Control factors for mobility of dense and dilute debris flows

Abstract

<p indent=0mm>Debris flow is a commonly occurring geological hazard in mountainous areas. The solid-liquid coupling makes the debris flows distinct from other geophysical fluids such as debris avalanches and floods. Owing to the complexity of this solid-liquid interaction, current research of debris-flow mobility focuses mainly on dense debris flows with a volumetric solid concentration of ~0.6. However, few studies focus on the physical processes occurring in the transition from dense to dilute debris flows with volumetric solid concentrations of 0.6–0.4. The objectives of this study are to explore the effects of changes in volumetric solid concentration on pore water pressure response and debris-flow mobility and to reveal the mechanisms in the transition from dense to dilute debris flows. Taking the debris-flow volumetric solid concentration and fluid viscosity as variables, a series of well-instrumented flume experiments is conducted. The results show that the high mobility of debris flow is closely related to the state of liquefaction. In the transition from dense to dilute debris flows, the shear dilatancy (state of solid particles) shifts from positive (dilation) to negative (contraction). As a result, the increase in pore water pressure causes the debris flow to liquefy, and the mobility surges accordingly. According to the calculated and measured shear stresses, it is apparent that the shear resistance of dense (dilute) debris flows is dominated by friction between the particles (viscous drag). An employed rheological model shows that the shear resistance in dense debris flows is insufficient for describing the rheological behavior of dilute flows. Models considering the interaction between particles, including friction and collision, and the viscosity of the fluid phase are warranted in future research.