What Can We Learn About Avalanche Dynamics From the Avalanche at Rigopiano in 2017?

What is it about?

A large avalanche descended from Monte Siella, in the Apennines (central Italy), in January 2017. It broke or uprooted the entire forest in the gully over a length of 1 km, then completely destroyed the spa hotel Rigopiano in the run-out area and killed 29 of the 40 persons waiting there for evacuation. In June 2017, a field survey of the release area and the track yielded interesting insights that are analyzed in this paper. Several velocity estimates could be obtained from the trimlines in gully bends ("superelevation", i.e., the outer flowmarks are higher up than the inner ones due to the centrifugal force) and the overflowing of a terrain shoulder. Lower limits on the pressure follow from the destruction of the forest and the hotel as well as from the displacement of the hotel debris. The estimates obtained from very simple mechanical considerations, i.e., without simulation with a numerical model, are in quite good agreement with simulations carried out by other groups. From data and photos in the media as well as the uprooting of trees, one may infer some limits on the released and deposited mass of this avalanche. Most likely, the avalanche eroded most of the snow cover from the upper track to the deposit area, but completely entrained only a limited part of this mass; much snow (and tree debris) was only dragged along for a limited distance and deposited again. This picture is supported by comparing the shear stress exerted by the avalanche to the momentum transfer rate that is needed to accelerate the eroded snow and debris to avalanche speed. In the same period, avalanches occurred in at least three neighboring paths. We combined this data with information on earlier avalanche events from local residents and found that the statistical correlation between path steepness and run-out angle in this part of the Apennine seems to be similar to the correlation found in Norway, despite significant topographic and climatic differences. Finally, preliminary information on the location of the rescued persons and the victims could be combined with photo-based estimates of the degree of damage to the building at these locations. This gives a first hint at the probability of surviving in a building that is destroyed by an avalanche to some given degree. These so-called lethality curves are important when it comes to assessing not only the hazard but also the risk due to an avalanche. The estimated curve can be improved if data from many more events is added.

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Photo by Anjali Mehta on Unsplash

Photo by Anjali Mehta on Unsplash

Why is it important?

In the case of major disaster avalanches―like the Rigopiano avalanche―a large amount of data is often collected. Surprisingly, however, that information is usually not analyzed in a systematic manner that takes into account restrictions due to simple mechanical considerations. Conversely, in many avalanche events that did not cause significant damage, usually at most the run-out distance and perhaps the width of the release area and some (average?) deposit depth are documented. This paper shows that it would be worthwhile to analyze disaster avalanches in more detail and to collect additional data from non-disastrous avalanches because much can be learnt about the avalanche behavior by such analyses. Moreover, if data on many avalanches in a region is collected, statistical analyses can be carried out that will be valuable for avalanche hazard mapping and risk assessments.

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