TRANSFORMATION OF WATER FLOODS TO DEBRIS FLOWS: LARGE-SCALE EXPERIMENTS

Six experiments at the USGS debris-flow flume yielded new insight to the dynamics of water floods transforming to debris flows. In each experiment 6-8 m³ of clear water was discharged as either an abrupt dam-break flood or steady flood lasting 60 s. Floods entered the head of a 2-m wide, 70-m-long rectangular channel with a rough concrete bed inclined 31° and lined almost uniformly with variable amounts (up to 17 m³) of moist, loose, silty, gravelly sand (porosity 0.4, internal friction angle 35-40°). The floods' sediment entrainment efficiency (defined as volume of sediment entrained / volume of sediment available) ranged from 0.61 to 0.99, and all floods transformed at least partly to debris flows. Entrainment efficiency decreased as flood duration increased, but did not decrease systematically as available sediment volume increased. Entrained sediment increased the resistance to flow but also increased the heights of bore fronts, thereby enabling the bores to move as rapidly as clear-water bores descending the same slope. Entrainment was not limited to bore fronts, however; electronic scour sensors located 33 and 66 m downslope from the flume head showed that entrainment peaked after flow stage peaked, and that scour propagated into the bed at rates ~5 cm/s. Remarkably, in all dam-break experiments, scour stripped a nearly uniform thickness of sediment from the entire bed, thereby preserving minor irregularities in the distribution of pre-flood bed sediment. In contrast, steady-flood experiments caused nonuniform erosion and sediment redistribution, and produced evolving, anastomosing channels that carried intermittent debris flows. All experimental flows formed depositional fans on the runout surface at the base of the flume, and all deposits showed features reminiscent of both debris-flow and fluvial sedimentation. Deposits of steady floods were thicker, more compact, and better sorted than deposits from dam-break floods.