Experiments on the relative mobility of muddy subaqueous and subaerial debris flows, and their capacity to remobilize antecedent deposits

Subaqueous debris flows share many similarities with their subaerial counterparts, but also differ in striking ways. The present treatment is devoted to an experimental comparison of paired subaqueous and subaerial debris flows. The debris slurry consisted of 39% water, 25% kaolin, 24% silt and 12% sand by weight. The debris flows ran down a rectangular channel with a length of 10 m and a width of 0.20 m. In some of the runs, the channel bottom was 'hard' (i.e., a fixed, rough inerodible surface) and in others it was 'soft' (i.e., an antecedent debris-flow deposit over which the debris flow was allowed to run). On the 'hard' bottom, the subaqueous debris flows ran farther downslope than their subaerial counterparts with the same rheology. Their deposits also displayed extensional features not seen in the subaerial case, and were much thinner than that which would be expected based on yield strength. On the 'soft' bottom, the subaerial debris flows extensively remobilized antecedent debris-flow deposits, whereas the corresponding subaqueous debris flows ran over antecedent debris-flow deposits with no detectable remobilization. The reason for these differences is hypothesized to be the incorporation of a thin layer of ambient water underneath a subaqueous debris flow as its head hydroplanes slightly above the bed. The lubricating layer appears to prevent the transmission of shear stress between the two layers.