Avalanche Mobility Research Articles

Scale‐Dependent Processes and Runout in Bidisperse Granular Flows: Insights From Laboratory Experiments and Implications for Rock/Debris Avalanches

AbstractThe bidispersity observed in the particle‐size distribution of rock avalanches and volcanic debris avalanches (rock/debris avalanches) has been proposed as a factor contributing to their long runout. This has been supported by small‐scale analog experimental studies, which observe that a small proportion of fine particles mixed with coarser particles enhances granular avalanche runout. However, the mechanisms enabling this phenomenon and their resemblance to rock/debris avalanches have not been directly evaluated. Here, binary mixture granular avalanche experiments are employed to constrain the processes and conditions under which bidispersity enhances the runout of granular avalanches in experiments. Structure‐from‐motion photogrammetry is used to measure center of mass displacement and assess energy dissipation. Subsequently, this study evaluates the dynamic scaling and flow regimes in the lab and field to assess whether the runout‐enhancing mechanism is applicable to rock/debris avalanches. In small‐scale experiments, the granular mass propagates under a collisional regime, enabling kinetic sieving and size segregation. Fine particles migrate to the base where they reduce frictional areas between coarse particles and the substrate and encourage rolling. The reduced energy dissipation increases the kinetic energy conversion and avalanche mobility. However, rock/debris avalanches are unlikely to acquire a purely collisional regime; instead, they propagate under a frictional regime. The size segregation which is essential for the process observed at the lab‐scale is prohibited by the frictional regime, as evident by the sedimentology of rock/debris avalanche deposits. The proposal of bidispersity as a runout‐enhancing mechanism overlooks that scale‐dependent behaviors of natural events are often omitted in small‐scale experiments.

Effects of slope topography on rock avalanche mobility and deposition: A 2-D discrete element method investigation

The impact of rock avalanches is a major concern in mountain areas. However, fundamental understanding and reliable prediction of the depositional area of rock avalanches remains inadequate. Here, an idealized numerical study of granular materials flowing down a slope is carried out using two-dimensional (2-D) discrete element method simulations. The effects of the slope geometry on the mobility and deposit morphology of the granular flow are studied in detail. It is found that the granular flow mobility increases with the curvature radius of the circular ramp, leading to a longer runout distance, which is explained from the viewpoint of energy transfer and dissipation. The results are expected to enhance understanding of the travel and deposition mechanisms of rock avalanches and other related geophysical flows.

Effect of structural setting of source volume on rock avalanche mobility and deposit morphology

Abstract. Deposit morphologies and sedimentary characteristics are methods for investigating rock avalanches. The characteristics of structural geology of source volume, namely the in-place rock mass structure, will influence these two deposit characteristics and rock avalanche mobility. In this study, a series of experiments were conducted by setting different initial configurations of blocks to simulate different characteristics of structural geology of source volume, specifically including the long axis of the blocks perpendicular to the strike of the inclined plate (EP), parallel to the strike of the inclined plate (LV), perpendicular to the inclined plate (LP), randomly (R) and without the blocks (NB) as a control experiment. The experimental materials comprised both cuboid blocks and granular materials to simulate large blocks and matrixes, respectively, in natural rock avalanches. The results revealed that the mobility of the mass flows was enhanced in LV, LP and R configurations, whereas it was restricted in the EP configuration. The mobility decreased with the increase in slope angles at LV configurations. Strand protrusion of the blocks made the elevation of the deposits at LV configuration larger than that at EP, LP and R configurations. A zigzag structure is created in the blocks resulting from the lateral spreading of the deposits causing the blocks to rotate. Varying degrees of deflection of the blocks demonstrated different levels of collision and friction in the interior of the mass flows; the most intensive collision was observed at EP. In the mass deposits, the blocks' orientation was affected by their initial configurations and the motion process of the mass flows. This research would support studies relating characteristics of structural geology of source volume to landslide mobility and deposit morphology.

Effect of structural setting of source volume on rock avalanche mobility and deposit morphology

Effect of structural setting of source volume on rock avalanche mobility and deposit morphology

Detrainment and braking of snow avalanches interacting with forests

Abstract. Mountain forests provide natural protection against avalanches. They can both prevent avalanche formation in release zones and reduce avalanche mobility in runout areas. Although the braking effect of forests has been previously explored through global statistical analyses on documented avalanches, little is known about the mechanism of snow detrainment in forests for small and medium avalanches. In this study, we investigate the detrainment and braking of snow avalanches in forested terrain, by performing three-dimensional simulations using the material point method (MPM) and a large-strain elastoplastic snow constitutive model based on critical state soil mechanics. First, the snow internal friction is evaluated using existing field measurements based on the detrainment mass, showing the feasibility of the numerical framework and offering a reference case for further exploration of different snow types. Then, we systematically investigate the influence of snow properties and forest parameters on avalanche characteristics. Our results suggest that for both the cold and warm snow parameterized in our simulations, the detrainment mass decreases with the square of the avalanche front velocity before it reaches a plateau value. Furthermore, the detrainment mass significantly depends on snow properties. It can be as much as 10 times larger for warm snow compared to cold snow. By examining the effect of forest configurations, it is found that forest density and tree diameter have cubic and square relations with the detrainment mass, respectively. The outcomes of this study may contribute to the development of improved formulations of avalanche–forest interaction models in popular operational simulation tools and thus improve hazard assessment for alpine geophysical mass flows in forested terrain.

A two-fluid model for immersed granular avalanches with dilatancy effects

When a deposited layer of granular material fully immersed in a liquid is suddenly inclined above a certain critical angle, it starts to flow down the slope. The initial dynamics of these underwater avalanches strongly depends on the initial volume fraction. If the granular bed is initially loose, i.e. looser than the critical state, the avalanche is triggered almost instantaneously and exhibits a strong acceleration, whereas for an initially dense granular bed, i.e. denser than the critical state, the avalanche's mobility remains low for some time before it starts flowing normally. This behaviour can be explained by a combination of geometrical granular dilatancy and pore pressure feedback on the granular media. In this contribution, a continuum formulation is presented and implemented in a three-dimensional continuum numerical model. The originality of the present model is to incorporate dilatancy as an elasto-plastic normal stress or pressure and not as a modification of the friction coefficient. This allows an explanation of the two different behaviours of initially loose and dense underwater avalanches. It also highlights the contribution from each depth-resolved variable in the strongly coupled transition to a flowing avalanche. The model compares favourably with existing experiments for the initiation of underwater granular avalanches. Results reveal the interplay between shear-induced changes of the granular stress and fluid pressure in the dynamics of avalanches. The characteristic time of the triggering phase is nearly independent of the local rheological parameters, whereas the initial drop in pore pressure and the surface velocity at steady state still strongly depend on them. Finally, the multidimensional capabilities of the model are illustrated for the two-dimensional Hele-Shaw configuration and some of the observed differences between one-dimensional simulations and experiments are clarified.

Solid-liquid interaction caused by minor wetting in gravel-ice mixtures: A key factor for the mobility of rock-ice avalanches

Rock-ice avalanches pose serious risks to humans and infrastructures due to their extremely large volumes and high mobility in glacierized mountain regions. The mobility of rock-ice avalanches is supposed to be influenced by water, while the solid-liquid interactions induced by minor wetting remain an open issue. A series of flume tests was performed to investigate the influence of a small amount of water (no more than 4% of the gravel weight) on the behavior and mobility of gravel-ice mixtures. The test results show that intergranular refreezing bonds were created with a preliminary addition of water. The breakup of some weak refreezing bonds during particle rearrangement likely dissipated some kinetic energy, and thus, the mixture mobility was reduced. As the water content increased, the thermal conductivity could likely not remain efficient enough to refreeze all the trapped water and thus allowed unfrozen water to lubricate particle corners, which enhanced mixture mobility. With a further increase in the water content, suction-induced cohesion cemented particles as clusters and reduced mixture mobility. However, a high ice content resulted in mobility enhancement with increasing water content because more gravel was refrozen into the ice matrix, and thus, the basal friction was lowered by the occupation of ice cubes at the bottom. The redistribution pattern of trapped water during mass propagation changed at a characteristic water content. When the water content was lower than the characteristic content, superficially cohered water detached from the surface asperity of one particle and transferred to another during particle rearrangement, i.e., local redistribution of trapped water. When the water content was higher than the characteristic content, the downward transfer of a considerable amount of trapped water was triggered by particle rearrangement under the influence of gravity, i.e., bulk redistribution of trapped water. The water experiencing bulk redistribution influenced the mixture mobility by exerting intergranular drag force via the build-up and break-up of liquid bridges, while locally redistributed water influenced mixture mobility by altering the surface properties of particles. This study is expected to be helpful in better understanding the propagation mechanisms of water-poor rock-ice avalanches and in improving relevant hazard assessments. • Water influence on rock-ice avalanche mobility was studied via a small flume test. • Lubrication of unfrozen water enhanced mixture mobility. • Suction-induced cohesion reduced mixture mobility. • The refreezing of minor trapped water might reduce mixture mobility. • Water redistribution pattern changed at a typical water content of 2.2%.

Rock avalanche mobility: The role of path material

Rock avalanches are high velocity flows of fragmented rock that can dramatically alter landscapes, and impact people and infrastructure far from their source. These catastrophic events have been studied for over a century, however, a consensus regarding the mechanism(s) that govern their motion has yet to emerge. This work details the results of the back-analysis of 45 rock avalanche case histories using a semi-empirical runout model that simulates motion over 3D terrain. These simulations account for topographic effects and bulk basal shear resistance variation during motion. For a subset of the cases, we find that a volume-dependent mechanism appears to influence basal resistance in the source zone. However, once the material has vacated the source zone, our results suggest that the character of the path material strongly controls mobility, and appears to dominate over other, potentially volume-dependent, mechanisms that may also be at work. The results presented in this paper support the longstanding hypothesis that the interaction of flowing debris with the underlying substrate is an important consideration in rock avalanche runout prediction, and that this mechanism warrants further research.

Rock avalanche mobility: optimal characterization and the effects of confinement

A Central Asia rock avalanche (rockslide) inventory that includes about 600 case studies with measured parameters is used to present a quantitative characterization of rock avalanche mobility and its dependence on the controlling parameters such as volume and height drop. The mobility can be characterized by runout and angle of reach, as well as by the affected area. The database was analyzed after dividing the rock avalanches into three confinement categories—frontally confined, laterally confined, and unconfined. In addition to the traditionally used slope failure volume and height drop, we correlated mobility parameters with their product that is proportional to the potential energy released during rock avalanche emplacement. It was found that coefficients of determination of the regressions of the affected area with volume and, especially, with its product with maximal height drop are higher than those of volume with runout and much higher than those of volume with angle of reach for all confinement conditions. Thus, these relationships provide an optimal characterization of the rock avalanche mobility. Relative influence of failure volume and height drop on rock avalanche mobility was also analyzed. The regression equations obtained in this study were successfully applied to several case studies from other mountain regions. The proposed regression equations can be used to predict the exposure of elements at risk threatened by anticipated rock avalanches. They can also be used to estimate the parameters of rockslide dams that might originate at the sites with anticipated large-scale rock slope failures, if their volumes and height drops can be assessed. The confinement conditions are statistically proved to influence the rock avalanche mobility and must be taken into consideration during hazard and risk assessment.

Spreading and Deposit Characteristics of a Rapid Dry Granular Avalanche Across 3D Topography: Experimental Study

Aiming to understand the propagation and deposit behaviours of a granular avalanche along a 3D complex basal terrain, a new 3D experimental platform in 1/400 scale was developed according to the natural terrain of the Xiejiadianzi rock avalanche, with a series of laboratory experiments being conducted. Through the conduction of these tests, parameters, including the morphological evolution of sliding mass, run-outs and velocities of surficial particles, thickness contour and centre of final deposit, equivalent frictional coefficient, and energy dissipation, are documented and analysed, with the geomorphic control effect, material grain size effect, drop angle effect, and drop distance effect on rock avalanche mobility being discussed primarily. From the study, some interesting conclusions for a better understanding of rock avalanche along a 3D complex basal topography are reached. (1) For the granular avalanche tested in this study, great differences between the evolutions of the debris along the right and left branch valleys were observed, with an obvious geomorphic control effect on avalanche mobility presented. In addition, some other interesting features, including groove-like trough and superelevation, were also observed under the control of the topographic interferences. (2) The equivalent frictional coefficients of the granular avalanches tested here range from 0.48 to 0.57, which is lower than that reached with a set-up composed of an inclined chute and horizontal plate and higher than that reached using a set-up composed of only an inclined chute. And the higher the drop angle and fine particle content, the higher the equivalent frictional coefficient. The effect of drop distance on avalanche mobility is minor. (3) For a granular avalanche, momentum transfer plays an important role in the motion of mass, which can accelerate the mobility of the front part greatly through delivering the kinetic energy of the rear part to the front.

Unraveling driving factors for large rock–ice avalanche mobility

ABSTRACTLarge rock–ice avalanches have attracted attention from scientists for decades and some of these events have caused high numbers of fatalities. A relation between rock slope instabilities in cold high mountain areas and climate change is currently becoming more evident and questions about possible consequences and hazard scenarios in densely populated high mountain regions leading beyond historical precedence are rising. To improve hazard assessment of potential rock–ice avalanches, their mobility is a critical factor. This contribution is an attempt to unravel driving factors for the mobility of large rock–ice avalanches by synthesizing results from physical laboratory experiments and empirical data from 64 rock–ice avalanches with volumes >1x106 m3 from glacierized high mountain regions around the world. The influence of avalanche volume, water and ice content, low‐friction surfaces, and topography on the apparent coefficient of friction (as a measure of mobility) is assessed. In laboratory experiments granular ice in the moving mass was found to reduce bulk friction up to 20% while water led to a reduction around 50% for completely saturated material compared with dry flows. Evidence for the effects of water as a key driving factor to enhance mobility was also found in the empirical data, while the influence of the ice content could not be confirmed to be of much relevance in nature. Besides liquefaction, it was confirmed that mobility increases with volumes and that frictional surface characteristics such as flow paths over glaciers are also dominant variables determining mass movement mobility. Effects of the topography along the flow path as well as channeling are assumed to be other critical factors. The results provide an empirical basis to roughly account for different path and flow characteristics of large rock–ice avalanches and to find appropriate ranges for friction parameters for scenario modeling and hazard assessments. Copyright © 2011 John Wiley & Sons, Ltd.

Large mobility of dry snow avalanches: Insights from small-scale laboratory tests on granular avalanches of bidisperse materials

This paper reports small-scale laboratory tests on granular avalanches of bidisperse materials made of fine particles and larger ones. These experiments were motivated by a recent study on the rheology of dense flowing snow which provided evidence for relevant similarities in flow behavior between bidisperse granular materials and dry cold snow [Rognon and others, J. Rheol., 52, 3 ([32])]. The mass proportion of fine particles in the initial binary mixture was systematically varied at constant initial released volume, and we measured the resulting flow depth, the avalanche front velocity and the final avalanche runout. In particular, we show that the avalanche mobility is largely increased, about 40% in our tests, when the mass proportion in fine particles reaches a critical value, around 0.25 in our tests. The avalanche deposit is shallow and lengthened for this critical mass proportion in fine particles. The experimental results are interpreted by the existence of different avalanche mobility regimes on the basis of a heuristic model previously reported in literature. Finally, we discuss their possible implications for the dynamics of full-scale dry snow avalanches.

Complete dynamic modeling calibration for the Thurwieser rock avalanche (Italian Central Alps)

Few rock avalanches have been witnessed and no exact real time description is available. The 2.5 M m 3 Punta Thurwieser rock avalanche (September 2004, Italian Central Alps) is a unique event for which many quantitative observations have been gathered. This is the first case ever of a natural rock avalanche being recorded on video, and for which an almost exact value of the total time duration, from initial detachment to final deposition, is available. The landslide travelled over 2.9 km from its source partially in a glacial environment. The initial failure and the time for the complete evacuation of the source area were studied and assessed by a series of photos. The mean front velocities along the path have been estimated from the video. The final geometry and the characteristics of the deposit have been obtained by GPS measurements, aerial photos, field surveys and laboratory tests. A quasi-3D continuum dynamic analysis of the landslide motion has been performed assuming both a Voellmy and a frictional rheology in the model DAN 3D. For the first time the landslide front velocities have been used for back analyses and calibration of the rheological parameters, together with the final shape and thickness of the deposit, and its developing extension. The dynamic flow resistance parameters, initially calibrated with DAN, were adopted and validated in DAN 3D, showing good consistency. They are consistent with the existing database for calibrated landslides of similar type and scale. Flow velocities on the path segment underlain by glacial deposits (up to 55–65 m s − 1 ) are more accurately predicted by adopting a frictional rheology (up to 50–55 m s − 1 ) with zero pore pressure and a 26° friction angle than by a Voellmy rheology (up to 40–45 m s − 1 ). Both observations and modelling show that glacial ice plays a substantial role in increasing the rock avalanche mobility.

Avalanche mobility induced by the presence of an erodible bed and associated entrainment

The partial fluidization model developed by Aranson and Tsimring (2002) is used to simulate the spreading of a 2D circular cap of granular material over an erodible bed made of the same material. Numerical results show that the presence of even a very thin layer of granular material lying on the solid bed strongly increases the mobility of granular flows. Furthermore, as the thickness of the granular layer increases, the dynamics of the flowing mass changes from a decelerating avalanche to a traveling wave. Numerical simulation suggest that surges are generated if enough mass is entrained, increasing the energy of the flowing material and balancing the energy lost by friction.

Comparison of pyroclastic flow and debris avalanche mobility : Hayashi, J N; Self, S J Geophys ResV97, NB6, June 1992, P9063–9071

Comparison of pyroclastic flow and debris avalanche mobility : Hayashi, J N; Self, S J Geophys ResV97, NB6, June 1992, P9063–9071

A comparison of pyroclastic flow and debris avalanche mobility

Many pyroclastic flows have runout distances longer than expected from basic frictional arguments. In these cases the ratio of the height descended H and the runout L are as small as ∼0.2, a feature which has been attributed to fluidization. However, significant fluidization by gases is now considered unlikely during emplacement of debris avalanches, where H/L and volume are inversely related. We determine whether pyroclastic flows are more mobile than debris avalanches of the same volume and if different emplacement mechanisms are needed to describe their runout. H/L and volume data were compiled for 34 nonvolcanic and 40 volcanic debris avalanche deposits, as well as 15 pyroclastic flow deposits. Systematic differences in the volume versus H/L plots were explored by statistical analysis. Results show an inverse relationship between H/L and volume for both nonvolcanic and volcanic debris avalanche deposits, as expected, and also for pyroclastic flow deposits. Regression lines for plots of pyroclastic flow and volcanic debris avalanche data are statistically indistinguishable and the non‐volcanic debris avalanche line is parallel to the other two. We argue that the nonvolcanic and volcanic debris avalanche differences are due to material properties rather than emplacement mechanisms. Therefore since pyroclastic flows and volcanic debris avalanches have indistinguishable H/L versus volume, it is unlikely that pyroclastic flows have a significantly different primary emplacement mechanism. Based on the data in hand, we conclude that it is unnecessary to invoke gas fluidization to explain the mobility of the types of pyroclastic flows examined or the excess runout of large‐volume debris avalanches.

The debris of the Frank Slide and theories of rockslide–avalanche mobility

The Frank Slide, a 30 × 106 m3 rockslide–avalanche of Palaeozoic limestone, occurred in April 1903 from the east face of Turtle Mountain in the Crowsnest Pass region of southwestern Alberta, Canada.The reconstruction of the Canadian Pacific Railway (CPR) line created a cut up to 16 m high across the deposit, giving a unique cross section nearly the depth of the debris. The debris shows vertical sorting. The base material is crushed limestone, mainly of sand and gravel size, and contains rounded pebbles from till or alluvial deposits on the surface of separation. The upper surface of the debris is an accumulation of large, predominantly angular boulders. Grain-size analyses by sieving and by "area-by-number" counts demonstrate a gradual increase in grain size with height above the base of the cut. Such inverse grading with fines concentrated at the base of the debris indicates that the landslide was not fluidized by gas pore pressures.The base material has run ahead of the coarse debris of the slide, which usually forms a second distinct scarp up to 300 m from the edge of the slide debris. Lateral ridges and distal rims have formed at only three places on the slide's margins, all close to crests of steep slopes the debris has run up. The Frank Slide then is not a slide of Shreve's Blackhawk type. Support for boulders in the debris probably came from dispersive forces and motion-induced vibration.