High-mobility of unconstrained rock avalanches: Numerical simulations of a laboratory experiment and an Argentinian event

Rock avalanches are one of the most hazardous landslide phenomena. They involve huge, extremely rapid flowing masses, carrying a strong power of destruction and reaching high mobility. The present article shows results of back-analysis of the Potrero de Leyes case in the Argentinian Andes. This rock avalanche has travelled on an unconstrained topography showing a great mobility, even without any channelizing effects. Simulations are carried out with VolcFlow, a continuum model, first on an unconstrained granular flow experiment and then on Potrero de Leyes. Results show that frictional model works well for the experiment, but that it can only reproduce the longitudinal runout in the field, and this provided that the basal friction angle used is close to the corresponding measured Fahrböschung. Here, where the rock avalanche starts as a coherent mass that disaggregates and shatters only during its propagation, lateral spreading is overestimated and a viscous behavior produces better results. characterized by a nearly oval, trapezoidal or tongue shape (see figure 1b). The Frank slide (fig. 1b), which took place in 1903 in the southern Rocky Mountains of Canada, is an example of this kind of rock avalanche (Cruden and Martin, 2007, Charrière et al., 2016). Finally, the low mobility ones are characterized by a reverse T-shape (see figure 1c), caused by the high energy dissipating impact with the opposite slope. A typical example is the Valpola rock avalanche (see figure 1c), which took place in Italy in 1987 (Azzoni et al., 1992). Figure 1. Sketches of typical shapes and picture of related examples of rock avalanches according to the classification of Nicoletti and Sorriso-Valvo (1991) and Corominas (1996): a) Hour glass shape and Pandemonium creek rock avalanche (source: Province of British Columbia); b) trapezoidal shape and Frank slide rock avalanche (source: Alberta Community Development); c) inverse T shape and Valpola rock avalanche (source: Azzoni et al, 1992). Adapted from Manzella (2008). Several models have been developed to try to simulate the propagation of these dangerous and high-mobile phenomena. In particular, because of their flow-like behaviour, continuum mechanics models based on the St Venant equations of unsteady flow and on the work of Savage and Hutter (1989) are often used to model their propagation, e.g. Hungr (1995); Mangeney-Castelnau et al. (2005); Pirulli and Mangeney (2008); Pudasaini and Hutter (2007). In order to be able to use them in a hazard predictive perspective, models validation on laboratory experiments and back-analysis of cases history become crucial, e.g. (Constantinescu et al., 2011). They improve our understanding of the most suitable rheology and of the propagation and emplacement processes. They help us detecting advantages and limits of the different codes, refining their development and employment and consequently improving modelling and assessment of area at risk. In the present study an unconstrained granular flow experiment carried out at the University of Geneva and the case of Potrero de Leyes rock avalanche were considered. This case history was chosen because, even if data available are limited and the accuracy is not very high, it represents a rare case of a well-preserved unconstrained rock avalanche. This event is very interesting to be studied since even if there is not a channelizing effect and it belongs to the aforementioned category of intermediate mobility flows, it happened to have a very low Fahrböschung. In addition unconstrained granular flows pose often problems to numerical modelers because of the difficulty in simulating lateral spreading with the lack of topographical constrains. To back-analyse these data a continuum mechanics model developed at the Laboratoire Magmas et Volcans (Clermont-Ferrand, France), VolcFLow (Kelfoun and Druitt, 2005, Kelfoun et al., 2009), was used. Main parameters were changed and different rheologies were tested to try to fit not only the runout and spreading of the mass but also the thickness of the final deposit. This has allowed improving our understanding of unconstrained type of flow and to have an idea of the kind of rheology that could be used for a first approximation hazard assessment in the region of interest.