CR5.2

Snow avalanche dynamics: from driving processes to mitigation strategies

This session is devoted to the dynamics of dense and powder snow avalanches and their accompanying transitional regimes. One focus is their interaction with, and impact on, vulnerable elements, such as buildings, protection dams, forests, and roads. We welcome novel experimental and computational contributions including, but not limited to the topics of avalanche dynamics and related processes, physical vulnerability of structures impacted by snow avalanches, avalanche hazard zoning and avalanche mitigation strategies. These include field, laboratory and numerical studies that rely on new methods and techniques (radars, drone, satellite, etc.) as well as practical case studies.

Furthermore, we solicit novel contributions from the area of granular flows, viscoplastic flows, density currents, turbulent flows, as well as contributions from other gravitational mass flows communities, which can improve our understanding and modeling of snow avalanche propagation and their interaction with natural or man-made structures.

While the main focus of this session is on snow avalanche dynamics from basic knowledge to mitigation strategies, it is closely linked to CR session entitled "Snow avalanche formation: from snow mechanics to avalanche detection" which addresses avalanche formation, detection and forecasting.

Co-organized by NH1
Convener: Thierry Faug | Co-conveners: Jan-Thomas Fischer, Florence Naaim-Bouvet, Betty Sovilla
vPICO presentations
| Tue, 27 Apr, 09:00–10:30 (CEST)

vPICO presentations: Tue, 27 Apr

Chairpersons: Betty Sovilla, Jan-Thomas Fischer, Thierry Faug
09:00–09:05
Measurements and Field experiments, Avalanche-Obstacle interactions
09:05–09:07
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EGU21-16562
Andreas Fichtner, Pascal Edme, Patrick Paitz, Nadja Lindner, Michael Hohl, Pierre Huguenin, Betty Sovilla, Pere Roig-Lafon, Emma Surinach, and Fabian Walter

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.

09:07–09:09
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EGU21-16361
Anselm Köhler, Lai Bun Lok, and Jan-Thomas Fischer

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.

09:09–09:11
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EGU21-16560
Jean-luc Velotiana Ralaiarisoa, Florence Naaim, Kenji Kosugi, Masaki Nemoto, Yoichi Ito, Alexandre Valance, Ahmed Ould el Moctar, and Pascal Dupont

Aeolian transport of particles occurs in many geophysical contexts such as wind-blown sand or snow drift and is governed by a myriad of physical mechanisms. Most of drifting particles are transported within a saltation layer and has been largely studied for cohesionless particles whether for snow or for sand. Thus, the theoretical description of aeolian transport has been greatly improved for the last decades. In contrast cohesive particles-air system have received much less attention and there remain many important physical issues to be addressed.  

In the present study, the characteristics of drifting cohesive snow phenomena is investigated experimentally. Several wind tunnel experiments were carried out in the Cryopsheric Environment simulator at Shinjo (Sato et al., 2001). Spatial distribution of wind velocity and the mass flux of drifting snow were measured simultaneously by an ultrasonic anemometer and a snow particle counter. Compacted snow was sifted on the floor and left for a determined duration time to become cohesive by sintering. Two kinds of snow beds with different compression hardness were used (“hard snow” with a compression hardness of about 60 kPa and “semi hard snow” with a compression hardness of about 30 kPa). Wind tunnel velocity varied from 7 m/s to 15 m/s. Moreover steady snow drifting can be produced by seeding snow particles at a constant rate at the upwind of the test section.

It was shown that :

- on hard snow cover, aerodynamic entrainment does not occur and saltating particles from the seeder just rebounded without splashing particles composing the snow surface (Kosugi et al.,2004). At a given transport rate, the characteristic decay length lν,which can be seen as an estimation of the height of the saltating layer, exhibits a quadratic dependence with the air friction speed, u*. It is in agreement with results obtained by Ho (2011) with saltating sand on non-erodible bed. More surprisingly, lν increases with snow particles diameter, which means that restitution coefficient over hard snow cover also increases with snow particles diameters.

 - On loose snow cover, without seeder, data analysis from  Sugiura et al. (1998), shows that lv is proportional to u* to the power 1.4. This results therefore supports the idea that cohesionless snow doesn’t exist: on erodible sand bed configuration, the decay length is invariant (Ho, 2012).

-on semi hard snow cover, without seeder, the inter-particle cohesion makes the transport unsteady and spatially inhomogeneous. lv is proportional to u* to the power 1.6. It is therefore an intermediate case between “loose” and “hard “snow. Restitution coefficient on semi-hard snow is higher than on loose snow cover but smaller than on hard snow cover.  Particles are mainly lifted through aerodynamic entrainment so that saturation length is not obtained in the wind-tunnel : the transport rate  is two orders of magnitude lower than   the maximum transport rate observed for loose snow.

-on semi hard snow cover, with seeder, the drifting snow flux dramatically increases, even for low wind speed, leading sometimes to snow cover vanish. Experimental results provide evidence that impacting particles are efficient to lift cohesive snow particles : the transport rate increases to nearly 10.

How to cite: Ralaiarisoa, J.-V., Naaim, F., Kosugi, K., Nemoto, M., Ito, Y., Valance, A., Ould el Moctar, A., and Dupont, P.: Influence of cohesion on drifting snow investigated in cold wind-tunnel , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16560, https://doi.org/10.5194/egusphere-egu21-16560, 2021.

09:11–09:13
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EGU21-14504
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ECS
Corina Todea and Olimpiu Pop

Snow avalanches (SAs) are a widespread natural hazard in the Carpathians, damaging forests and threatening properties, tourism infrastructures and people. In Şureanu Mountains (Southern Carpathians), SA activity is not documented in the historical archives and consequently information regarding the SA frequency and their spatial extent is lacking. Along the forested avalanche paths, disturbed trees record selectively in their annual rings evidence of past events. Tree rings represent therefore a natural archive which can provide valuable information about the past SA activity. The aim of the present study is to reconstruct the occurrence and spatial extent of past SA activity with tree rings in Şureanu Mts. For this purpose, two avalanche paths adjacent to a ski area located in the central part of Şureanu Mts., have been investigated. Samples (cores and discs) collected from 121 and 141 Norway spruce (Picea abies (L.) Karst.) trees damaged by SAs along both paths have been analyzed. Tree-growth anomalies (e.g. scars, callus tissues, onset sequences of tangential rows of traumatic resin ducts, compression wood and growth suppression sequences) associated with the mechanical impact produced by SAs on trees were identified and used to reconstruct the SA history. Within the investigated paths, the reconstructed SA chronology spans the period of the last century. The minimum SA frequency and maximum extent reconstructed served to define the return periods within the two paths investigated. Tree-ring derived records provided the most consistent SA chronology in the study area, and can further be integrated in the avalanche hazard zoning assessment.

How to cite: Todea, C. and Pop, O.: Tree-ring reconstruction of snow avalanches in Şureanu Mountains (Southern Carpathians, Romania), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14504, https://doi.org/10.5194/egusphere-egu21-14504, 2021.

09:13–09:15
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EGU21-4097
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ECS
From avalanche-obstacle interaction processes to physics-based impact pressure calculations – an insight from DEM simulations
(withdrawn)
Michael Lukas Kyburz, Betty Sovilla, Johan Gaume, and Christophe Ancey
09:15–09:17
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EGU21-11518
Thierry Faug

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.

Advances in modeling and simulation
09:17–09:19
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EGU21-11202
Christian Jaedicke, Dieter Issler, Kjersti Gleditsch Gisnås, Sean Salazar, Kate Robinson, Peter Gauer, Henrik Langeland, Ulrik Domaas, Zhongqiang Liu, Sylfest Glimsdal, Frode Sandersen, Katrine Mo, Regula Frauenfelder, Håkon Heyerdahl, Hedda Breien, and Graham Gilbert

Snow avalanches are a significant natural hazard and common phenomenon in Norway. Applied research on avalanches and their societal impact has been conducted at the Norwegian Geotechnical Institute (NGI) for nearly half a century.

Recent activities within the applied avalanche research group at NGI have focused on four areas: (1) Improved understanding of is sought through the application of simple probabilistic release models and local wind modelling. Encouraging results are obtained by analysing and refining publicly available climate time series for temperature, snow depth and precipitation on a 1 km² grid. A major remaining challenge in view of elaborating realistic large-area avalanche hazard indication maps is the a priori determination of the size of release areas as a function of return period. (2) Different aspects of are investigated by means of a wide array of experimental technologies at the Ryggfonn full-scale test site, application of aerial survey methods to derive snow distribution, and investigation of the scaling behaviour of avalanches with extreme runouts in many different paths. The results of all these analyses point towards the need for a departure from modelling avalanches with Voellmy-type models in favour of models encompassing multiple flow regimes, a more realistic rheology and entrainment as well as deposition. (3) To improve risk assessment and mitigation measures, with structures are studied by documenting destructive avalanche events, constructing vulnerability curves for persons inside buildings based on historic avalanche events, improving methods for evaluation of individual risks, and development of criteria for physical mitigation measures against powder-snow avalanches. (4) Current efforts in focus on the one hand on simple block models for studying scaling behaviour on idealised and natural slopes and on the other hand on an advanced multi-flow-regime model that also incorporates different effects of the snow cover. Ongoing work aims, among others, at an entrainment and deposition model that is dynamically consistent and only depends on measurable snow properties. This contribution will present an overview of recent activities and advancements in applied avalanche research in Norway. It is hoped that it will serve to facilitate future international collaborative efforts to address challenges in applied avalanche research.

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.

09:19–09:21
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EGU21-6110
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ECS
Daria Romanova and Margarita Egiit

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.

09:21–09:23
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EGU21-6045
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ECS
Xingyue Li, Betty Sovilla, Chenfanfu Jiang, and Johan Gaume

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.

09:23–09:25
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EGU21-6560
Felix Oesterle, Anna Wirbel, Matthias Tonnel, and Jan-Thomas Fischer

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.

09:25–09:27
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EGU21-10762
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ECS
Hippolyte Kern, Vincent Jomelli, Nicolas Eckert, and Delphine Grancher

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.

Model development
09:27–09:29
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EGU21-4781
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ECS
Kseniya Ivanova, Yves Bühler, and Perry Bartelt

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.

09:29–09:31
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EGU21-7454
Daniel Zugliani, Giorgio Rosatti, and Stefania Sansone

Snow avalanche models are commonly based on a continuum fluid scheme, on the assumption of shallow flow in the direction normal to the bed, on a depth-averaged description of the flow quantities and on different assumptions concerning the velocity profile, the friction law, and the pressure in the flow direction (see Bartelt et al, 1999, Barbolini et al., 2000, for an overview). The coordinate reference system is commonly local, i.e., for each point of the domain, one axis is normal to the bed while the other two axes lay in a tangent plane. When the bed is vertical and the flow is not aligned with the steepest direction (e.g., in case of a side wall), the flow depth is no longer defined considering the normal direction and the model based on the local coordinate system is no longer valid. In near-vertical conditions, numerical problems can be expected.

Another critical point, for numerical models based on finite volume schemes and Godunov fluxes, is the accurate treatment of the source term in case of no-motion conditions (persistence, starting and stopping of the flow) due to the presence of velocity-independent, Coulomb-type terms in the bed shear stress. 

In this work, we provide a numerical approach for a Voellmy-fluid based model, able to overcome the limits depicted above, to accurately simulate analytical solutions and to give reliable solutions in other cases (Zugliani & Rosatti, 2021). Firstly, differently from the other literature models, the chosen coordinate reference system is global (an axis opposite the gravity vector and the other two orthogonal axes lay in the horizontal plane) and therefore, the relevant mass and momentum equations have been derived accordingly. Secondly, these equations have been discretized by using a finite volume method on a Cartesian square grid where the Godunov fluxes has been evaluated by mean of a modified DOT scheme (Zugliani & Rosatti, 2016) while source terms in conditions of motion have been discretized by using an implicit operator-splitting technique. Finally, a specific algorithm has been derived to deal with the source term to determine the no-motion conditions.  Several test cases assess the capabilities of the proposed approach.

 

References:

Barbolini, M., Gruber, U., Keylock, C.J., Naaim, M., Savi, F. (2000), Application of statistical and hydraulic-continuum dense-snow avalanche models to five real European sites. Cold Regions Science and Tech. 31, 133–149.

Bartelt, P., Salm, B., Gruber, U. (1999), Calculating dense-snow avalanche runout using a voellmy-fluid model with active/passive longitudinal straining. Journal of Glaciology 45, 242-254.

Zugliani D., Rosatti G. (2021), Accurate modeling of two-dimensional dense snow avalanches in global coordinate system: the TRENT2D approach. Paper under review.

Zugliani, D., Rosatti, G. (2016), A new Osher Riemann solver for shallow water flow over fixed or mobile bed, Proceedings of the 4th European Congress of the IAHR, pp. 707–713.

How to cite: Zugliani, D., Rosatti, G., and Sansone, S.: A numerical approach for simulating two-dimensional dense-snow avalanches in global coordinate systems, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7454, https://doi.org/10.5194/egusphere-egu21-7454, 2021.

09:31–09:33
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EGU21-16564
Dieter Issler

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.

09:33–09:35
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EGU21-7978
Julia Zayko and Margarita Eglit

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.

09:35–09:37
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EGU21-11983
Guillaume Chambon, Thierry Faug, and Mohamed Naaim

Wet snow avalanches present distinctive features such as unusual trajectories, peculiar deposit shapes, and a rheological behavior displaying a combination of granular and pasty features depending on the actual snow liquid water content. Complex transitions between dry (cold) and wet (hot) flow regimes can also occur during a single avalanche flow. In an attempt to account for this complexity, we report on numerical simulations of avalanches using a frictional-cohesive rheology implemented in a depth-averaged shallow-flow model. Through extensive sensitivity studies on synthetic and real topographies, we show that cohesion plays a key role to enrich the physics of the simulated flows, and to represent realistic avalanche behaviors. First, when coupled to a proper treatment of the yielding criterion, cohesion provides a way to define objective stopping criteria for the flow, independently of the issues incurred by artificial diffusion of the numerical scheme. Second, and more importantly, the interplay between cohesion and friction gives raise to a variety of nontrivial physical effects affecting the dynamics of the avalanches and the morphology of the deposits. The relative weights of frictional and cohesive contributions to the overall stress are investigated as a function of space and time during the propagation, and related to the formation of specific features such as lateral levées, hydraulic jumps, etc. This study represents a first step towards robust avalanches simulations, spanning the wide range of possible flow regimes, through shallow-flow approaches. Future improvements involving more refined cohesion parameterizations will be discussed.

How to cite: Chambon, G., Faug, T., and Naaim, M.: Numerical simulations of wet snow avalanches: interplay between cohesion and friction, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11983, https://doi.org/10.5194/egusphere-egu21-11983, 2021.

09:37–10:30