Frictional behavior of granular gravel–ice mixtures in vertically rotating drum experiments and implications for rock–ice avalanches
Highlights
► First friction coefficient and pore water pressure measurements of gravel–ice mixtures. ► Friction coefficient is reduced by up to 20% for increasing volumetric ice contents. ► Relation between volumetric ice content and friction coefficient is linear. ► Friction coefficient is continuously reduced with rising pore water pressures.
Introduction
Cryospheric systems are sensitive to climate change and generally respond quickly (Haeberli et al., 1997, Noetzli and Gruber, 2009, Salzmann et al., 2007). The decay of glaciers and degradation of permafrost can cause slope instabilities and large rapid mass movements in steep high mountain areas (Davies et al., 2001, Dramis et al., 1995, Geertsema et al., 2006, Gruber and Haeberli, 2007, Haeberli et al., 1997, Haeberli et al., 2003, Harris et al., 2001, Harris et al., 2003). The number of large slope failures in glaciated high mountain areas has increased in the last two decades as compared to the 20th century and may further increase in future (Fischer, 2009, Geertsema et al., 2006, Huggel et al., 2010, Van Der Woerd et al., 2004). While there is a broad variety of possible effects causing high mobility of large rapid mass movements (see discussion in Erismann and Abele, 2001, Korup et al., in press), slope failures from glacial environments are often subject to an additionally enhanced mobility for several reasons (e.g. Evans and Clague, 1988): 1) due to their origin, the moving mass usually contains or entrains a considerable proportion of snow and ice which reduces friction within the moving mass, 2) the transported snow and ice continuously supply meltwater due to frictional heating and convective mixing with non-frozen ground material and air, reducing the shear resistance of the flowing material as it reaches lower regions, and 3) propagation over a glacier which serves as a low friction surface can strongly increase the avalanche velocity and hence its momentum, resulting in an extended runout distance.
The rock avalanches on Sherman glacier on March 27, 1964 (Shreve, 1966), from Mt. Munday around June, 1997 (Evans and Clague, 1998), from Aoraki/Mt. Cook in 1991 (McSaveney, 2002), and the earthquake-triggered large multiple landslides and rock avalanches at Black Rapids on November 3, 2002 (Jibson et al., 2006) are a few examples for large and/or long-runout events on glacial surfaces. The enormous rock and ice masses which detached from Huascarán in Peru on January 10, 1962 and May 31, 1970 have caused a total death toll of 7000 people ((Evans et al., 2009a), with older estimations reaching as high as 22,000 casualties (Plafker and Ericksen, 1978)), and have dramatically shown the catastrophic potential of combined rock–ice avalanches if they reach populated regions. On September 22, 2002, the extreme mobility of gravel-ice mixtures was again tragically demonstrated by the Kolka glacier failure in the Russian Caucasus (Evans et al., 2009b, Kotlyakov et al., 2004). Both events were characterized by extremely high velocities, high ice contents, and flow transformations (multi-phase movement) along the flow path, to debris-flows that traveled a great distance downstream (Petrakov et al., 2008). The unexpected and sudden initiation of large rock–ice avalanches makes any direct physical measurements in the field impossible. Therefore, the current knowledge of rock–ice avalanches is largely based on post-event documentation, using remote sensing data and some field investigations of the source zone, travel path, and deposition area. While a broad range of case studies exist, there is no physical quantification of the effects of ice on frictional characteristics available.
In this study we focus on the frictional characteristics of different gravel–ice mixtures and on the development of an inter-granular fluid (water) phase by using two large rotating drums. The first aspect considers the influence of the proportion of granular ice on the bulk friction coefficient (tangent of friction angle) while the second aspect concentrates on the time-evolution of the friction angle when the ice is melting, mimicking the evolution of a rock–ice avalanche during its runout. Rotating drums have been used for debris-flow rheology studies (Huizinga, 1996, Kaitna and Rickenmann, 2007a), measurements of bedrock erosion by debris-flows (Hsu et al., 2007, Hsu et al., 2008), abrasion of fluvially transported grains (Kodama, 1994, Mikoš and Jaeggi, 1995), observations of grain-size segregation (Henein et al., 1985, Hsu, 2010), and for investigations on flow characteristics of dry granular material (Chou and Lee, 2009), but not for granular flow experiments containing gravel and ice. An advantage of drum experiments is that experimental devices allow measurements for pre-defined time spans at a given rotational velocity and enable long periods of observation so that flow transformations related to the melting of ice can be observed. The experimental setup in rotating drums is suited to study the flow process in a quasi-stationary regime, but neither initiation nor deposition.
Section snippets
Drum characteristics and velocity scaling
Because full dynamic similarity for geometrically similar flows at different sizes is probably impossible to achieve (Iverson et al., 2010), the best way to reduce this problem is to use the largest possible scale (Hsu, 2010). The laboratory experiments were performed in two similar vertically rotating drums with diameters of 246 cm in Vienna and 399 cm in Berkeley (Fig. 1 and Table 1). Each drum had its own advantages: The rotational drum in Berkeley was the largest facility available, while the
Smaller drum (Vienna)
Measurements at the smaller drum in Vienna (Fig. 4) include torque at the drum axis, flow depth (via 2 lasers), normal and shear force (via 4 load cells), temperature of the mixture and rotational velocity of the drum. The lasers and load cells were arranged in two identical sensor groups called ‘left’ and ‘right’ (Fig. 4) and the signals of all sensors are recorded at a frequency of 800 Hz. Additionally, a video camera is installed near the drum axis to record the frontal region of the flow.
Measured and observed flow characteristics
Here we present and discuss the results for the first 5 min of 12 experiments in the smaller drum in Vienna (V01-050 to V12-100) with ice contents between 0 and 100% by volume. The experiment numbers increase chronologically followed by a number representing the ice content (see also Table 2). We focused on the frictional behavior during the first 5 min when the flow was largely dry. Fig. 8 shows images from the video camera in the drum (see Fig. 4, letter i) for all runs with varying ice
Comparison of drum experiments and natural events
A major difference between the rotating flume and straight chute experiments or natural events is related to the longitudinal flow characteristics. Kern et al. (2009) found supercritical flow behavior (Fr > 1) for snow avalanche fronts and subcritical behavior (Fr < 1) for the avalanche tails. This is not possible in rotating drums, because the avalanche tail is artificially held at the same velocity by higher inclination angles. Instead, the shear rates and Froude numbers rise to the tail due to
Conclusions
The main objective of this study was to quantify the influence of ice on friction in rapid granular mass movements and on related phase transitions when ice is melting. The experiments provide new laboratory evidence to illustrate these effects and our results suggest that the presence of ice in the moving mass is important both as a part of the moving material and as supplier of water for the flow itself. However, scale effects are difficult to quantify and need to be considered if friction
Acknowledgments
This work was financially supported by the Swiss National Foundation grant NF 200021-121823/1 (Rock–ice avalanches: A systematic investigation of the influence of ice) and the Austrian Science Foundation grant J2837-N10. The rotational drums were provided by the Department of Earth and Planetary Science at the University of California Berkeley (UCB) in cooperation with the National Center for Earth-surface Dynamics (NCED) and by the Institute of Mountain Risk Engineering (IAN) at the University
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