Journal articles on the topic 'Snow rheology'
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1
Meyer, Colin R., Kaitlin M. Keegan, Ian Baker, and Robert L. Hawley. "A model for French-press experiments of dry snow compaction." Cryosphere 14, no. 5 (May 5, 2020): 1449–58. http://dx.doi.org/10.5194/tc-14-1449-2020.
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Abstract. Snow densification stores water in alpine regions and transforms snow into ice on the surface of glaciers. Despite its importance in determining snow-water equivalent and glacier-induced sea level rise, we still lack a complete understanding of the physical mechanisms underlying snow compaction. In essence, compaction is a rheological process, where the rheology evolves with depth due to variation in temperature, pressure, humidity, and meltwater. The rheology of snow compaction can be determined in a few ways, for example, through empirical investigations (e.g., Herron and Langway, 1980), by microstructural considerations (e.g., Alley, 1987), or by measuring the rheology directly, which is the approach we take here. Using a French-press or cafetière-à-piston compression stage, Wang and Baker (2013) compressed numerous snow samples of different densities. Here we derive a mixture theory for compaction and airflow through the porous snow to compare against these experimental data. We find that a plastic compaction law explains experimental results. Taking standard forms for the permeability and effective pressure as functions of the porosity, we show that this compaction mode persists for a range of densities and overburden loads. These findings suggest that measuring compaction in the lab is a promising direction for determining the rheology of snow through its many stages of densification.
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Sturm, Matthew, and Jon Holmgren. "Differences in compaction behavior of three climate classes of snow." Annals of Glaciology 26 (1998): 125–30. http://dx.doi.org/10.3189/1998aog26-1-125-130.
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In a recent paper (Sturm and others, 1995), a global seasonal snow-cover classification system was developed with each class defined by snow properties like grain-size and type. Here, characteristic bulk density vs time curves are assigned to three classes using snow-course data from Alaskan and Canadian sites. Within each class, curves have similar slopes and intercepts but between classes they are different. The relationship between slope, intercept and snow rheology has been investigated using a finite-difference model in which snow layers are assumed to behave as viscous fluids. Using observed slopes, the density-dependent compactive viscosity of each class has been determined. These are consistent with published values. Results indicate that load and load history are less important to the compaction behavior than grain and bond characteristics, snow temperature and wetness. The study suggests that differences in compaction behavior arise primarily from differences in rheology, the result of climatically controlled differences in the character of the snow. This finding explains why regional snow densities have been successfully predicted from air temperature and wind speed alone, without considering snow depth.
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Sturm, Matthew, and Jon Holmgren. "Differences in compaction behavior of three climate classes of snow." Annals of Glaciology 26 (1998): 125–30. http://dx.doi.org/10.1017/s0260305500014683.
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In a recent paper (Sturm and others, 1995), a global seasonal snow-cover classification system was developed with each class defined by snow properties like grain-size and type. Here, characteristic bulk density vs time curves are assigned to three classes using snow-course data from Alaskan and Canadian sites. Within each class, curves have similar slopes and intercepts but between classes they are different. The relationship between slope, intercept and snow rheology has been investigated using a finite-difference model in which snow layers are assumed to behave as viscous fluids. Using observed slopes, the density-dependent compactive viscosity of each class has been determined. These are consistent with published values. Results indicate that load and load history are less important to the compaction behavior than grain and bond characteristics, snow temperature and wetness. The study suggests that differences in compaction behavior arise primarily from differences in rheology, the result of climatically controlled differences in the character of the snow. This finding explains why regional snow densities have been successfully predicted from air temperature and wind speed alone, without considering snow depth.
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Hutter, Kolumban. "Schnee-und Gletscherrheologie / Snow and Glacier Rheology." Applied Rheology 7, no. 6 (December 1, 1997): 266–76. http://dx.doi.org/10.2478/arh-1997-070607.
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Kern, M. A., F. Tiefenbacher, and J. N. McElwaine. "The rheology of snow in large chute flows." Cold Regions Science and Technology 39, no. 2-3 (October 2004): 181–92. http://dx.doi.org/10.1016/j.coldregions.2004.03.006.
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Upadhyay, Agraj, Amod Kumar, and Arun Chaudhary. "Velocity measurements of wet snow avalanche on the Dhundi snow chute." Annals of Glaciology 51, no. 54 (2010): 139–45. http://dx.doi.org/10.3189/172756410791386580.
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AbstractWet snow avalanches in India are common during the mid- and late winter in the Pir Panjal Range (2000–3000ma.s.l.) and during the late winter in the Great Himalayan Range (3000 ma.s.l. and above). Although it is well known that the presence of liquid water in snow makes the flow behaviour of wet snow avalanches different from that of dry snow avalanches, there exist few actual flow measurements with wet snow. The aim of this investigation is to understand the dynamics of wet snow avalanches by conducting medium-scale experiments (volumes of 3, 6 and 11 m3) on the Dhundi snow chute in Himachal Pradesh, India. We measured flow velocities using video data, as well as optical velocity sensors installed on the side walls and running surface. Measurement results relating to the slip velocity of the front and tail of the moving snow mass, as well as the average slip velocity, are presented. In addition, we use the results of the vertical velocity profile measurements to calculate the effective viscosity of snow at two locations within the flow. We identified a shear thinning type of behaviour, suggesting that a single avalanche rheology cannot describe wet snow avalanche behaviour.
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Buser, Othmar, and Perry Bartelt. "Production and decay of random kinetic energy in granular snow avalanches." Journal of Glaciology 55, no. 189 (2009): 3–12. http://dx.doi.org/10.3189/002214309788608859.
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AbstractAny model of snow avalanches must be able to reproduce velocity profiles. This is a key problem in avalanche science because the profiles are the result of a multitude of snow/ice particle interactions that, in the fend, define the rheology of flowing snow. Recent measurements on real-scale avalanches show that the velocity profiles change from a highly sheared profile at the avalanche front to a plug-like profile at the avalanche tail, preventing the application of a single, simple rheology to the avalanche problem. In this paper, we model not only the velocity profiles but also the evolution of the velocity profiles, by taking into account the production and decay of the kinetic energy of the random motion of the snow granules. We find that the generation of this random energy depends on the distribution of viscous shearing within the avalanche. Conversely, the viscous shearing depends on the magnitude of the random energy and therefore its collisional dissipation. Thus, there is a self–consistency problem that must be resolved in order to predict the amount of random energy and therefore the velocity profiles. We solve this problem by stating equations that describe the production and decay of random energy in avalanches. An important guide to the form of these equations is that the generation of random energy is irreversible. We show that our approach successfully accounts for measured profiles in natural avalanches.
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Issler, Dieter, and Manuel Pastor Pérez. "Interplay of entrainment and rheology in snow avalanches: a numerical study." Annals of Glaciology 52, no. 58 (2011): 143–47. http://dx.doi.org/10.3189/172756411797252031.
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AbstractA one-dimensional evolution equation for the slope-normal velocity profile of a streamwise uniform avalanche over an entrainable bed is derived. The boundary conditions are no slip at the bed, a stress-free surface and constant bed shear stress equal to the shear strength of the snow cover. The resulting equation is solved numerically by means of finite differences on a regular grid with a superposed fine grid near the erosion front that is adjusted at each time-step. The first exploratory simulations yield realistic entrainment rates and show that the entrainment rate tends towards a constant value while the flow depth and the velocity increase linearly with time for all investigated rheologies. It is shown that there indeed exists a rheology-independent asymptotic solution to the equation of motion of an entraining slab if the bottom friction is equal to the bed shear strength; the asymptotic acceleration is found to be half the downslope gravitational acceleration. The model can easily be extended to general path profiles, non-uniform flows and variable snow properties.
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Rognon, Pierre G., François Chevoir, Hervé Bellot, Frédéric Ousset, Mohamed Naaïm, and Philippe Coussot. "Rheology of dense snow flows: Inferences from steady state chute-flow experiments." Journal of Rheology 52, no. 3 (May 2008): 729–48. http://dx.doi.org/10.1122/1.2897609.
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Avanzi, Francesco, Simone Gabellani, Fabio Delogu, Francesco Silvestro, Edoardo Cremonese, Umberto Morra di Cella, Sara Ratto, and Hervé Stevenin. "Snow Multidata Mapping and Modeling (S3M) 5.1: a distributed cryospheric model with dry and wet snow, data assimilation, glacier mass balance, and debris-driven melt." Geoscientific Model Development 15, no. 12 (June 27, 2022): 4853–79. http://dx.doi.org/10.5194/gmd-15-4853-2022.
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Abstract. By shifting winter precipitation into summer freshet, the cryosphere supports life across the world. The sensitivity of this mechanism to climate and the role played by the cryosphere in the Earth's energy budget have motivated the development of a broad spectrum of predictive models. Such models represent seasonal snow and glaciers with various complexities and generally are not integrated with hydrologic models describing the fate of meltwater through the hydrologic budget. We present Snow Multidata Mapping and Modeling (S3M) v5.1, a spatially explicit and hydrology-oriented cryospheric model that simulates seasonal snow and glacier evolution through time and that can be natively coupled with distributed hydrologic models. Model physics include precipitation-phase partitioning, snow and glacier mass balances, snow rheology and hydraulics, a hybrid temperature-index and radiation-driven melt parametrization, and a data-assimilation protocol. Comparatively novel aspects of S3M are an explicit representation of the spatial patterns of snow liquid-water content, the implementation of the Δh parametrization for distributed ice-thickness change, and the inclusion of a distributed debris-driven melt factor. Focusing on its operational implementation in the northwestern Italian Alps, we show that S3M provides robust predictions of the snow and glacier mass balances at multiple scales, thus delivering the necessary information to support real-world hydrologic operations. S3M is well suited for both operational flood forecasting and basic research, including future scenarios of the fate of the cryosphere and water supply in a warming climate. The model is open source, and the paper comprises a user manual as well as resources to prepare input data and set up computational environments and libraries.
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Viroulet, Sylvain, Chris Johnson, and Nico Gray. "Modelling erosion and deposition in geophysical granular mass flows." Europhysics News 52, no. 1 (2021): 29–32. http://dx.doi.org/10.1051/epn/2021106.
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During hazardous geophysical mass flows, such as rock or snow avalanches, debris flows and volcanic pyroclastic flows, a continuous exchange of material can occur between the slide and the bed. The net balance between erosion and deposition of particles can drastically influence the behaviour of these flows. Recent advances in describing the non-monotonic effective basal friction and the internal granular rheology in depth averaged theories have enabled small scale laboratory experiments (see fig. 1) to be quantitatively reproduced and can also be implemented in large scale models to improve hazard mitigation.
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Keshari, Ashok K., Deba P. Satapathy, and Amod Kumar. "The influence of vertical density and velocity distributions on snow avalanche runout." Annals of Glaciology 51, no. 54 (2010): 200–206. http://dx.doi.org/10.3189/172756410791386409.
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AbstractA one-dimensional avalanche dynamics model accounting for vertical density and velocity distributions is presented. Mass and momentum flux distribution factors are derived to incorporate the effect of density and velocity variations within the framework of depth-integrated models. Using experiments of avalanche flows on an inclined snow chute at Dhundhi, Manali, India, we conceptualize snow flow rheology as a Voellmy fluid where the distribution of internal shearing is given by a Newtonian fluid (NF) or Criminale–Ericksen–Filbey fluid (CEFF). Then the generalized mass and momentum distribution factors are computed for these two fluid models for different density stratifications. Numerical solutions are obtained using a total variation diminishing Lax–Friedrichs (TVDLF) finite-difference method. The model is validated with the experimental results. We find that the flow features of the chute experiments are simulated well by the model. The velocities and runout distances are obtained for the Voellmy model with both NF and CEFF extensions for various input volumes, and the optimum values of the model parameters, namely, coefficients of dynamic and turbulent friction, are determined.
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Eglit, Margarita, Alexander Yakubenko, and Julia Zayko. "A Review of Russian Snow Avalanche Models—From Analytical Solutions to Novel 3D Models." Geosciences 10, no. 2 (February 20, 2020): 77. http://dx.doi.org/10.3390/geosciences10020077.
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The article is a review of mathematical models of snow avalanches that have been proposed since the middle of the 20th century and are still in use. The main attention is paid to the work of researchers from the Soviet Union and Russia, since many of their works were published only in Russian and are not widely available. Mathematical models of various levels of complexity for avalanches of various types—from dense to powder-snow avalanches—are discussed. Analytical solutions including formulas for the avalanche front speed are described. The results of simulations of the movement of avalanches are given that were used to create avalanche hazard maps. The last part of the article is devoted to constructing models of a new type, in which avalanches are considered as laminar or turbulent flows of non-Newtonian fluids, using the full (not depth-averaged) equations of continuum mechanics. The results of a numerical study of the effect of non-Newtonian rheology and mass entrainment on the avalanche dynamics are presented.
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Pajtášová, Mariana, Zuzana Mičicová, Darina Ondrušová, Slavomíra Božeková, Róbert Janík, Beáta Pecušová, and Lukáš Raník. "Use of waste materials in rubber matrix." MATEC Web of Conferences 157 (2018): 07009. http://dx.doi.org/10.1051/matecconf/201815707009.
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The presented paper deals with the use of waste materials as ecological fillers into rubber matrix. Waste materials were used as partial replacement of the commercial filler – carbon black, designated as N339. These prepared rubber compounds were characterized on the basis of the rheology and vulcanization characteristics – minimum torque (ML), maximum torque (MH), optimum time of vulcanization (t(c90)), processing safety of compound (ts), rate coefficient of vulcanization (Rv). In the case of the prepared vulcanizates, physical-mechanical properties (tensile strength, tensibility and hardness) and dynamic-mechanical properties (storage modulus, loss modulus, loss angle tan δ) were investigated. Using the dependency of loss angle on temperature, the selected properties for tyre tread vulcanizates were evaluated, including traction on snow and ice, traction on the wet surface and rolling resistance.
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Edwards, A. N., S. Viroulet, B. P. Kokelaar, and J. M. N. T. Gray. "Formation of levees, troughs and elevated channels by avalanches on erodible slopes." Journal of Fluid Mechanics 823 (June 16, 2017): 278–315. http://dx.doi.org/10.1017/jfm.2017.309.
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Snow avalanches are typically initiated on marginally stable slopes with a surface layer of fresh snow that may easily be incorporated into them. The erosion of snow at the front is fundamental to the dynamics and growth of snow avalanches and they may rapidly bulk up, making them much more destructive than the initial release. Snow may also deposit at the rear, base and sides of the flow and the net balance of erosion and deposition determines whether an avalanche grows or decays. In this paper, small-scale analogue experiments are performed on a rough inclined plane with a static erodible layer of carborundum grains. The static layer is prepared by slowly closing down a flow from a hopper at the top of the slope. This leaves behind a uniform-depth layer of thickness $h_{stop}$ at a given slope inclination. Due to the hysteresis of the rough bed friction law, this layer can then be inclined to higher angles provided that the thickness does not exceed $h_{start}$, which is the maximum depth that can be held static on a rough bed. An avalanche is then initiated on top of the static layer by releasing a fixed volume of carborundum grains. Dependent on the slope inclination and the depth of the static layer three different behaviours are observed. For initial deposit depths above $h_{stop}$, the avalanche rapidly grows in size by progressively entraining more and more grains at the front and sides, and depositing relatively few particles at the base and tail. This leaves behind a trough eroded to a depth below the initial deposit surface and whose maximal areal extent has a triangular shape. Conversely, a release on a shallower slope, with a deposit of thickness $h_{stop}$, leads to net deposition. This time the avalanche leaves behind a levee-flanked channel, the floor of which lies above the level of the initial deposit and narrows downstream. It is also possible to generate avalanches that have a perfect balance between net erosion and deposition. These avalanches propagate perfectly steadily downslope, leaving a constant-width trail with levees flanking a shallow trough cut slightly lower than the initial deposit surface. The cross-section of the trail therefore represents an exact redistribution of the mass reworked from the initial static layer. Granular flow problems involving erosion and deposition are notoriously difficult, because there is no accepted method of modelling the phase transition between static and moving particles. Remarkably, it is shown in this paper that by combining Pouliquen & Forterre’s (J. Fluid Mech., vol. 453, 2002, pp. 133–151) extended friction law with the depth-averaged $\unicode[STIX]{x1D707}(I)$-rheology of Gray & Edwards (J. Fluid Mech., vol. 755, 2014, pp. 503–544) it is possible to develop a two-dimensional shallow-water-like avalanche model that qualitatively captures all of the experimentally observed behaviour. Furthermore, the computed wavespeed, wave peak height and stationary layer thickness, as well as the distance travelled by decaying avalanches, are all in good quantitative agreement with the experiments. This model is therefore likely to have important practical implications for modelling the initiation, growth and decay of snow avalanches for hazard assessment and risk mitigation.
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Eckert, N., M. Naaim, and E. Parent. "Long-term avalanche hazard assessment with a Bayesian depth-averaged propagation model." Journal of Glaciology 56, no. 198 (2010): 563–86. http://dx.doi.org/10.3189/002214310793146331.
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AbstractWhile performing statistical–dynamical simulations for avalanche predetermination, a propagation model must reach a compromise between precise description of the avalanche flow and computation times. Crucial problems are the choice of appropriate distributions describing the variability of the different inputs/outputs and model identifiability. In this study, a depth-averaged propagation model is used within a hierarchical Bayesian framework. First, the joint posterior distribution is estimated using a sequential Metropolis–Hastings algorithm. Details for tuning the estimation algorithm are provided, as well as tests to check convergence. Of particular interest is the calibration of the two coefficients of a Voellmy friction law, with model identifiability ensured by prior information. Second, the point estimates are used to predict the joint distribution of different variables of interest for hazard mapping. Recent developments are employed to compute pressure distributions taking into account the rheology of snow. The different steps of the method are illustrated with a real case study, for which all possible decennial scenarios are simulated. It appears that the marginal distribution of impact pressures is strongly skewed, with possible high values for avalanches characterized by low Froude numbers. Model assumptions and results are discussed.
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Baker, J. L., C. G. Johnson, and J. M. N. T. Gray. "Segregation-induced finger formation in granular free-surface flows." Journal of Fluid Mechanics 809 (November 9, 2016): 168–212. http://dx.doi.org/10.1017/jfm.2016.673.
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Geophysical granular flows, such as landslides, pyroclastic flows and snow avalanches, consist of particles with varying surface roughnesses or shapes that have a tendency to segregate during flow due to size differences. Such segregation leads to the formation of regions with different frictional properties, which in turn can feed back on the bulk flow. This paper introduces a well-posed depth-averaged model for these segregation-mobility feedback effects. The full segregation equation for dense granular flows is integrated through the avalanche thickness by assuming inversely graded layers with large particles above fines, and a Bagnold shear profile. The resulting large particle transport equation is then coupled to depth-averaged equations for conservation of mass and momentum, with the feedback arising through a basal friction law that is composition dependent, implying greater friction where there are more large particles. The new system of equations includes viscous terms in the momentum balance, which are derived from the $\unicode[STIX]{x1D707}(I)$-rheology for dense granular flows and represent a singular perturbation to previous models. Linear stability calculations of the steady uniform base state demonstrate the significance of these higher-order terms, which ensure that, unlike the inviscid equations, the growth rates remain bounded everywhere. The new system is therefore mathematically well posed. Two-dimensional simulations of bidisperse material propagating down an inclined plane show the development of an unstable large-rich flow front, which subsequently breaks into a series of finger-like structures, each bounded by coarse-grained lateral levees. The key properties of the fingers are independent of the grid resolution and are controlled by the physical viscosity. This process of segregation-induced finger formation is observed in laboratory experiments, and numerical computations are in qualitative agreement.
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Salamatin, Andrey N., Vladimir Ya Lipenkov, and Paul Duval. "Bubbly-ice densification in ice sheets: I. Theory." Journal of Glaciology 43, no. 145 (1997): 387–96. http://dx.doi.org/10.3189/s0022143000034961.
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AbstractDry snow on the surface of polar ice ice sheets is first densified and metamorphosed to produce firn. Bubbly ice is the next stage of the transformation process which takes place below the depth of pore closure. This stage extends to the transition zone where, due to high pressures and low temperatures. air trapped in bubbles and ice begins to form the mixed air clathrate hydrates, while the gas phase progressively disappears. Here we develop a model of bubbly-ice rheology and ice-sheet dynamics taking into account glacier-ice compressibility. The interaction between hydrostatic compression of air bubbles, deviatoric (uniaxial) compressive deformation of the ice matrix and global deformations of the glacier body is considered. The ice-matrix pressure and the absolute-load pressure are distinguished. Similarity theory and scale analysis are used in examine the resultant mathematical model of bubbly-ice densification. The initial rate of bubble compression in ice sheets appears to be relatively high, so that the pressure (density) relaxation process takes place only 150-200 m in depth (below pore close-off) to reach its asymptotic phase, wherein the minimal drop between bubble and ice pressures is governed by the rate of loading (ice accumulation). This makes it possible to consider densification under stationary (present-day) conditions of ice formation as a special case of primary interest. The computational tests performed with the model indicate that both ice-porosity and bubble-pressure profiles in ice sheets are sensitive to variations of the rheological parameters of pure ice. However, only the bubble-pressure distinguishes between the rheological properties at low and high stresses. The porosity profile at the asymptotic phase is mostly determined by the air content in the ice. In the companion paper (Lipenkov and others, 1997), we apply the model to experimental data from polar ice cores and deduce, through an inverse procedure, the rhelogical properties of pure ice as well as the mean air content in Holocene and glacial ice sediments at vostok Station (Antarctica).
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Salamatin, Andrey N., Vladimir Ya Lipenkov, and Paul Duval. "Bubbly-ice densification in ice sheets: I. Theory." Journal of Glaciology 43, no. 145 (1997): 387–96. http://dx.doi.org/10.1017/s0022143000034961.
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AbstractDry snow on the surface of polar ice ice sheets is first densified and metamorphosed to produce firn. Bubbly ice is the next stage of the transformation process which takes place below the depth of pore closure. This stage extends to the transition zone where, due to high pressures and low temperatures. air trapped in bubbles and ice begins to form the mixed air clathrate hydrates, while the gas phase progressively disappears. Here we develop a model of bubbly-ice rheology and ice-sheet dynamics taking into account glacier-ice compressibility. The interaction between hydrostatic compression of air bubbles, deviatoric (uniaxial) compressive deformation of the ice matrix and global deformations of the glacier body is considered. The ice-matrix pressure and the absolute-load pressure are distinguished. Similarity theory and scale analysis are used in examine the resultant mathematical model of bubbly-ice densification. The initial rate of bubble compression in ice sheets appears to be relatively high, so that the pressure (density) relaxation process takes place only 150-200 m in depth (below pore close-off) to reach its asymptotic phase, wherein the minimal drop between bubble and ice pressures is governed by the rate of loading (ice accumulation). This makes it possible to consider densification under stationary (present-day) conditions of ice formation as a special case of primary interest. The computational tests performed with the model indicate that both ice-porosity and bubble-pressure profiles in ice sheets are sensitive to variations of the rheological parameters of pure ice. However, only the bubble-pressure distinguishes between the rheological properties at low and high stresses. The porosity profile at the asymptotic phase is mostly determined by the air content in the ice. In the companion paper (Lipenkov and others, 1997), we apply the model to experimental data from polar ice cores and deduce, through an inverse procedure, the rhelogical properties of pure ice as well as the mean air content in Holocene and glacial ice sediments at vostok Station (Antarctica).
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Woodhouse, M. J., A. R. Thornton, C. G. Johnson, B. P. Kokelaar, and J. M. N. T. Gray. "Segregation-induced fingering instabilities in granular free-surface flows." Journal of Fluid Mechanics 709 (August 21, 2012): 543–80. http://dx.doi.org/10.1017/jfm.2012.348.
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AbstractParticle-size segregation can have a significant feedback on the bulk motion of granular avalanches when the larger grains experience greater resistance to motion than the fine grains. When such segregation-mobility feedback effects occur the flow may form digitate lobate fingers or spontaneously self-channelize to form lateral levees that enhance run-out distance. This is particularly important in geophysical mass flows, such as pyroclastic currents, snow avalanches and debris flows, where run-out distance is of crucial importance in hazards assessment. A model for finger formation in a bidisperse granular avalanche is developed by coupling a depth-averaged description of the preferential transport of large particles towards the front with an established avalanche model. The coupling is achieved through a concentration-dependent friction coefficient, which results in a system of non-strictly hyperbolic equations. We compute numerical solutions to the flow of a bidisperse mixture of small mobile particles and larger more resistive grains down an inclined chute. The numerical results demonstrate that our model is able to describe the formation of a front rich in large particles, the instability of this front and the subsequent evolution of elongated fingers bounded by large-rich lateral levees, as observed in small-scale laboratory experiments. However, our numerical results are grid dependent, with the number of fingers increasing as the numerical resolution is increased. We investigate this pathology by examining the linear stability of a steady uniform flow, which shows that arbitrarily small wavelength perturbations grow exponentially quickly. Furthermore, we find that on a curve in parameter space the growth rate is unbounded above as the wavelength of perturbations is decreased and so the system of equations on this curve is ill-posed. This indicates that the model captures the physical mechanisms that drive the instability, but additional dissipation mechanisms, such as those considered in the realm of flow rheology, are required to set the length scale of the fingers that develop.
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Lavigne, Frank, and Jean-Claude Thouret. "Les lahars; depots, origines et dynamique." Bulletin de la Société Géologique de France 171, no. 5 (September 1, 2000): 545–57. http://dx.doi.org/10.2113/171.5.545.
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Abstract A lahar is a flowing mixture of rock debris and water (other than normal streamflow) from a volcano, which encompasses a continuum from debris flows (sediment concentration > or =60% per volume) to hyperconcentrated streamflows (sediment concentration from 20 to 60% per volume). Debris flow deposits are poorly sorted and massive with abundant clasts. Lahars can be either syn-eruptive, post-eruptive or have a non-eruptive origin. Four types of lahars can be generated during an eruption, based on distinct sources of water (i.e. ice, snow, crater lake, river, and rain) that allow the sediments to be removed and incorporated in the lahar (e.g., Mount St.-Helens in 1980, Nevado del Ruiz in 1985). Post-eruptive lahars, which are rain-triggered, occur during several years after an eruption (e.g., still occurring at Pinatubo). Non-eruptive lahars are flows generated on volcanoes without eruptive activity, particularly in the case of a debris avalanche or a lake outburst (e.g., Kelud or Ruapehu). Lahars flow as pulses, whose velocity and discharge are much higher than those of streamflows, including catchments similar in size. Sediment transport capacity of lahars is exceptional, owing to buoyancy, dispersive pressure, and to the amount of cohesive clay and silt. However, the finding of recent experimental works indicates that even clay-rich lahar mixtures have little true cohesion. Therefore, the typical classification of lahars into "cohesive" and "non cohesive" seems to be inappropriate at present. Besides, past work on lahar mechanics used models based on the Bagnold's or the Bingham's theories. Recent advances in experimentation show that a lahar has specific rheological properties: it moves as a surge or series of surges, driven by gravity, by porosity fluctuation, and by pore fluid pressures, in accordance with the Coulomb grain flow model. Grain size distribution and sorting control pore pressure distribution. Lahar mechanics depend on much more than steady-state rheology, because lahars are highly unsteady and typically heterogeneous flows. Lahar can show a succession of debris flow phases, hyperconcentrated flow phases, and sometimes transient streamflow phases. Therefore, some fluids-mechanics concepts and terminology, such as "viscous", "laminar" or "non-Newtonian" are inappropriate to describe the mechanical properties of lahars. Processes of deposition are complex and poorly known. Interpretation of massive and unsorted lahar deposits commonly ascribe the deposition regime to a freezing en masse process. However, recent laboratory experiments highlight that debris-flow deposits may result from incremental deposition processes.
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Rocha, F. M., C. G. Johnson, and J. M. N. T. Gray. "Self-channelisation and levee formation in monodisperse granular flows." Journal of Fluid Mechanics 876 (August 5, 2019): 591–641. http://dx.doi.org/10.1017/jfm.2019.518.
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Dense granular flows can spontaneously self-channelise by forming a pair of parallel-sided static levees on either side of a central flowing channel. This process prevents lateral spreading and maintains the flow thickness, and hence mobility, enabling the grains to run out considerably further than a spreading flow on shallow slopes. Since levees commonly form in hazardous geophysical mass flows, such as snow avalanches, debris flows, lahars and pyroclastic flows, this has important implications for risk management in mountainous and volcanic regions. In this paper an avalanche model that incorporates frictional hysteresis, as well as depth-averaged viscous terms derived from the $\unicode[STIX]{x1D707}(I)$-rheology, is used to quantitatively model self-channelisation and levee formation. The viscous terms are crucial for determining a smoothly varying steady-state velocity profile across the flowing channel, which has the important property that it does not exert any shear stresses at the levee–channel interfaces. For a fixed mass flux, the resulting boundary value problem for the velocity profile also uniquely determines the width and height of the channel, and the predictions are in very good agreement with existing experimental data for both spherical and angular particles. It is also shown that in the absence of viscous (second-order gradient) terms, the problem degenerates, to produce plug flow in the channel with two frictionless contact discontinuities at the levee–channel margins. Such solutions are not observed in experiments. Moreover, the steady-state inviscid problem lacks a thickness or width selection mechanism and consequently there is no unique solution. The viscous theory is therefore a significant step forward. Fully time-dependent numerical simulations to the viscous model are able to quantitatively capture the process in which the flow self-channelises and show how the levees are initially emplaced behind the flow head. Both experiments and numerical simulations show that the height and width of the channel are not necessarily fixed by these initial values, but respond to changes in the supplied mass flux, allowing narrowing and widening of the channel long after the initial front has passed by. In addition, below a critical mass flux the steady-state solutions become unstable and time-dependent numerical simulations are able to capture the transition to periodic erosion–deposition waves observed in experiments.
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Pogorzelski, Stanisław J., Paweł Rochowski, Maciej Grzegorczyk, and Katarzyna Boniewicz-Szmyt. "Snowpack-stored atmospheric surface-active contaminants traced with snowmelt water surface film rheology." Environmental Science and Pollution Research, September 23, 2020. http://dx.doi.org/10.1007/s11356-020-10874-1.
Full textAbstract:
Abstract The aim of the study was to quantify the adsorptive and thermo-elastic properties of snowmelt water surface films and their spatial-temporal evolution with snowpack structure characteristics and the entrapped surface-active organic composition. Surface pressure–area (π-A)T isotherms, surface pressure-temperature (π-T)A isochors, and stress–relaxation (π-t) measurements were performed using a Langmuir trough system on snowmelt water samples collected in a large-scale field studies performed at several industrialized and rural Tricity (Gdansk, Poland) areas at various environmental conditions and subsequent stages of the snowpack melting progress. Since the snow-melted water composition and concentrations of surface active organic matter fractions therein are largely undetermined, the force-area isotherm scaling formalisms (2D virial equation and 2D film scaling theory of polymeric films) were adapted to the complex mixture of surfactants. The surface film parameters and their spatial and temporal evolution turned out to be unequivocally related to principal signatures of the film-forming materials: surfactant concentrations (π, Alim), surface activity (Eisoth, |E|), film material solubility (R), surface material miscibility and 2D architecture complexity (y, βs), molecular thermal mobility (πk), and a timescale of the relaxation processes within the film (τi, |E|). Moreover, the parameters appeared to be correlated with snowpack structure characteristics (snow density ρ, specific snow area SSA, snow cover thickness), sample age time, and anthropogenic atmospheric contamination pressure source locations. In particular, Eisoth was found to be related to ρ and SSA, while R correlated with the solubility of film-forming organics which turned out to be long-chain fatty acids; similarly, spatial profiles of Eisoth revealed the peak values next to the areas being under a severe anthropogenic air pollution pressure. Snowmelt water films stand for a structurally heterogeneous (y > 10) interfacial system where several transition processes of differentiated time-scales (relaxation times from 7 to 63 s) took place leading to the apparent surface viscoelasticity. To sum up, the established surface rheological parameters could serve as novel indicators, based solely on physical attributes, allowing to follow the snowpack evolution, and its melting polymorphism in order to test or improve the existing snow-entrapped organics release models based on chemical analyses. The cross-correlation functional dependences of practical value remain to be established on the larger data set.