Frictional behavior of granular gravel–ice mixtures in vertically rotating drum experiments and implications for rock–ice avalanches

Abstract

Rapid mass movements involving large proportions of ice and snow can travel significantly further downslope than pure rock avalanches and may transform into debris-flows as the ice melts and as water from the stream network or water-saturated debris is incorporated. Currently, ice is thought to have three distinctive effects: 1) reduction of the friction within the moving mass itself, 2) increase of pore pressure as the ice melts and consequent reduction of the shear resistance of the flowing material, and 3) reduction of boundary friction where the failing mass travels on a glacier. However, measurement-based evidence to support these hypotheses is largely missing. In this study, laboratory experiments on the first two mechanisms were carried out in two partially-filled large rotating drums, one in Vienna (Austria) and a second in Berkeley (USA). Varying proportions of cold gravel and gravel-sized ice were mixed and added to the rotating drum running at constant rotational velocity until all ice had melted. Flow behavior was recorded with flow depth, normal force, shear force, pore-water pressure, and temperature sensors. The bulk friction coefficient was found to decrease linearly with increasing ice content by ~ 20% in the early phase of the experiments, before significant portions of the ice transformed into water. For ice contents larger than 40% by volume, the transformation from a dry granular flow to debris-flow-like movement or hyperconcentrated flow was observed when pore-water pressures rose and approached the normal forces along the flow profile. Pore-water pressure from melting ice developed within several minutes after the start of the experiments and, as it increased, progressively reduced the friction coefficient. The results emphasize that the presence of ice in granular moving material can significantly reduce the friction coefficient of both dry and partially-saturated debris. Due to size effects and the absence of other factors reducing friction (e.g. surfaces with low friction and rock comminution), the absolute measured friction coefficients from the laboratory experiments were larger than those found from natural events. However, the relative changes in friction coefficients depending on the ice and water content may also be considered in real-scale hazard assessments of rapid mass movements in high mountain environments. coefficients from the laboratory experiments were larger than those found from natural events. However, the 31 relative changes in friction coefficients depending on the ice and water content may also be considered in real-32 scale hazard assessments of rapid mass movements in high mountain environments. 33

Unraveling driving factors for large rock–ice avalanche mobility

“…The rotating drum experiments revealed bulk friction values between 0.50 and 0.72 for varying volumetric ice contents while real events having an estimated ice content of 10–100% reach much lower apparent coefficients of friction between 0.11 and 0.52 (Figure ). The friction coefficient in the laboratory experiments linearly decreases with increasing ice content, leading to a ~20% reduction of the friction for dry granular pure ice compared with gravel only (Schneider et al , ). The field data again does not obviously reveal such a trend except for the events with multi‐phase flow behavior (squares) where a slight trend for lower apparent coefficients of friction with higher ice contents could be demonstrated.…”

“…(), and the experiments with the gravel–ice mixtures by Schneider et al . (). Herein we used measurements of the dynamic bulk friction coefficients of cold ( T ≈ −5 °C) and dry granular gravel–ice mixtures with varying volumetric ice contents (0.0856 m 3 material in the smaller drum and 0.4 m 3 in the larger drum).…”

“…While it is evident that glacial environments increase the runout distance of rapid mass movements, individual involved processes are difficult to separate. Besides the mentioned interactions at the boundary, snow and ice also have effects on internal friction when they are incorporated into the granular mass either as ice or water (Schneider et al , ). Three general possibilities exist about how ice can be incorporated in the moving mass, while most events include combinations of them: Failure of steep glaciers.…”

Using Stereo Satellite Imagery to Account for Ablation, Entrainment, and Compaction in Volume Calculations for Rock Avalanches on Glaciers: Application to the 2016 Lamplugh Rock Avalanche in Glacier Bay National Park, Alaska

“…Changes in the material characteristics of rock avalanches and fluidization during emplacement can lead to increased mobility (Deline, ; Hungr & Evans, ). Laboratory experiments (Schneider et al, ) and numerical modeling (Sosio et al, ) have demonstrated a reduction in friction of rock avalanche material upon the incorporation of ice. However, a lack of data and the difficulty of quantifying the amount of snow and ice in rock avalanche deposits contribute to a poor understanding of the degree to which ice entrainment affects the travel distance and overall emplacement characteristics of rock avalanches.…”

Mean and fluctuating basal forces generated by granular flows: Laboratory observations in a large vertically rotating drum

“…While monosized or narrow size distributions can be described adequately by a single particle diameter, D p , for wide‐grain‐size distributions there is no agreement on the best effective diameter [ Stock and Dietrich , ]. Several studies have proposed that for a wide‐grain‐size distribution, a higher percentile than the median, such as the 84th percentile of the grain‐size distribution ( D 84 , 1 standard deviation above the mean grain size) may be a better effective grain size for quantifying inertial stresses [e.g., Stock , ; Schneider et al , ; Yohannes et al , ].…”