Abstract
Granular debris flows in nature are composed of a wide range of solids and viscous pore fluids, moving at high velocities down sloping channels. The pore fluids in a granular debris flow affect the interactions between the solid and fluid phases and thus govern the debris-flow mobility. Study of the pore fluid effect (i.e., excess pore water pressures correlated to solid structures, and the viscous shearing and dragging) is essential for understanding the high flow mobility of granular debris flows. This study critically reviews two dimensionless numbers with clear physical meanings, then demonstrates a new application of field monitoring data for identifying natural debris flows on large scales (i.e., surge and continuous debris-flows, respectively). This study illustrates that, the pore fluid viscous shearing stress dominates solid inertial stress due to solids collision. It is also found that different to continuous debris-flows, the high pore fluid pressures generated in surge debris-flow body dissipate quite slowly and mostly influence particle contact behaviour significantly. A new scientific criterion for identification of continuous and surge debris flow in nature can be given by this study.
Highlights
Debris flows occur when masses of poorly sorted sediment, agitated and saturated with water, surge down a slope in response to gravitational attraction [1]
We reinterpret the recorded field data of viscous debris flows in Dongchuan, China, and identify the different flowing regimes of natural granular debris flows using two dimensionless numbers based on the relative importance of solids collision, pore fluid viscous drag and pore fluid pressures
To distinguish the different debris flows based on the field observation data, a new criterion correlating with the flow duration and pore fluid pressure dissipation is developed
Summary
Debris flows occur when masses of poorly sorted sediment, agitated and saturated with water, surge down a slope in response to gravitational attraction [1]. Bagnold [11] reported that flows of solid-fluid mixtures can be divided into grain-inertial flows and macro-viscous flows. These flows depend on the relative significance of solid collisions and pore fluid viscosity, and a dimensionless number called the Bagnold number was developed for identifying flow regimes in this way. We reinterpret the recorded field data of viscous debris flows in Dongchuan, China, and identify the different flowing regimes of natural granular debris flows using two dimensionless numbers based on the relative importance of solids collision, pore fluid viscous drag and pore fluid pressures. A uniform criterion which has clear physical meanings is developed to clarify different natural debris flows
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