Mobility of long-runout rock flows: a discrete numerical investigation

In the perspective of studying the mobility and deposit geometry of long-runout flows, discrete numerical simulations of model granular avalanches over simple topography are performed. Analysing the behaviour of the ratio H/L depending on the parameters varied in the simulations, we show its poor sensitivity to the topographic parameters, namely the initial height of the flowing mass and the slope. On the contrary, a correlation between H/L and the frictional properties of the material is established. However, the existence of variations of the effective friction induced by the dynamics are not reflected by the value of H/L. As an alternative to the H/L description of the flow behaviour, we propose a new scaling for the runout distance taking into account the lateral extension L-0 of the relief. Forming the normalized runout L/L-0, and defining the aspect ratio a of the topography, we show the dependence of L/L-0 on a, thus generalizing previous analysis of Martian data. Analysing the correlation between the normalized runout and the front velocity for the numerical simulations, we show how L/L-0 depends both on the frictional properties of the material and on the dynamics of the flow. This result suggests the necessity of evaluating the lateral extension of the relief L-0 in the case of natural flows, in order to define a 'horizontal travel index' including the topography in an attempt to better understand the flow dynamics. Analysing the deposit for both simulations and real flow data we evidence a common behaviour suggesting that the geometric factors prevail in the spreading dynamics irrespective of the details of the flow context and conditions. We moreover assert the relevance of discrete simulations to the discussion of real cases.