Academic literature on the topic 'Snow rheology'

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.

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.

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.

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.

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.

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.

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.

The interaction between snow avalanches and structures represents a topic of interest both from a scientific point of view, since different study domains and knowledge are involved (structural mechanics, fluid dynamics…), and due to its applicability in practice for a correct design of structures located in avalanche risk areas. In this thesis the interaction between the snow avalanches and structures is investigated together with the avalanche dynamics. Chapter 1 deals with the state of the art of the avalanche dynamics and interaction between snow in movement and structures. The snow avalanches are classified, giving the basics concepts. Secondly the different approaches to study the interaction between avalanches and structures are analysed. The observations of the damages caused on structures by real events are not sufficient to understand all the complex processes inner the dynamics itself and the impact strictly. Furthermore experiments are carried in order to analyse deeper velocity profiles, to which pressure ones are linked, entrainment of snow, from which the volumes involved depended as well as the pressure behaviour. In fact pressure values evolve in time and in space and change with the obstacle shape. Experimental studies are made at real scale avalanches, in the test sites, or at reduced scale, in laboratory chutes. To translate the results from the small scale to the real one similitude criteria have to be satisfied. Hence the dimensional analysis is proposed. Another approach to study the problem in object is to use analytical and numerical models. For this reason a summary of the state of art of dynamics models is proposed, focusing the attention on those taking into account the erosion and the interaction with obstacles. From both experimental and theoretical analysis recommendations are born in order to help the expert to correctly design the structures in avalanche areas. In Chapter 2 a new model is described, able to provide the pressure and the velocity in all the points of the avalanche, without impose a proportional relationship between them. The model describe the evolution of the avalanche shape thanks to the level set method, suitable for free-boundary problems, and the Navier-Stokes equations, since the avalanche is considered a fluid. A first validation on experimental data of a laboratory chute is given. Afterwards the attention is set at the avalanche bottom. In particular the boundary condition of the slip velocity is analysed, giving an analytical justification. The slip condition, coupled with a non-newtonian fluid, is able to correctly describe the velocity profile. Finally a new model for the erosion is proposed, starting from general continuum mechanics hypothesis. In particular both the avalanche and the snow at rest are considered as the same fluid having a viscosity depending from the shear rate. It is shown as the model is in agreement with other theories in the literature and takes into account the influence of snow and avalanche properties, the avalanche depth, the slope angle, and the position in the avalanche (front or tail). Chapter 3 focuses the attention on the definition of a model to describe the impact of an avalanche with obstacles. Different approaches can be pursued: a stationary and a transient ones, as well as a two-dimensional analysis in the avalanche depth plane, in the slope plane and a three-dimensional one. Some preliminary simulations are shown and qualitatively compared with the state of the art concerning the impact pressure. For instance the pressure profile along the avalanche depth, the influence on the obstacle shape and dimension, and the dependence on the relative position obstacle-avalanche (directly or not directly exposed) are investigated. In Chapter 4 the new Italian P.ta Seehore test site is described. Its peculiarity is to study the small-medium avalanches that occurred with high frequency, since artificially triggered for safe reasons. The attention is focused on the design of an obstacle, located in the avalanche track, to study the interaction between snow in movement and structure. The static and dynamic test carried to characterise it are shown as well as its instrumentation. Finally an overview of the surveys is proposed focusing the attention on the measurements carried in some events. Chapter 5 deals with the analysis of the measurement data concerning experiments in the P.ta Seehore test site from different point of view. Firstly, the erosion and deposition processes are analysed, using laser scan data, analytical and numerical methods and presenting a new cheap test to detect the net erosion and deposition. Secondly, a commercial dynamics model is applied to obtain the flow density and velocity at the obstacle, data not experimentally recorded. Thirdly, our dynamics model is used for instance to simulate the creation of a dihedral shape upward the obstacle, experimentally measured and to give information on the pressure. Finally analytical approaches are used to describe the pressure, applying for instance the Mohr-Coulomb criterion, to simulate the pressure in the avalanche tail. Concepts reported in the available recommendations, as for instance the compressibility of the snow during the impact are used too. In Chapter 6 applications concerning the impact against houses destroyed in 15th of December 2008 are reported. In particular both the transient and stationary models (in their two and three-dimensional versions) are applied and compared with a back-analysis of damages. General laws for the influence of the impact angle on the pressure are respected as well as the areas of positive and negative pressure. In addition, the protection role played by a house on the structures downstream, especially in term of reduced pressures, is analysed. The Conclusions and outlooks finalize the work.