Correlating Snow Micro-Structure with Snow Shear Strength (original) (raw)
Preliminary characterization of Alpine snow using SnowMicroPen
Cold Regions Science and Technology, 2009
Objective and accurate observations of snowpack layering and properties are essential for avalanche warning. Human observations are helpful, but they are often highly subjective and therefore inconsistent making them difficult to interpret. In this paper we address this problem by applying a high-resolution penetrometer to determine snowpack properties. We develop an algorithm to characterize the different snow microstructure classes using the signal obtained from the penetrometer measurements. For this purpose a database consisting of various snow profiles of Alpine snow was created, which contains the records of the mean, standard deviation and coefficient of variation of penetration resistance over 1 mm depth. The algorithm is able to characterize the major snow classes, namely, new snow, faceted snow, depth hoar, rounded grains and melt-freeze with an acceptable accuracy for warning purposes. We find that new snow, depth hoar and meltfreeze snow layers are characterized better than the rounded grains and faceted snow layers. However, knowledge-based rules, formulated from the experience of field researchers, can further improve the automated snow profiling procedure. For a comprehensive prediction of snow classes with better accuracy, it is proposed in future to include more expert rules and to enlarge the database of measurements. This preliminary approach could be helpful in validating numerical simulation models of snowpacks and subsequently numerical avalanche forecasting. The potential application of snow class information derived from the SMP to infer the specific surface area of snow layers is also discussed.
Local snowpack measurements and local stability tests are currently the basis for the assessment of snowpack stability in most avalanche warning operations. The SnowMicroPen (SMP), a high-resolution penetrometer for snow, measures penetration resistance of snow. In order for SMP measurements to be useful for stability evaluation (or avalanche forecasting purposes), stability information needs to be derived from the SMP signal. It was shown that structural and mechanical parameters derived from the SMP signal for known (or manually observed) failure interfaces were related to snowpack stability. The dataset contained 66 parallel SMP and manual snow profiles with stability tests from five winter seasons 2001-02 to 2005-06 in Switzerland. It was balanced between stable (35) and unstable (31) profiles. The manual failure layer determination in the SMP signal was improved. Micro structural and mechanical parameters were derived from the SMP signal using two models describing the interaction of the SMP tip with an idealized snow structure. The parameters from the improved model are compared with snow stability data for the first time. The new model slightly improved the results of the statistical analysis and the classification accuracy of a failure interface from a SMP profile. The analysis confirms the potential of the SnowMicroPen operational use in avalanche forecasting services.
Testing a new shear loading apparatus for in-situ studies of weak snow layers
Here we show preliminary results of shear tests carried out with a new shear test apparatus designed and constructed by Politecnico di Torino, DISEG (Italy). The ultimate objective of the work is to develop an instrument for in situ measurements of the mechanical properties of weak snow layers. At the present, it is a portable force-controlled apparatus with adjustable shear-loading rate and normal pressure for specimen dimensions 0.16 m × 0.16 m × 0.08 m. The first series of cold laboratory experiments were focused on calibration, experimental methodology, and, in particular, on sintering (conducted at CEN / Météo-France). As an illustration of the instrument's performance, here we show a set of tests dedicated to sintering effects on snow interface strengthening with time. The protocol of experiments could be briefly described as follows: two snow blocks were placed on top of one another and immediately subjected to rapid horizontal loading at a constant rate of about 19 N s -1 (i.e., effectively cutting the sintering time to zero); then the same samples were left for four hours and re-tested; finally, the same procedure was repeated after 16 more hours. For each test, shear displacements and force were recorded with high-frequency gauges. Interfacial strength evolved rapidly under constant normal pressure, air temperature and relative humidity (about 0.1 kPa, -10 °C, and 70 %, respectively) and corresponded to an increase of failure load by an order of magnitude (about 0.3 kPa h -1 ; that is comparable to field measurements by . Following further cold laboratory testing with various types of snow and, possibly, some technical modifications, we intend to continue the development of the instrument for its future usage in the field at slab avalanche release zones.
The First Wetting of Snow: Micro-Structural Hardness Measurements Using a Snow Micro Penetrometer
Wet snow avalanches are responsible for avalanche fatalities but more frequently they threaten infrastructure, as mountain pass roads, in spring. A weak snowpack base consisting of persistent grains is considered one of the causes for these avalanches. In three field experiments in the Grisons Mountains in eastern Switzerland the evolution of water flow and the loss of micro-structural hardness with first wetting were investigated focusing on layers of facets and depth-hoar crystals. The Snow Micro Pen (SMP) measures the micro-structural hardness (bond strength) of snow. Based on a total of 91 SMP measurements, snow hardness always decreased at water contents of about 3%. The loss in snow hardness was significant in a facet grain -depth hoar layer reaching 16% of initial dry snow hardness (water content < 6 %). The two other investigated layers also showed changes: hardness reduced by 36% to 21% (water content < 3%). These results indicate that the loss of strength with first introduction of water begins at very low water contents. Rapid hardness decrease may influence wet snow stability and be indeed one of the keys for deep wet slab avalanche release during spring snow melt.
Force-controlled shear experiments with snow samples
Natural dry-snow slab avalanches start with a failure within a weak snow layer. In order to understand the mechanical behaviour and the failure mechanism, we performed loading experiments with homogeneous and layered snow samples under controlled conditions in a cold laboratory. For simulating loading conditions similar to the natural snow pack, we designed and built an apparatus where a snow sample can be tilted by a 'slope angle' and is loaded via the gravitational force. The deformation within the snow sample was measured optically with a pattern recognition algorithm. Shortly before macroscopic failure (breaking of the whole sample) we observed a concentration of deformation (softening) within the weak layer. Additionally, we measure acoustic emissions during the shear experiments to quantify the microscopic failure (breaking of bonds) in the snow sample before the complete failure. Our results support the assumption that the dominating mechanisms for snow deformation are the competing processes of breaking and sintering of bonds between grains.
On the objective analysis of snow microstructure
1987
ABSTRACT It is a long-standing problem to find an appropriate small set of parameters that best describes snow micro structure as observed on sections or thin slices, for the purposes of relating physical properties of snow to its electromagnetic, thermal, or mechanical properties. The stereological parameters point density and intercept length (uniquely related to surface area per unit volume) are likely members of the minimal set of descriptors, but a measure of their distribution, such as the variance, is also needed.
Measured shear rates in large dry and wet snow avalanches
Journal of Glaciology, 2009
We present estimates of internal shear rates of real-scale avalanches that are based on velocity measurements. Optical velocity sensors installed on the instrument pylon at the Swiss Vallée de la Sionne test site are used to measure flow velocities at different flow heights of three large dry and wet snow avalanches. Possible sources of error in the correlation analysis of the time-lagged reflectivity signals measured by optical sensors are identified for real-size avalanches. These include spurious velocities due to noise and elongated peaks. An appropriate choice of the correlation length is essential for obtaining good velocity estimates. Placing restrictions on the maximum possible accelerations in the flow improves the analysis of the measured data. Coherent signals are found only in the dense flowing cores. We observe the evolution of shear rates at different depths between the front and tail of the flowing avalanche. At the front, large shear rates are found throughout the de...
Storm Snow Avalanches: Characteristics and Forecasting
Proceedings 2012 International Snow Science Workshop Anchorage Alaska, 2012
At ski areas, a majority of avalanches fail in storm snow. We investigate these avalanches using stability tests and avalanche observations from California and Alaska. Collapse amplitudes during fracture, measured using particle tracking, were 1 mm for a failure layer of precipitation particles and 7 mm for a layer of unrimed sectored plates. Stability test results showed little dependence on slope angle, suggesting that both precipitation particles and older faceted crystals (persistent weak layers) fail as described by the anticrack model, with collapse providing energy. Using observations from avalanche control work at Mammoth Mountain, CA USA, a large coastal ski area where 9/10 avalanches fail in storm snow, we examined Extended Column Test (ECT) results and their relation to avalanche activity. ECT propagation was a powerful predictor; days with ECTs that propagated had significantly more and larger avalanches. Since other studies have shown that the ECT is an effective predictor of avalanches involving persistent weak layers, we suggest that the ECT is an effective test to predict both types of avalanches, those that fail in storm snow and those that fail on persistent weak layers.
Snowpack stability information derived from the SnowMicroPen signal
Cold Regions Science and Technology, 2007
Snowpack measurements and stability tests are, next to recent avalanche activity and weather history, currently the basis for snowpack stability assessment in most avalanche warning operations. The SnowMicroPen (SMP), a high-resolution penetrometer for snow, measures penetration resistance force or snow hardness. In order to be useful for an avalanche warning service, stability information needs to be provided and must be derivable from the SMP signal. SMP profiles (25 on slopes, 14 on flat sites) were taken together with manual snow profiles and stability tests, such as Rutschblock and compression tests. The data are from three winter seasons of the years 2001-2002 to 2003-2004 in the Swiss Alps. According to their stability test score and failure interface properties the manual profiles were classified as stable or unstable. Based on the manual observations the failure interfaces were identified in the SMP profiles and possible indicators of instability were derived from the SMP signals at these interfaces. The distinct indicators of instability were the failure layer micro-structural length and hardness, the difference in structural length across the failure interface and the failure layer macro-elastic modulus. The cross-validated accuracy of classification into stable or unstable failure interfaces gained from SMP parameters was comparable to the classification accuracy from manual profile parameters (about 65%). It remains to be tested if stability information can be derived from a SMP measurement without knowing the location of the failure interface found by a stability test. If this can be done successfully and reliably, avalanche warning operations could definitely benefit from the instrument.
Snow stability on uniform slopes: implications for avalanche forecasting
This research investigated whether single snowpits can reliably represent snowpack stability on uniform slopes. The study utilized seven carefully selected slopes, three each in the Bridger and Madison Ranges of Southwest Montana, and one in the Columbia Mountains near Rogers Pass, British Columbia. Teams performed ten Quantified Loaded Column Tests in each of five snowpits within a 900 m 2 plot at a slope, measuring shear strength in a single weak layer. Collection of slab shear stress data enabled the calculation of a strength/stress stability ratio. Altogether, eleven stability-sampling trials were performed during 2000/2001 and 2001/2002, testing several weak layer types exhibiting a wide range of strengths. Of the 54 snowpits completed, 26 pits (48%) represented plot-wide stability and 28 pits (52%) did not. One plot collapsed prior to completion of a 55 th pit. Two of the eleven plots did contain full complements of five representative snowpits. The results of this study suggest the importance of improving our understanding of the processes affecting the variability of snowpack stability on any given day.
Effects of flow regime and sensor geometry on snow avalanche impact-pressure measurements
Journal of Glaciology, 2011
Impact pressures of snow avalanches have been measured at the Swiss Vallée de la Sionne experimental test site using two kinds of sensor placed at different locations in the avalanche flow. Pressures measured in a fast dry-snow avalanche and a slow wet-snow avalanche are compared and discussed. The pressures recorded using the two types of sensor in the dense flow of a dry-snow avalanche agree well, showing negligible dependence on the measurement device. On the other hand, significantly different pressures are measured in the slow dense flow of a wet-snow avalanche. This is attributed to the slow drag and bulk flow of this type of avalanche, leading to the formation and collapse of force-chain structures against the different surfaces of the sensors. At a macroscopic scale, limit state analysis can be used to explain such a mechanism by a shear failure occurring between freely flowing snow and a confined snow volume against the sensor, according to a Mohr–Coulomb failure criterion....
Granulometric investigations of snow avalanches
Journal of Glaciology, 2009
Avalanche deposits consist of rounded granules composed of aggregates of snow and ice particles. The size of the granules is related to vertical shear gradients within the flow; studying the granule-size distribution may be useful in understanding the flow and stopping of avalanches. We applied a sediment-size sampling method to measure snow granule-size distributions at different depositional environments on two dry and two wet avalanche deposits at three field sites. The granule-size distributions are approximately log-normal, similar to many natural sediment deposits. The median granule size in the wet and dry avalanches varies between 65 and 162 mm. Wet avalanches tend to produce more large granules than dry avalanches, indicating both smaller flow velocities and near-surface shear gradients. Granule size is similar in frontal lobes and levee deposits, suggesting that levee formation occurs independently of the size segregation at the avalanche front.
Field measurements of snow stability provide the only direct evidence of snow stability except avalanche observation. In-situ snow stability measurements are reviewed. Field data on shear strength of weak layers and tensile strength of slabs are described, as well as the appropriate techniques to collect the data. An overview of presently applied snow stability tests (rutschblock, compression test etc.) is given. New possibilities to measure the mechanical properties (e.g. SnowMicroPen) relevant for assessing avalanche danger are discussed.
Instruments and Methods Stability algorithm for snow micro-penetrometer measurements
2009
Information on snow-cover stability is important for predicting avalanche danger. Traditionally, stability evaluation is based on manual observations of snow stratigraphy and stability tests, which are time-consuming. The SnowMicroPen (SMP) is a high-resolution, constant-speed penetrometer to measure penetration resistance. We have analysed the resistance signal to derive snow stability. The proposed stability algorithm was developed by comparing 68 SMP force-distance profiles with the corresponding manual profiles, including stability tests. The algorithm identifies a set of four potentially weak layers by taking into account changes in structure and rupture strength of microstructural elements that make up snow layers as derived from the SMP signal. In 90% of the cases, one of the four potentially weak layers proposed by the algorithm coincided with the failure layer observed in the stability test. To select the critical layer from the four potential weaknesses was more difficult. With fully automatic picking of the critical layer, agreement with the failure layer observed in the stability test was reached in 60% of the cases. To derive a stability classification, we analysed weaklayer as well as slab properties. These predictor variables allow the SMP signal to be classified into two stability classes, poor and fair-to-good, with an accuracy of $75% when compared with observed stability. The SMP, in combination with the proposed algorithm, shows high potential for providing snow-cover stability information at high resolution in time and space.
Density, velocity and friction measurements in a dry-snow avalanche
Annals of Glaciology
A small avalanche path near the Bridger Bowl ski area in southwestern Montana has been instrumented to measure density, velocity and dynamic friction in a flowing avalanche. These measurements, made by an array of sensors mounted in the avalanche path, have been carried out for several dry-snow avalanches. Measurements of density were made using a capacitance probe that measures the dielectric constant of any material that passes in front of it. Through a calibration procedure, the dielectric constant of a given type of snow can be related to the density of that snow. Optical sensors were used to measure light reflected from the avalanche as it passed by the sensors. Signals from adjacent optical sensors were cross-correlated to determine velocity. Density and velocity measurements were made at several heights in the avalanche, with particular attention directed near the running surface. Results indicate that avalanche deformation is concentrated near the running surface where the s...
Cold Regions Science and Technology, 2009
Compression tests are snow stability tests that are widely used by avalanche professionals and snow researchers to identify potential weak snowpack layers. Describing fracture character in addition to the number of taps required to initiate a fracture improves the interpretation of compression test results, since certain types of fractures, i.e. sudden fractures, are more often associated with skier-triggered avalanches. Digital snowpack penetrometers provide high resolution penetration resistance data of the snow cover with depth. The SnowMicroPen (SMP) was used to measure high resolution penetration resistance profiles in the snowpack next to compression tests. A reliable method to automatically detect the snow surface in the SMP signals was introduced. Furthermore, a method based on the autocorrelation of the penetration resistance signal was developed to identify the failure layers, identified using compression tests, in the penetration resistance profiles. Using field data from 190 penetration resistance measurements, each collected between two compression tests, micro-structural parameters associated with different types of fractures were identified. More than 550 fractures were classified as either Progressive Compression (1.3%), Resistant Planar (12.1%), Sudden Planar (50.4%), Sudden Collapse (26.8%) or non-planar Break (9.4%). Measurement and analysis were focussed on micro-structural properties of the failure layer, the layer adjacent to the failure layer and the slab above the failure layer. Sudden collapse fractures were found to have typical micro-structural snowpack parameters that are generally associated with unstable snow conditions, such as large differences in penetration resistance between the failure layer and the adjacent layer.
Reviews of Geophysics, 2003
1] Snow avalanches are a major natural hazard, endangering human life and infrastructure in mountainous areas throughout the world. In many countries with seasonally snow-covered mountains, avalanche-forecasting services reliably warn the public by issuing occurrence probabilities for a certain region. However, at present, a single avalanche event cannot be predicted in time and space. Much about the release process remains unknown, mainly because of the highly variable, layered character of the snowpack, a highly porous material that exists close to its melting point. The complex interaction between terrain, snowpack, and meteorological conditions leading to avalanche release is commonly described as avalanche formation. It is relevant to hazard mapping and essential to short-term forecasting, which involves weighting many contributory factors. Alternatively, the release process can be studied and modeled. This approach relies heavily on snow mechanics and snow properties, including texture. While the effect of meteorological conditions or changes on the deformational behavior of snow is known in qualitative or semiquantitative manner, the knowledge of the quantitative relation between snow texture and mechanical proper-ties is limited, but promising developments are under way. Fracture mechanical models have been applied to explain the fracture propagation, and micromechanical models including the two competing processes (damage and sintering) have been applied to explain snow failure. There are knowledge gaps between the sequence of processes that lead to the release of the snow slab: snow deformation and failure, damage accumulation, fracture initiation, and fracture propagation. Simultaneously, the spatial variability that affects damage, fracture initiation, and fracture propagation has to be considered. This review focuses on dry snow slab avalanches and shows that dealing with a highly porous media close to its melting point and processes covering several orders of scale, from the size of a bond between snow grains to the size of a mountain slope, will continue to be very challenging.
On snow avalanche flow regimes: Inferences from observations and measurements
Mixed dry-snow avalanches are commonly thought to consist of a dense core and a dilute suspension layer, even though observations and measurements from Canada and Russia have long indicated the presence of an intermediate-density layer ("light flow" or "saltation layer"). We summarize field observations and measurements from Norway and Switzerland, both from spontaneous events and from avalanches released at the test sites Ryggfonn and Valí ee de la Sionne. Deposition patterns, high-frequency impact pressure records and radar measurements show that a substantial mass fraction of mixed dry-snow avalanches is flowing in this "fluidized" regime, par-ticularly the head. Based on mechanical considerations, we suggest close correspon-dence with the grain-inertia regime observed in granular flows; however, the role that the interstitial air plays in avalanches is not clarified at present. Distinguishing between three avalanche flow regimes instead of only two ...