CR5.2

Avalanche research requires comprehensive measurements of sudden and rapid snow mass movement that is hard to predict. Automatic cameras, radar and infrasound sensors provide valuable observations of avalanche structure and dynamic parameters, such as velocity. Recently, seismic sensors have also gained popularity, because they can monitor avalanche activity over larger spatial scales. Moreover, seismic signals elucidate rheological properties, which can be used to distinguish different types of avalanches and flow regimes. To date, however, seismic instrumentation in avalanche terrain is sparse. This limits the spatial resolution of avalanche details, needed to characterise flow regimes and maximise detection accuracy for avalanche warning.

As an alternative to conventional seismic instrumentation, we propose Distributed Acoustic Sensing (DAS) to measure avalanche-induced ground motion. DAS is based on fibre-optic technology, which has previously been used already for environmental monitoring, e.g., of snow avalanches. Thanks to recent technological advances, modern DAS interrogators allow us to measure dynamic strain along a fibre-optic cable with unprecedented temporal and spatial resolution. It therefore becomes possible to record seismic signals along many kilometres of fibre-optic cables, with a spatial resolution of a few metres, thereby creating large arrays of seismic receivers. We test this approach at an avalanche test site in the Valleé de la Sionne, in the Swiss Alps, using an existing 700 m long fibre-optic cable that is permanently installed underground for the purpose of data transfer of other, independent avalanche measurements.

During winter 2020/2021, we recorded numerous snow avalanches, including several which reached the valley bottom, travelling directly over the cable during runout. The DAS recordings show clear seismic signatures revealing individual flow surges and various phases/modes that may be associated with roll waves and avalanche arrest. We compare our observations to state-of-the-art radar and seismic measurements which ideally complement the DAS data.

Our initial analysis highlights the suitability of DAS-based monitoring and research for avalanches and other hazardous granular flows. Moreover, the clear detectability of avalanche signals using existing fibre-optic infrastructure of telecommunication networks opens the opportunity for unrivalled warning capabilities in Alpine environments.

How to cite: Fichtner, A., Edme, P., Paitz, P., Lindner, N., Hohl, M., Huguenin, P., Sovilla, B., Roig-Lafon, P., Surinach, E., and Walter, F.: Observing avalanche dynamics with Distributed Acoustic Sensing, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16562, https://doi.org/10.5194/egusphere-egu21-16562, 2021.

Radar imaging has become increasingly important in either the detection of avalanches or in the scientific characterization of the avalanche flow. Detection radars usually base on the Doppler radar principle that is sensitive to the velocity of moving object but lack sufficient resolution which can be circumvented with frequency-modulated pulsed radars. We present a new radar device as a follow up of the successful GEODAR radar that suit the need for both applications – low resolution detection and high resolution observation. While the original GEODAR is permanently installed in the full-scale avalanche Test site “Vallée de la Sionne" in Valais, Switzerland, our new device is much smaller and can be deployed in fast response to the metrological forecast and avalanche situation. Currently the mGEODAR is installed at Nordkette ski resort above Innsbruck that is known for its frequent artificial avalanche releases. The new radar features a versatile frequency generation scheme using direct digital synthesis and can be quickly reprogrammed into a low-resolution detection mode for continuous data recording that switches to a high-resolution observation mode as soon as an avalanche is detected.

Beside the radar system itself, avalanche data are presented of the winter season 2020/21. A focus is on small to medium sized avalanches that are just on the limit to develop into a powder snow avalanche which is characterized by surging in the intermittent frontal region. Connecting the radar data of the dynamic flow evolution with snow conditions will lead to drivers of this flow regime transition. The snow conditions are taken from nearby weather stations and manual snow profiles but also from the radar itself. A continuous scanning of the resting snow cover in the avalanche path during the season further allows snow cover monitoring. This application allows to identify new snow fall, warming and wetting of surface layers as well as diurnal melt-freeze cycles.

The radar data should be consequently used for model validation, calibration, and development. With the current measurement campaign of small to mid-sized avalanches, we hope to close a gap in data availability for those events as they are increasingly the object of simulation scenarios, but current modelling and simulation tools are usually calibrated for larger and extreme events. In the future, we expect the mGEODAR radar to be deploy on other gravitational mass-movements phenomena like soil slides and rock-fall.

How to cite: Köhler, A., Lok, L. B., and Fischer, J.-T.: mGEODAR – a new mobile radar for avalanche mass movement monitoring, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16361, https://doi.org/10.5194/egusphere-egu21-16361, 2021.

Predicting the magnitude of the impact force that snow avalanches can exert on structures still remains a challenging question.

In fast flow regimes, the impact pressure is mainly driven by the kinetic energy of the flow: it scales as one-half the product of the flow density and the square of the avalanche speed, and the effect of the shape of the structure is encapsulated in the so-called drag coefficient. Recent measurements on well-documented snow avalanches that have impacted different types of structures have confirmed the existence of another impact force regime at lower speed for which the pressure exerted on the obstacle is independent of the avalanche speed but rather controlled by the lithostatic pressure associated with the typical flow thickness. These measurements have also shown that the depth-dependent force could reach values that are many times greater than the lithostatic force.

The present paper proposes a general analytic form for the impact force of dense avalanches on structures. The approach is based on the application of mass and momentum conservation equations, in their depth-averaged forms, to a control-volume which surrounds the influence zone of the obstacle. A criterion to distinguish between the depth-dependent force regime and the velocity-square force regime can be derived. It is demonstrated that the size of the influence zone of the obstacle, relative to the dimension of the obstacle and/or the avalanche thickness, is a key ingredient (in addition to the traditional Froude number) to demarcate the depth-dependent impact forces from the velocity-square impact forces. Further developments are needed to unravel the size and shape of the influence zone (of any kind obstacle for any type of flowing snow), and then to be able to hone the proposed criterion. However, the present study takes a step forward for a better characterization of avalanche impact forces on structures.

How to cite: Faug, T.: Dense avalanches hitting structures: demarcating depth-dependent impact pressures from velocity-squared impact pressures, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11518, https://doi.org/10.5194/egusphere-egu21-11518, 2021.

How to cite: Jaedicke, C., Issler, D., Gleditsch Gisnås, K., Salazar, S., Robinson, K., Gauer, P., Langeland, H., Domaas, U., Liu, Z., Glimsdal, S., Sandersen, F., Mo, K., Frauenfelder, R., Heyerdahl, H., Breien, H., and Gilbert, G.: Recent advances in applied avalanche research in Norway, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11202, https://doi.org/10.5194/egusphere-egu21-11202, 2021.

The work is devoted to the comparison of different approaches for modeling the dynamics of dense and powder snow avalanches. Various 3D and 2D approaches are considered. The accuracy of determining the avalanche run-out zone, the interaction of the flow with obstacles, the front speed, and various distributed parameters are evaluated. As objects for comparison, an experiment on the interaction of a slushflow with a combination of protective structures and a powder snow avalanche in the Khibiny mountains are modeled.

Taking into account the advantages and disadvantages of various approaches based on basic solutions available in the OpenFOAM package, a specialized software avalancheFoam is being developed for three-dimensional modeling of the dynamics of snow avalanches, taking into account the complex turbulent regime and multiphase structure of the flow. Machine learning techniques are used to refine turbulent stresses. The neural network is trained on a dataset obtained from high-precision supercomputer simulation of the flow, and sets the form of additional refinement members of the mathematical model of less computational complexity. Various avalanche sites in the Khibiny mountains are modeled to validate the developed software.

How to cite: Romanova, D. and Egiit, M.: Modeling of snow avalanche dynamics using open source software OpenFOAM, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6110, https://doi.org/10.5194/egusphere-egu21-6110, 2021.

Various dynamics models can reproduce the motion of avalanches from release to deposition. These models often simulate a conceptual avalanche, adopt depth-averaged approaches and do not resolve variations along flow depth direction, and thus have clear limitations. This study presents three-dimensional, full-scale modeling of dense snow avalanches performed using the complex real terrain of the Vallée de la Sionne avalanche test site in Switzerland. We use the material point method (MPM) and a large-strain elastoplastic constitutive law for snow based on a Modified Cam Clay model. In our simulations, various and transient avalanche flow regimes are simulated by setting distinct snow properties. Snow avalanches are investigated from release to deposition. Detailed simulation results include the initial failure patterns, the mechanical behavior during the flow, and the characteristics of the final avalanche deposits. More specifically in the release zone, we can observe brittle and ductile fractures depending on the defined snow properties. During the flow phase, we monitor the temporal and spatial variations of snow density in the avalanche. In particular, cohesionless granular flows, cohesive granular flows, and plug flows are associated with snow fracture, compaction, and expansion. Finally, we can observe the structure of the avalanche deposit surfaces which show distinguishable differences in terms of smoothness, granulation, and compacting shear planes. This new model can offer a quantitative analysis for studying avalanches in different regimes and provide a powerful tool for exploring the dynamics of full-scale avalanches on complex real terrain, with high physical detail.

How to cite: Li, X., Sovilla, B., Jiang, C., and Gaume, J.: The material point method for simulating dense snow avalanches over complex real terrain, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6045, https://doi.org/10.5194/egusphere-egu21-6045, 2021.

Testing and benchmarking avalanche models is a crucial step in developing models as well as assessing their applicability. This is not only limited to the representation of physical processes within models, be it via first principles or using empirical relationships, but also concerns their computing environment, including compilers, hardware used, programming language, among others. 

Test, benchmarking, and comparison strategies can aim at different issues, among others: numerics, the implementation thereof, plausibility, verification, or evaluation. However, they always require reference or expected results. References can come from observations, analytical results, comparison to other models, known physical processes or material properties that cannot be changed – e.g., “avalanches cannot fly”. The question is: which characteristics or properties do we test and how to design appropriate tests?  

To facilitate this, as part of the newly developed opensource avalanche framework - AvaFrame -, we started providing commonly accessible tools to make testing and developing easier. This ranges from tools to import data, generate input parameters to automatic analysis and plotting. Not only do we provide the infrastructure for testing, but we also provide a set of test cases complete with all necessary input data, reference results, and run script examples. These tests so far include idealized (generic) topographies, specific test cases for numerical questions, and 6 real world avalanches with distinct characteristics. 

In this contribution we present this freely available set of tests and benchmarks suitable to assess various aspects and properties of a shallow water model solver for a dense flow avalanche model, one of the core computing modules of AvaFrame (com1DFA). We highlight how we utilize the entire range of tests in our continuous model development to assure the quality and applicability / validity of our development. Showing results from comparison to existing models, but also how to extend and apply our strategies to other models or research questions, we invite other researchers and developers to make full use of these tools.

How to cite: Oesterle, F., Wirbel, A., Tonnel, M., and Fischer, J.-T.: AvaFrame as a testing and benchmarking environment for avalanche simulations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6560, https://doi.org/10.5194/egusphere-egu21-6560, 2021.

Snow avalanche deposit volume is an important characteristic that determines vulnerability to snow avalanche. However, there is insufficient knowledge about snow and meteorological variables controlling deposit volumes. Our study focuses on the analysis of 1986 deposit volumes from 182 paths located in different regions of the French Alps including Queyras, and Maurienne valleys, between 2003 and 2017. This work uses data from the Permanent Avalanche Survey (EPA) database, an inventory of avalanche events occurring at well-known, delineated and mapped paths in France. We investigated relationships between snow deposit volumes and meteorological quantities, such as precipitation and temperature determined from SAFRAN reanalyses and snow-depth and wet snow-depth estimated from CROCUS reanalyses at a daily time scale at 2100m a.s.l. Analysis was conducted at an annual and seasonal time scale considering winter (November-February) and spring (March-May) between the mean deposit volumes and the mean meteorological and snow conditions. 

Results do not show any significant relationship between deposit volumes and meteorological or snow conditions at an annual time scale or for spring season. However, correlations between deposit volumes and meteorological and snow variables are high in winter (R²=0.78). The best model includes two snow variables: mean snow-depth and maximal wet snow-depth. We suggest that these two important snow variables reflect variations in the snow cover characteristics later influencing the nature of the flow and the deposit volumes. Dividing the studied paths sample into several classes according to their morphology (i.e: surface area or mean slope) increases the significance of the relationship for both seasons and highlights more complex relationships with meteorological and snow variables.

How to cite: Kern, H., Jomelli, V., Eckert, N., and Grancher, D.: Testing the influence of snow and meteorological conditions on snow avalanche deposit volumes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10762, https://doi.org/10.5194/egusphere-egu21-10762, 2021.

Two different mathematical models of fluid mechanics are now being investigated  at the WSL Institute for Snow and Avalanche Research in Davos to model powder-snow avalanches. The first approach is to solve the full three-dimensional multiphase (ice-dust, air) incompressible Navier-Stokes equations; the second approach is to apply depth-averaged models to simulate both the formation and independent propagation of the powder cloud. The final goal of both models is to predict the dynamics of powder avalanches in three-dimensional terrain and specifically cloud impact pressures. Both models are driven by the same set of terrain dependent mass and momentum exchanges defined by the flow state (speed, density, height) of the avalanche core. The great advantage of the depth-average approach is computational speed, allowing the investigation of different hazard scenarios involving variable release locations, snow temperature and entrainment depths. This fact has allowed the widespread application of the depth-average model to many historical case studies, including the avalanches measured at the Vallée de la Sionne (VdlS) test site. However, a central modelling problem needs to be resolved: both air-entrainment (cloud height and density) and drag (cloud speed) are intimately linked to the turbulence created during the cloud formation phase.

In this presentation, we present a depth-averaged turbulence model proposed by V. M. Teshukov [1] and extended by Richard and Gavrilyuk [2] and Gavrilyuk et al. [3], Ivanova et al. [5, 6]. The mathematical model is a 2D hyperbolic non-conservative system of equations that is mathematically equivalent to the Reynolds-averaged model of barotropic turbulent flows. The system is non-conservative, extending the classical shallow water equations to contain three independent components of the symmetric Reynolds stress tensor. We simulate the measured powder cloud heights of two VdlS avalanches using both the incompressible Navier-Stokes and turbulent shallow-water models, capturing the unsteady formation of billow height and width measured by ground based photogrammetry [4]. This can only be achieved by making air-entrainment dependent on the vorticity predicted by the turbulence model. We conclude by summarizing why we believe shallow-water type models can be applied for practical hazard engineering problems.

References:

[1] V. M. Teshukov in "Gas-dynamics analogy for vortex free-boundary flows.", J. Appl. Mech. Tech. Phys., 2007.

[2] G. L. Richard, S. L. Gavrilyuk in "A new model of roll waves: comparison with Brock’s experiments", Journal of Fluid Mechanics, 2012.

[3] S.L. Gavrilyuk, K.A. Ivanova, N. Favrie in "Multi-dimensional shear shallow water flows : problems and solutions", Journal of Computational Physics, 2018.

[4] Dreier, L., Bühler, Y., Ginzler, C., and Bartelt, P.: Comparison of simulated powder snow avalanches with photogrammetric measurements, Annals of Glaciology, 57, 371 - 381, 10.3189/2016AoG71A532, 2016.]

[5] K.A. Ivanova, S.L. Gavrilyuk, ”Structure of the hydraulic jump in convergent radial flows”,Journal of Fluid Mechanics, Volume 860, 10 February2019 , pp. 441-464.

[6] K.A. Ivanova, S.L. Gavrilyuk, B. Nkonga, G.L. Richard, ”Formation and coarsening of roll-waves in shear shallow water flows down an inclinedrectangular channel”,Computers& Fluids, 159, pp 189203, 2017

How to cite: Ivanova, K., Bühler, Y., and Bartelt, P.: Modelling powder snow avalanches using a depth-averaged turbulent shear model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4781, https://doi.org/10.5194/egusphere-egu21-4781, 2021.

On physical grounds, the rate of bed entrainment in gravity mass flows should be determined by the properties of the bed material and the dynamical variables of the flow. Due to the complexity of the process, most entrainment formulas proposed in the literature contain some ad-hoc parameter not tied to measurable snow properties. Among the very few models without free parameters are the Eglit–Grigorian–Yakimov (EGY) model of frontal entrainment from the 1960s and two formulas for basal entrainment, one from the 1970s due to Grigorian and Ostroumov (GO) and one (IJ) implemented in NGI’s flow code MoT-Voellmy. A common feature of these three approaches is their treating erosion as a shock and exploiting jump conditions for mass and momentum across the erosion front. The erosion or entrainment rate is determined by the difference between the avalanche-generated stress at the erosion front and the strength of the snow cover. The models differ with regard to how the shock is oriented and which momentum components are considered. The present contribution shows that each of the three models has some shortcomings: The EGY model is ambiguous if the avalanche pressure is too small to entrain the entire snow layer, the IJ model neglects normal stresses, and the GO model disregards shear stresses and acceleration of the eroded mass. As they stand, neither the GO nor the IJ model capture situations―observed experimentally by means of profiling radar―in which the snow cover is not eroded progressively but suddenly fails on a buried weak layer as the avalanche flows over it. We suggest a way to resolve the ambiguity in the EGY model and sketch a more comprehensive model combining all three approaches to capture gradual entrainment from the snow-cover surface together with erosion along a buried weak layer.

How to cite: Issler, D.: Towards an Improved Parameter-Free Entrainment Model for Snow Avalanches, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16564, https://doi.org/10.5194/egusphere-egu21-16564, 2021.

Hydrodynamic instability of open flows down inclines is an important phenomenon which leads perturbation growth, turbulence, roll waves formation etc. It has been widely studied for flows of Newtonian rheology with respect to longitudinal perturbations (perturbations that spread along the flow velocity vector), for example, see works [1 - 4]. From mathematical point of view, the study of the stability of open flow down an inclined planes with respect to two- or three-dimensional perturbations (i.e., with respect to oblique perturbations, spreading under an arbitrary angle to the flow velocity vector) is quite difficult, especially, if the fluid has non-Newtonian rheological properties, which can be important in the context of geophysical applications. Nonetheless, works exist, where these two factors (non-Newtonian rheology of the moving medium and arbitrary angle of spreading of perturbations) are taken into account, e.g., [5,6]. In more recent work [5], the problem of downslope flow linear stability is solved in complete formulation (continuity and momentum equations are used with no averaging over the depth, stability with respect to 3D perturbations is studied); this significant work uses complex mathematics, and can be difficult for applications.

This abstract is based on the work [6], where linear stability analysis was first conducted for the downslope flow that is described by hydraulic equations, but 1) the rheology of the flow and flow regime (laminar or turbulent) were arbitrary, 2) oblique perturbations were taken into account. The stability criterion is obtained analytically, it contains basic flow characteristics and can be applied to the flow if it's depth-averaged velocity u, depth h, relation between the bottom friction and h, u (u is the velocity modulus), slope angle are known. It is shown that the flow can be unstable (i.e., small perturbations grow, and this can lead, for example, to roll waves formation, or turbulisation of the flow) to oblique perturbations, even if standard stability criterion for longitudinal 1D perturbations is satisfied. This takes place, e.g., for dilatant fluids with power law index greater than 2).

The result is important not only for experimentalists, but for researchers who use numerical modeling, because knowledge of the flow behavior (for example, possible roll waves development) plays crucial role when choosing a computational scheme that will allow one to get the correct result.

[1] Benjamin T.B. Wave formation in laminar flow down an inclined plane. J. Fluid Mech. 1957. V. 2. P. 554 – 574.

[2] Yih C-S. Stability of liquid flow down an inclined plane. Phys. Fluids. 1963. V. 6(3). P. 321 – 334.

[3] Trowbridge J.H. Instability of concentrated free surface flows. J. Geophys. Res. 1987. V. 92(C9). P. 9523 – 9530.

[4] Coussot P. Steady, laminar, flow of concentrated mud suspensions in open channel. J. Hydraul. Res. 1994. V. 32. P. 535 – 559.

[5] Mogilevskiy E. Stability of a non-Newtonian falling film due to three-dimensional disturbances. Phys. Fluids. 2020. V. 32. 073101.

[6] Zayko J., Eglit M. Stability of downslope flows to two-dimensional perturbations. Phys. Fluids. 2019. V. 31. No. 8. 086601.

How to cite: Zayko, J. and Eglit, M.: Hydrodynamic instability of downslope flow with respect to two-dimensional perturbations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7978, https://doi.org/10.5194/egusphere-egu21-7978, 2021.