A two‐phase mechanical model for rock‐ice avalanches

Abstract

Rock‐ice avalanche events are among the most hazardous natural disasters in the last century. In contrast to rock avalanches, the solid phase (ice) can transform to fluid during the course of the rock‐ice avalanche and fundamentally alter mechanical processes. A real two‐phase debris flow model could better address the dynamic interaction of solid (rock and ice) and fluid (water, snow, slurry, and fine particles) than presently used single‐phase Voellmy‐ or Coulomb‐type models. We present a two‐phase model capable of performing dynamic strength weakening due to internal fluidization and basal lubrication and internal mass and momentum exchanges between the phases. Effective basal and internal friction angles are variable and correspond to evolving effective solid volume fraction, friction factors, volume fraction of the ice, true friction coefficients, and lubrication and fluidization factors. Benchmark numerical simulations demonstrate that the two‐phase model can explain dynamically changing frictional properties of rock‐ice avalanches that occur internally and along the flow path. The interphase mass and momentum exchanges are capable of demonstrating the mechanics of frontal surge head and multiple other surges in the debris body. This is an observed phenomenon in a real two‐phase debris flow, but newly simulated here by applying the two‐phase mass flow model. Mass and momentum exchanges between the phases and the associated internal and basal strength weakening control the exceptional long runout distances, provide a more realistic simulation especially during the critical initial and propagation stages of avalanche, and explain the exceptionally high and dynamically changing mobility of rock‐ice avalanches.

Lie symmetry solutions for two-phase mass flows

“…Here, we consider the general two-phase debris flow model [1] and reduce it to one-dimensional inclined channel flow [12,17] and construct Lie group solutions for flows in an inclined channel. For simplicity, here we begin by assuming that for the solid-phase the drag force is dominated by gravity, friction and pressure-gradient, which also implies that the virtual mass force is negligible.…”

“…Gravity mass flows and gravity currents, including avalanches, debris flows, debris floods, floods, and water waves, play important role both in environment, geophysics and engineering [2][3][4][5][6][7][8][9][10][11][12]. Debris flows are extremely destructive and dangerous natural hazards, so there is a need for reliable methods for predicting the dynamics, runout distances, and possible areas hit by such events.…”

“…As observed in natural debris flows, the solid and fluid phase velocities may deviate substantially from each other, essentially affecting flow mechanics. For this reason, there has been extensive field, experimental, theoretical, and numerical investigations on the dynamics, consequences, and mitigation measures as well as industrial applications of these flows [12][13][14][15][16].…”

“…An important aspect of the model is the strong interactions between the solid and the fluid phases, e.g., through the hydraulic pressure gradients, buoyancy, and solid volume fraction, that control the flow dynamics. The equations are formulated as a set of well-structured partial differential equations in conservative form [1,12,17]. Here, we apply the Lie group and symmetry method to this two-phase mass flow model.…”

Thixotropic and anti-thixotropic behaviors of fine-grained soils in various flocculated systems

“…In subaerial and subaqueous landslides that occur in high-water-content sediments, the failed fine-grained mass transforms into a mud flow as they experience large deformations (Locat and Lee, 2002). To describe the flow transformations that resulted in enhanced mobility, a twophase model capable of performing dynamic strength weakening was presented (Pudasaini and Krautblatter, 2014). The debris of the St-Jean-Vianney landslide, that resulted from the sudden failure of a high water content, high sensitivity (but not thixotropic) sediment with a relatively low liquidity index (less than 2) and flowed approximately 3 km down a deep, narrow valley at a speed of approximately 26 km/h (Saihi et al, 2002;Tavenas et al, 1971), provides a dramatic example of the applicability of such rheological studies.…”