Results 2016 from SP 4 FoU Snøskred: Work Package 1 Ryggfonn and Avalanche Dynamics

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1 Page: 1 Results 2016 from SP 4 FoU Snøskred: Work Package 1 Ryggfonn and Avalanche Dynamics Project nr : Title : Ryggfonn and avalanche dynamics Total budget (knok) From Dept. Of Oil and Energy (knok) Costs per (knok) Task 1: Avalanche experiments at the Ryggfonn test site The objective is providing experimental data from full scale avalanche experiments to: improve the understanding of the behavior of the avalanches with a focus on flow regime changes, obtain data of sufficient quality for model calibration, gain in-depth understanding of the interaction of snow avalanches with catching dams. Task 2: Avalanche Dynamics The objective is to provide improved tools for avalanche hazard mapping (with a focus on flow regime changes). The main results are published or are going to be published in refereed journals and conference proceedings. Har prosjektet oppnådd de oppsatte mål: Ja: X Nei: Begrunnelse for eventuelle avvik og beskrivelse av korrigerende tiltak: Task 2: New insight into the processes leading to fluidization required a fresh assessment of the theoretical basis for an advanced avalanche model. For this reason, it was not opportune to force the numerical implementation of the earlier mathematical model before this process is concluded. Hence, Deliverable D1.6 (Recommendations for the use of advanced models) could not be produced in Date Workpackage leader Date Discipline leader Peter Gauer Christian Jaedicke

2 Page: 2 Title: Project Manager: Project Members: WP1 Ryggfonn and model development Peter Gauer Krister Kristensen, Erik Lied, Dieter Issler TASK 1: AVALANCHE EXPERIMENTS AT THE RYGGFONN TEST SITE Subtask 1.1: Maintenance of Ryggfonn Under this task, necessary repairs and updating of the data acquisition system at the Ryggfonn avalanche test site were carried out so that the site is ready for the winter season 2016/2017. Due to lightning some damage occurred at the data acquisition system during summer. Last year, it was decided to buy a 3 m high pylon equipped with temperature sensors at a spacing of 10 cm. It is first installed near the instrument cabin a few hundred meters downstream from the catching dam and will serve a dual purpose of recording temperature profiles and indicating the actual snow depth, with data transmission to Stryn and Oslo by Internet. Later on, installation near the starting zone of the Ryggfonn avalanche path is an option. Subtask 1.2: Avalanche measurements at Ryggfonn Four small to medium-size spontaneous avalanches occurred during the winter No weather situation occurred that held promise for artificially releasing a sizeable avalanche and obtaining good-quality measurements. In end-april, an artificial release was nevertheless attempted, but resulted only in a small avalanche that stopped in the upper track. More detailed information on all five events can be found in Deliverable D1.4. In the course of the last years, new techniques for analyzing the data from avalanche measurements have been developed at NGI. They made it worthwhile to reanalyze the data collected since the first experiments at Ryggfonn almost four decades ago. While not of the same quality as recent measurements with more sophisticated equipment, they make it possible to compare avalanches of widely different sizes and to study the probability distribution functions of several interesting quantities. This reanalysis is almost completed for the time being and a comparison with measurements and observations from other sites was started (Gauer, 2016a). TASK 2: AVALANCHE DYNAMICS During 2016, four different subtasks were pursued: (i) D. Issler served as co-adviser for a MSc thesis at NTNU on the braking effect of forests on small to medium-size avalanches (Kahrs, 2016). In addition, a new version of MoT-Voellmy was created to take into account the braking effect of forests of different densities. (ii) The physical and mathematical content of extensions to SLF's model RAMMS, as proposed in a series of papers by Bartelt and coworkers, was critically investigated (Issler et al., 2017). (iii) A dynamic model taking into account the fluidizing effect of air expelled from the overrun

3 Page: 3 snow cover was formulated mathematically. (iv) Rewriting of a manuscript analyzing field observations from three special avalanches in Switzerland in 1995 (Issler et al., submitted a,b) was started. Braking effect of forests The objective of the MSc thesis (Kahrs, 2016) was to determine experimentally how the run-out distance, velocity and flow depth of laboratory-scale granular flows changes as a function of the areal density and diameter of thin cylindrical obstacles representing trees. These data were then to be analyzed in terms of a Voellmy-type model in order to see whether the effect of the obstacles can be captured by modifying the two friction parameters of the model. Much of the conceptual preparations for the experiments was carried out in (Kahrs, 2015), as reported in the Annual Report A chain of circumstances made this MSc less successful than expected: In the starting phase, the construction of the chute was delayed by more than a month and some unfortunate design decisions were made. As a consequence, much of the limited advising time from NGI's side had to be spent on help with the chute construction and instrumentation instead of developing the experimental procedures and data analysis methods. In particular, the use of a "forest" based on a regular quadratic grid aligned with the direction of steepest descent allowed fingers of the flow to pass through essentially unaffected by the obstacles. This circumstance precluded the planned analysis of the experimental results. Furthermore, fine sand that is more irregular and less elastic than the glass ballotini used in these experiments might have reduced the intensity of saltation that made it difficult to measure meaningful flow depths. On the positive side, this work has established which pitfalls need to be avoided in a future series of experiments. The Norwegian Forest and Landscape Institute, now part of NIBIO (Norwegian Institute of Bioeconomy Research), has published the map data set SAT-SKOG ( which provides geographic information on density, age, quality and species distribution of forest stands for nearly all of Norway. Despite some important limitations (the satellite data used in the analysis are about ten years old, and the uncertainty in a given pixel is rather high), this data set offers the opportunity to systematically take into account the effect of forest stands on the release probability and run-out distance of avalanches. This topic gained particular importance through the project "Nye aktsomhetskart snøskred", which NGI carried out for NVE in Most of this activity was outside FoU Snøskred; this concerns in particular the development of procedures for estimating the quantity n D, i.e., the product of n, the number of tree trunks per unit area, and D, the average diameter of trees at breast height (Gauer, 2016b), an analysis of how the presence of trees reduces the release probability (Gauer, 2016b), and a simple modification of the friction parameters µ and k of MoT-Voellmy as a function of n D and of the instantaneous local flow depth (Issler, 2016). FoU Snøskred contributed to the implementation of this modification in MoT-Voellmy and the testing of the new version.

4 Page: 4 Critical appraisal of the RKE-extensions to RAMMS In a series of papers, Bartelt and co-workers at SLF have proposed a number of extensions to the Voellmy-type code RAMMS, which presently is the most often used dynamical avalanche model in consulting (Bartelt and Buser, 2010; Bartelt et al., 2011, 2012, 2016; Buser and Bartelt, 2009, 2011, 2015). The central idea of these extensions is that the intensity of random collisions between snow particles determines the friction and the density of the avalanche flow. This notion is deeply rooted in the kinetic theory of granular flows and has, to some degree, already been implemented in the Norem Irgens Schieldrop model in the 1980s (Norem et al., 1987, 1989). The particular mathematical formulation employed by Bartelt and co-workers has been criticized (in vain) by many reviewers of these papers, but no systematic critique has ever been published. In view of the planned introduction of these extensions into the production version of RAMMS, it is of great importance to clarify these issues. A joint effort with J. Jenkins (Cornell University) and Jim McElwaine (Durham University) led to the following main conclusions: The proposed exponential decay of the Voellmy friction parameters µ and k on the granular temperature is purely ad hoc and leads to an unrealistic decrease of the effective friction with increasing velocity. The model with variable density ignores the direct relationship between granular temperature and dispersive pressure between grains that is firmly established by kinetic theory. Instead, a spurious evolution equation for dispersive pressure is obtained by wrong application of thermodynamic relations and erroneous mathematical manipulations. The most recent model adds a suspension layer to describe the powder-snow cloud. However, most of the well-established results of several decades of experimental and theoretical research on gravitational mass flows are ignored. For example, the effect of gravity on the suspension layer is completely neglected. These findings imply that these extensions of RAMMS should not be used, despite the claims by Bartelt and co-workers that they are able to simulate observed avalanches with unparalleled precision. A paper on this investigation will be submitted shortly (Issler et al., 2017a). Mathematical model of avalanche fluidization by air expulsion from the snow cover Several observations of mixed snow avalanches in 1995 (Issler et al., 2015a,b) made it clear that snow avalanches often exhibit an intermediate flow regime between dense and suspension flow, which can be of great relevance for the run-out and damaging effect of avalanches. Our most recent attempt at modeling the transition between the dense and fluidized flow regimes (Issler and Gauer, 2008) appealed to results from granular mechanics to extend the Norem Irgens Schieldrop model (Norem et al., 1987, 1989) to flows with variable density. Despite surprisingly good agreement between observations and simulations with this simple block model (without tuning the friction parameters!), it became clear that particle collisions alone are not sufficient to attain the low densities suggested by experiments at Ryggfonn and Vallée de la Sionne. At that time, we invoked aerodynamic lift to increase the degree of fluidization, but while such an effect should be present, it seems to be too localized to fully explain the discrepancy.

5 Page: 5 A decisive step forward was the realization that, as the avalanche flows over the snow cover, it exerts a pressure on the order of 1 10 kpa on the latter. The compression is, however, only possible if the pore air in the snow cover is expelled. As the air escapes through the snow cover and the avalanching mass, it exerts a bed-normal force on the avalanche whose magnitude is proportional to the avalanche weight. This force also depends on the relative depth and porosity of the (new-)snow cover and the avalanche, and it may probably amount to 50 80% of the avalanche weight in some cases. Such a reduction of the effective pressure combined with the dispersive pressure due to particle collisions during shearing makes the avalanching snow expand substantially. The degree of expansion depends both on the excess pore pressure and the dispersive pressure (and also on the suction due to the ambient air flowing over the avalanche). The more the avalanche expands, the more the hydraulic permeability increases and the faster the air can escape. This, in turn, regulates how far back from the avalanche front the fluidizing effect persists. First estimates (Issler, 2003) indicated that one may expect escape times (or pore-pressure dissipation times) of the order of s in natural avalanches. Essential ingredients in a model of this process are thus (i) the effective normal stress in the snow cover under simultaneous compression and shear, (ii) the permeability as a function of density and particle size, (iii) the suction due to flowing ambient air, and (iv) the dispersive pressure due to particle collisions under shear. In a depth-averaged flow model, one also has to decide whether to describe the vertical expansion by means of an equation of motion (2 nd order differential equation), relaxation to a (variable) equilibrium density (1 st order differential equation) or the instantaneous equilibrium (algebraic equation). These questions are discussed in (Issler, 2016b). Work on finalizing the equation system and implementing it in a 2D depth-averaged model will continue in Draft paper on observations of mixed avalanches After rejection of a two-part manuscript on three powder-snow avalanches observed in Switzerland in 1995 (Issler et al., 2015a,b) by Cold Regions Science and Technology, the text is being rewritten completely. So far, the purely observational part, which is intended to be published as Supplementary Materials, is finished (Issler et al., 2017b). The main text on the analysis and interpretation of the events is currently in the draft stage. References Bartelt, P. and O. Buser (2010). Frictional relaxation in avalanches. Annals Glaciol. 51(54), Bartelt, P., O. Buser, C. Vera Valero and Y. Bühler (2016). Configurational energy and the formation of mixed flowing/powder snow and ice avalanches. Annals Glaciol. 57(71), Bartelt, P., Y. Bühler, O. Buser, M. Christen and L. Meier (2012). Modeling mass-dependent flow regime transitions to predict the stopping and depositional behavior of snow avalanches. J. Geophys. Res. 117, F Bartelt, P., L. Meier and O. Buser (2011). Snow avalanche flow-regime transitions induced by mass and random kinetic energy fluxes. Annals Glaciol. 52(58),

6 Page: 6 Buser, O. and P. Bartelt (2009). Production and decay of random kinetic energy in granular snow avalanches. J. Glaciol. 55(189), Buser, O. and P. Bartelt (2011). Dispersive pressure and density variations in snow avalanches. J. Glaciol. 57(205), Buser, O. and P. Bartelt (2015). An energy-based method to calculate streamwise density variations in snow avalanches. J. Glaciol. 61(227), Gauer, P. (2016a). Selected Observations from Avalanche Measurements at the Ryggfonn Test Site and Comparisons with Observations from Other Locations. In: Proceedings of the International Snow Science Workshop 2016, Breckenridge, CO. Gauer, P. (2016b). Forest cover within Nye aktsomhetskart snøskred i Norge (NAKSIN). Norwegian Geotechnical Institute, Oslo, Norway, NGI Technical Note TN. Issler (2016a). Incorporation of forest effects in MoT-Voellmy. Norwegian Geotechnical Institute, Oslo, Norway, NGI Technical Note TN. Issler (2016b). Notes on fluidization of snow avalanches by air expulsion from the snow cover. Norwegian Geotechnical Institute, Oslo, Norway, NGI Technical Note TN. Issler, D., P. Gauer, M. Schaer and S. Keller (2015a). Three flow regimes in mixed snow avalanches (I): Field observations. Submitted to, and rejected by, Cold Reg. Sci. Technol. Issler, D., P. Gauer and M. Schaer (2015b). Three flow regimes in mixed snow avalanches (II): Analysis. Submitted to, and rejected by, Cold Reg. Sci. Technol. Issler, D., P. Gauer, M. Schaer and S. Keller (2017b). Field observations of three mixed snow avalanches. Manuscript to be submitted to Cold Reg. Sci. Technol. Issler, D., J. T. Jenkins and J. N. McElwaine (2017a). Critique of avalanche flow models based on extensions of the concept of random kinetic energy. (To be submitted to J. Glaciol.) Kahrs, K. (2015). The braking effect of trees on snow avalanches Design of an experimental study. Department of Civil and Transport Engineering, Norwegian University of Science and Technology (NTNU), Project Thesis TBA4510. Kahrs, K. (2016). The braking effect of trees on snow avalanches An experimental study. MSc Thesis, Department of Civil and Transport Engineering, Norwegian University of Science and Technology (NTNU). Norem, H., F. Irgens and B. Schieldrop (1987). A continuum model for calculating snow avalanche velocities. In: Salm, B. and H. Gubler, eds., Avalanche Formation, Movement and Effects. Proceedings of the Davos Symposium, September 1986, IAHS Press, Wallingford, Oxfordshire, UK, IAHS Publication vol. 162, Norem, H., F. Irgens and B. Schieldrop (1989). Simulation of snow avalanche flow in run out zones. Annals Glaciol. 13,

7 Page: 7 PUBLICATIONS IN 2016 Issler, D., Á. Jónsson, P. Gauer and U. Domaas (2016). Vulnerability of Houses and Persons under Avalanche Impact the Avalanche at Longyearbyen on Proceedings of the International Snow Science Workshop 2016, Breckenridge, CO. Gauer, P. (2016). Selected Observations from Avalanche Measurements at the Ryggfonn Test Site and Comparisons with Observations from Other Locations. Proceedings of the International Snow Science Workshop 2016, Breckenridge, CO. Gauer, P. & Kristensen, K. Four decades of observations from NGI's full-scale avalanche test site Ryggfonn Summary of experimental results. Cold Regions Science and Technology 125, PRESENTATIONS IN 2016 Issler, D.: Why do some avalanches not stop where they ought to? Invited oral presentation at the workshop geoflo16, Max Planck Institute for the Physics of Complex Systems, Dresden, Germany, 13 April Issler, D., Á. Jónsson, P. Gauer and U. Domaas: Vulnerability of Houses and Persons under Avalanche Impact the Avalanche at Longyearbyen on Oral presentation at the International Snow Science Workshop 2016, Breckenridge, CO, 3 7 October Gauer, P. Selected Observations from Avalanche Measurements at the Ryggfonn Test Site and Comparisons with Observations from Other Locations. Poster at the International Snow Science Workshop 2016, Breckenridge, CO, 3 7 October Gauer, P. and K. Kristensen: What have we learnt from the Ryggfonn experiments? Oral presentation at the Information meeting on the R&D project Snow Avalanches at Oslo Airport, Issler, D. New modeling tools under development. Oral presentation at the Information meeting on the R&D project Snow Avalanches at Oslo Airport, Høydal, Ø. A., H. Breien and P. Gauer: Snow avalanches: How shall forests be taken into account? Oral presentation at the Information meeting on the R&D project Snow Avalanches at Oslo Airport, PROJECT-RELATED REPORTS IN 2016 Kahrs, K. (2016). The braking effect of trees on snow avalanches an experimental study. MSc thesis, Department of Civil and Transport Engineering, Norwegian University of Science and Technology (NTNU), June 2016.

8 Page: 8 DELIVERABLE D1.5 DATA REPORTING AND DATA ANALYSIS FROM MEASUREMENTS AT RYGGFONN Spontaneous avalanches Four spontaneous avalanches were recorded on , , , and Only pressure data is available from these events. They are therefore of limited value for further analysis. On , an attempt at releasing an avalanche artificially resulted in a small avalanche that stopped in the upper track. The avalanche of stopped between the concrete wedge and the dam. Pressures on the uppermost pylon and the concrete wedge were less than 10 and 20 kpa, respectively, and lasted for less than 5 s (Figure 1). Figure 1. Avalanche test site Ryggfonn, spontaneous snow avalanche on Pressure measurements at the pylon and concrete wedge. The avalanche of (Figure 2) stopped between the concrete wedge and the dam. Pressures on the uppermost pylon and the concrete wedge were less than 40 kpa and 20 kpa, respectively, and lasted for less than 60 s (Figure 3).

9 Page: 9 Figure 2. Avalanche test site Ryggfonn, spontaneous avalanche on

10 Page: 10 Figure 3. Avalanche test site Ryggfonn, spontaneous wet snow avalanche on Pressure measurements at the pylon and concrete wedge. The wet-snow avalanche of (Figure 4) stopped on bare ground below the concrete wedge and short of the dam. Pressures on the uppermost pylon and the concrete wedge were less than 60 kpa, and lasted for about 30 to 50 s (Figure 5).

11 Page: 11 Figure 4. Avalanche test site Ryggfonn, spontaneous avalanche on

12 Page: 12 Figure 5. Avalanche test site Ryggfonn, spontaneous wet snow avalanche on Pressure measurements at the pylon and concrete wedge. The wet-snow avalanche of stopped below the concrete wedge and short of the dam (Figure 6). Pressures on the uppermost pylon and the concrete wedge were less than 100 kpa and 30 kpa, respectively, and lasted for about 30 to 50 s (Figure 7).

13 Page: 13 Figure 6. Avalanche test site Ryggfonn, spontaneous avalanche on

14 Page: 14 Figure 7. Avalanche test site Ryggfonn, spontaneous wet snow avalanche on Pressure measurements at the pylon and concrete wedge. Artificially released avalanche During the entire winter, no weather situation developed where the chance of releasing a sizeable avalanche was considered to be good. Towards spring, a campaign was started to dispose of the explosives stored in the Wyssen tower. Weather conditions made it possible to have some video observations from the RADAR point. The released avalanche was small, with an estimated volume of 1000 m 3. It stopped after a travel distance (measured from the fracture crown) of about 700 to 750 m, i.e., before leaving the bowl comprising the release area and entering the steeper middle track (Figure 8).

15 Page: 15 Figure 8. Artificially released avalanche at Ryggfonn, View from Radar position. The avalanche stopped in the flatter part of the bowl visible in the picture after about 41 s (shortly after this snapshot). Photo P. Gauer, NGI.

16 Page: 16 Figure 9. Avalanche test site Ryggfonn, artificially released avalanche on Longitudinal profile of front velocity derived from video analysis and timing: The blue line shows the best fit and the light blue area indicates the uncertainty range. The horizontal distance is scaled with Hsc = 357 m, the velocity with (ghsc) 1/2 = 59.2 m/s. The peak velocity thus is about 30 m/s. The red triangle shows the simulated position after 40 s. The inset depicts the observed (open circles) and estimated (full line) travel distance versus time. Due to the topography of the Ryggfonn path, the Doppler radar located at the foot of the opposing slope is not able to see avalanches before they enter the steeper, channeled middle section of the track. Therefore, there are no radar measurements of the internal and front velocity of the small artificially released avalanche of However, video recordings allow to make some estimates of the front velocity Uf along the path (Figure 9). Even though the uncertainty of the result is probably in the order of ±5 m/s, a fairly smooth curve is obtained assuming a retarding acceleration aret/g This quantity is the starting point for analyzing the dynamics of the avalanche. Preliminary back-calculations of the avalanche were carried out with the model RAMMS, using the observed release area and the estimated average release depth of 0.8 m, which gives a release mass of approx m 3. First, the standard calibration of RAMMS recommended by SLF for tiny avalanches with a return period of 10 years was used together with the standard entrainment model. Under these conditions, the avalanche velocity peaks at m/s (Figure 10, left panel), which is less than the estimated maximum front velocity of about 30 m/s (Figure 9). The velocity maximum occurs, however, at the correct location about 140 m from the fracture crown. The simulated avalanche creeps about 500 m farther along the path than observed (corresponding

17 Page: 17 to a vertical drop of some 400 m), at a velocity of about 5 m/s. This is a consequence of the relatively low value of the dry-friction parameter µ, which determines the maximum slope angle at which the avalanche flow can stop. There is a considerable time lag between the observed front location and the simulated one. A better result was obtained using a modified version of the alternative calibration proposed by Gauer (2014), see Figure 10, right panel. The dry-friction parameter µ is set to 0.44 a value closer to tan α, where α is the observed run-out angle. The turbulent friction parameter ξ of RAMMS is chosen as 20,000 m/s 2 in order to make its contribution rather small. This is in stark contrast to the standard calibration, which results in 0.32 < µ < 0.47 and ξ = m/s 2. Figure 10. Back-calculations of the avalanche that was artificially released at Ryggfonn on using RAMMS. Left panel: Friction parameters chosen according to SLF's calibration for tiny avalanches with return period 10 years. Right panel: Parameters chosen according to calibration based on run-out angle and observed time. REFERENCES Gauer, P. (2014). Comparison of avalanche front velocity measurements and implications for avalanche models. Cold Regions Sci. Technol. 97,