Propagation and final deposition of granular flow: dam-break experiments with water and gas-fluidized grains

摘要

Experimental results on propagation and final deposition of initially fluidized granular flows are presented and discussed. In the experiments, an initially fluidized bed of nearly spherical glass beads with sizes of about 45 to 90 μm is suddenly released into a horizontal channel where particles are finally deposited. During initial stages of granular flows, depth and front velocity behave similarly to those of an equivalent flow of a Newtonian fluid. This is proved by comparison with experiments, conducted in the same facility, in which water was released under the same conditions. A constant Froude number of about 2.58 is observed for the fluidized granular flows, which is somewhat lower than the value of about 3.2 observed for the water flows. This is explained in terms of reduced resistance in the latter case relative to the former. Fluidized granular flows generate deposits with a nearly horizontal bulge and a front wedge with a slope of about 2 to 4%. It is argued that fines-rich, dense pyroclas-tic flows may emplace as fluid gravity currents, except at late stages just before final deposition. Such fluid-like behavior, with negligible inter-particle friction, is likely to promote high flow mobility even on nearly horizontal slopes. Several characteristics of pyroclastic deposits are consistent with the present experimental results.