Investigation of avalanche flow and its interaction with the obstacles

Abstract

At present, mainly, avalanche forecasting and avalanche control techniques are used to manage the avalanche hazards. Avalanche forecasting is in general issued over a large mountain area. This technique is suitable where limited movement of people and vehicles is there and frequency of avalanching is very less. Avalanche control is another technique which involves either permanent control of avalanches through the structure control or temporary control of avalanches through the artificial triggering of the unstable snow mass in the formation zone of the avalanche through explosives. Avalanche control becomes essential for busy highways, railway tracks, electric towers, or snow bound areas where large human population is involved. In order to control the avalanche, different avalanche control structures can be used in its path. In order to prevent initiation of avalanche from the formation zone itself, snow bridges, snow rakes and snow net structures can be used. Sometimes, due to terrain conditions, road conditions or some other constraints, it is not possible to install avalanche control structures in the formation zone. In that case, avalanche control structures like snow gallery, diversion wall etc. are installed in the middle zone of the avalanche path. These structures divert the direction of the avalanche. Catch dam, mounds and wedge type structures are typically installed in the runout zone of an avalanche. These structures retard, stop. or split the avalanche. The present study was undertaken with a focus on the track zone and runout zone avalanche control structures. The present design of these structures is mainly based on the empirical guidelines. In order that these structures can be installed at a large number of avalanche prone areas, it is important to have optimum design of these middle zone and runout zone avalanche control structures. To achieve this objective, accurate knowledge of the avalanche flow parameters i.e., avalanche impact pressure, flow depth, velocity. runout distance, dynamic coefficient of snow friction, shear forces, normal forces etc. is very important. A number of measurements are available for the above parameters but these experimental studies are not comprehensive in nature and cannot be used for the design of all kinds of avalanche control structures under varying conditions of mountain terrains, altitude, and snow types. To counter these shortcomings, a number of avalanche dynamics models developed by various researchers exist, which can simulate most of the avalanche flow parameters mentioned above under different conditions. However, most of these models ignore the actual interaction of the avalanche with the obstacles/structures, which is vital for the accurate assessment of the avalanche impact pressures, velocity, drag coefficient, runout distance, lateral spread, debris deposition etc. Further, the available models for estimating the snow avalanche impact pressures are mostly one-dimensional (7-D), two-dimensional (2-D) or pseudo three-dimensional (3-D) in nature, which model the important avalanche flow parameters mentioned above with a large number of assumptions. In order to address some of these gaps, in the current work, 3-D non-Newtonian Navier-Stokes equations based simulation model has been developed. Further, the conventional no-slip fluid boundary condition was replaced by the slipwall boundary condition. The present model overcomes the limitations of previously used Newtonian fluid based models that fail to simulate the avalanche debris deposition and the depthaveraged models, which are not in position to accurately capture the avalanche-obstacle interaction process. Present simulation outcomes were observed in acceptable conformity with the experimental data with an average root mean squares error (RMSE) of 0.166 for the avalanche debris depth and RMSE of 1.48 for the avalanche front velocity. Further, for transient comparison, snow avalanche impact pressures were measured on an instrumented obstacle of 1 m height and 0.65 m width for high-density moist snow. This experimental set-up has been developed and installed on a 61 m long experimental facility i.e., snow chute at Dhundhi field research station located about 20 kms away from Manali, Himachal Pradesh (H.P.), India. Based on experimentations and simulations carried out in the current work, the measured and the simulated avalanche impact pressures were correlated, which can be used to estimate the avalanche impact pressures on the structures for the dense flow of avalanches. The root mean square error between the currently proposed model and the measured data is nearly 10.74, which is significantly lesser than the existing models for the estimation of the avalanche impact pressures on the obstacles. Further, the effective drag coefficient Cj for the avalanche flow and the instrumented obstacle, which takes into account the combined effects of the fluid, solid, granular, and compressibility effects of the flowing snow, is found in the range of 3.97 to 8.54, which is in agreement with the published studies. Due to better control on the experimental conditions, accuracy and repeatability of the data is also expected to be high. This work related to the model development and validation is presented in Chapter 3 of the thesis. In this model, average value of dynamic coefficient of Coulomb friction between the snow chute surface and the snow ur has been used as 0.12. This value was validated based on the thirty two measurements carried out during the period 2017-2020 for the shear force and normal force components of the avalanches. The measurements were carried out using a three component piezoelectric load cells based dynamometer, which in turn was installed on the 12° slope of the snow chute mentioned above. Details of this work are presented in Chapter 4 of the thesis. Lastly, an attempt has been made in the current work to simulate avalanche flow interaction with an Instrumented Tower installed in the path of an important avalanche site (named as MSP-10) at Dhundhi. Due to huge size of the mountain terrain, geometric and computational complexity was high. For this reason, the developed model was applied in the 2-D domain for these simulations. The present proposed model is able to simulate avalanche mass retention before the Instrumented Tower and avalanche impact pressure on its pylons. The simulated results are in agreement with the observations. The results gave the confidence that the proposed model can be used to simulate many such avalanche-obstacle situations for the better assessment of avalanche loads on the obstacles/structures. Details of this work are presented in Chapter 5 of the thesis. The thesis ends with the conclusions and the future scope of work. The present thesis may find its applications in assessment of avalanche impact pressures on the structures in the runout zone of the avalanche like catch dams, mounds etc. and numerical assessment of shear force and normal force components of an avalanche on snow sheds/galleries. However, present work may be more useful in case of high density wet snow conditions.