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New thermomechanical model for rock/ice avalanches

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22.05.2018  |  News

 

The SLF is well-known for its snow avalanche expertise. Increasingly, however, the SLF is being asked to analyse the hazard of large avalanches composed of ice or a mixture of rock and ice.

 

 

In mountainous regions of the world, avalanches containing a mixture of rock and ice are a dangerous natural phenomena. The runout of these avalanches are difficult to predict because they contain a mixture of different materials: snow, ice, water and rocky debris. One recent example is the co-seismic rock/ice avalanche that destroyed the village of Langtang, Nepal. Intense ground shaking during the Nepalese earthquake of 2015 destabilized a hanging ice glacier to start an immense avalanche. The avalanche entrained considerable amounts of rocky debris and snow before reaching the valley bottom. The village of Langtang was destroyed by the air-blast created by the avalanche, more than 240 people died.

Another recent example is the collapse of 3.1 mio m3 of rock on the north-east face of Pizzo Cengalo (Val Bregaglia, Switzerland) in 2017. The rock mass fell on a glacier, mobilizing an estimated 0.6 mio m3 of ice. Although this avalanche was accompanied by a huge dust cloud, similar to Langtang, the air-blast did little damage. Several secondary debris flows released out of the water-saturated deposits of the rock/ice avalanche. The debris flows reached the mountain village of Bondo.

 

These recent examples indicate that the hazard area of a rock/ice avalanche is enlarged due to flow transitions, depending on the composition of the avalanche (the amount of ice and snow) as well as the temperature of the material. Both dry fluidized (powder avalanches) and fluid lubricated flow regimes (debris flows) are possible. Knowing which flow regime can occur and when is essential to determine the potential hazard of the avalanche.  

Interestingly, the thermo-mechanical properties of the material entrained by the avalanche often determines which flow regime dominates. Modelling rock/ice avalanche speed and runout therefore requires a thermomechanical model which accounts for the melting and lubricating properties of the entrained material, usually a mixture of snow, ice, water and rocky debris.

 

Researchers at the SLF have developed a rock/ice avalanche model capable of modelling both extreme cases: avalanches containing mainly ice as well as avalanches that mainly consist of rock debris The model is embedded in the RAMMS software (Rapid Mass Movements Simulation) and was recently applied by an international consortium of scientists to simulate the collapse of a large ice glacier in Tibet.  Avalanche experts in Switzerland used the model to assess the hazard posed by ice avalanches releasing from the Eiger west face.

 

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