Enhanced run-out of dam-break granular flows caused by initial fluidization and initial material expansion

Abstract

We report results of the run-out of experimental dam-break flows in a horizontal channel generated from the collapse of columns of fine (75 μm) particles fluidized at various degrees. We find that the flow run-out (x) made dimensionless by the initial column length (xo) is a power function of the initial column height-to-length ratio (r ), as shown in previous works with non-fluidized flows. The run-out of flows initially fluidized at different degrees is accounted by x/xo = αr n. For initially non-fluidized flows, our values of α are significantly higher than those reported earlier for flows of coarser granular material (>0.15 mm), showing that finely grained flows have longer run-outs compared to their coarser counterparts. The coefficient α is a function of the initial degree of fluidization, with a higher growth above 93 % of fluidization, which coincides with the onset of bed expansion, and it accounts for a flow run-out increase being up to more than twice that of non-fluidized flows. The parameter α is well correlated with the amount of initial bed expansion, which undergoes a sharp transition at high degrees of fluidization that has shown to be an important mechanism for reducing flow friction. Our results are consistent with earlier findings that showed that bed expansion significantly increases pore pressure diffusion timescales in static columns, suggesting that the long run-out of initially expanded finely grained flows is due to their ability to diffuse pore pressure slowly.