THE PERFECT DEBRIS FLOW: AGGREGATED RESULTS FROM 28 LARGE-SCALE EXPERIMENTS

Data collected during 28 controlled experiments reveal robust patterns as well as sources of variability in debris-flow dynamics. In each experiment ~10 m³ of unsorted, water-saturated sediment composed mostly of sand and gravel discharged from behind a gate, descended a steep, 82-m flume, and formed a deposit on a nearly horizontal runout surface. Experiment subsets were distinguished by differing basal boundary conditions (1 vs. 16 mm roughness heights) and sediment mud contents (1 vs. 7 percent dry weight). Aggregated sensor measurements of evolving flow thicknesses, basal normal stresses, and basal pore-fluid pressures demonstrate that debris flows in all subsets developed dilated, coarse-grained, high-friction snouts, followed by bodies of nearly liquefied, finer-grained debris. Mud increased flow mobility by maintaining high pore pressures in flow bodies, and bed roughness reduced flow speeds but not distances of flow runout. Roughness had this effect because it promoted flow agitation that enhanced grain-size segregation, thereby aiding growth of lateral levees that channelized flow. Grain-size segregation also contributed to development of ubiquitous roll waves, which had diverse amplitudes exhibiting fractal number-size distributions. Despite the presence of these waves and other sources of variability, the aggregated data have well-defined patterns that constrain individual terms in a depth-averaged debris-flow model. The data imply that local flow resistance evolved together with global flow dynamics, contradicting the hypothesis that any consistent rheology applied. We conclude that new evolution equations, not new rheologies, are needed to model the emergence of archetypical debris-flow behavior from the interactions of debris constituents.