uniquely en masse. The depositional proce ss was recorded mainly by deposit morphology and surface texture and w as not faithfully registered by interior sedimentary texture; homogene ous internal textures could be misinterpreted as the result of en mass e emplacement by a single surge. Deposition of sediment by similar, ye t separate, debris flows produced a homogenous, massively textured com posite deposit having little stratigraphic distinction. Similar deposi t characteristics and textures are observed in natural debris-flow dep osits. Experimental production of massively textured deposits by progr essive sediment accre tion limits interpretations that can be drawn fr om deposit characteristics and casts doubt on methods of estimating fl ow properties from deposit thickness or from relations between particl e size and bed thickness.