ABSTRACT
Between 1981 and 1983 field measurements of dynamic and physical properties of channelized debris flows were obtained at two sites at Mount St. Helens. Motion picture photography, time lapse photography, acoustic rangefinding, timing drift, and hand sampling of flows were used to obtain velocity, depth, and compositional data for 10 debris flows.
All flows observed moved as surges downchannel, having a steep, bouldery flow fronts that impeded the flow of the more fluid slurry behind the fronts. Flows ranged in magnitude from about 1 to about 50 m3/s. Front velocities were as much as 4.4 m/s and thalweg surface velocities as much as 5.9 m/s for flow depths up to 2.3 m on channel slopes between 7° and 22°. Velocity of the boulder fronts appeared to be controlled largely by channel gradient and depth. Fluid velocity appeared to be influenced by slurry sediment concentration, as well as by depth and slope. Less concentrated flows tended to move at higher velocities than more concentrated flows. The unsteady flow of the debris slurries was at times supercritical and at times turbulent. The development of rigid plugs, a function of slurry yield strength, appeared to be controlled by the relative concentration of boulder-sized particles.
Sediment concentration of debris flow samples (slurry matrix), which was highest near the heads of flows, ranged from 76 to 86 percent by weight (55 to 70 percent by volume). When concentrations dropped below 74 percent by weight, slurry coherence was lost and flow transformed to hyperconcentrated streamflow.
