Abstract
Pore water pressure has been recognized as an important factor to enhance the mobility of debris flow moving in channel of very gentle slope. The creation and dissipation of pore water pressure are associated with interaction between grains. This study proposes a physical model for the pressure on mobility of flows with different granular configurations: the flow with overlying coarse-grained layer (i.e., inverse grading) and the flow with fully-mixed grains. The flow velocity is derived by the effective stress principle and the relationship between acceleration and pore water pressure is analyzed under different conditions. The results show that a high excess pore water pressure leads to high velocity of flow, and the pressure increases during the movement; and acceleration increases with time and flow depth under given pore water pressure. Moreover, compared with the flow with mixed grains, the flow with overlying coarse-grained layer is more effective to promote the excess pore water pressure and the liquefaction slip surface. Therefore, the internal drag reduction due to pore water pressure produces an acceleration effect on the flow.
Highlights
Debris flow is a complex multi-phase fluid characterized by wide-ranged granular materials; and the viscous debris flow is the most typical representative for the dynamical appearances (Chen, 1983; Pierson and Costa, 1987; Wu et al, 1990; Scott et al, 1995; Pérez, 2001; Kang et al, 2004)
In this paper, based on observations in JJG, we propose the effect of the overlying coarse-grained layer on pore water pressure and its implication in mobility maintenance of debris flow, establish the mechanism for the flow acceleration, and conduct a quantitative analysis of the proposed equations
Based on field observations of debris flow surges, this study proposes a dynamical model for debris flow acceleration due to pore water pressure associated with granular configurations
Summary
Debris flow is a complex multi-phase fluid characterized by wide-ranged granular materials; and the viscous debris flow (with density greater than 2.0 t/m3) is the most typical representative for the dynamical appearances (Chen, 1983; Pierson and Costa, 1987; Wu et al, 1990; Scott et al, 1995; Pérez, 2001; Kang et al, 2004). The fact that high-density debris flows move at a high velocity on gentle slopes has attracted general attentions (e.g., Rodine and Johnson, 1976; Pierson, 1981). Calculations of velocity mainly take the external characteristics into consideration. The increase in the flow velocity depends on the obstruction of the streambed, and on the granular characteristics of flow
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