( ) A. Mangeney 1,3, O. Roche 2, L. Tsimring 3, F. Bouchut 4, O. Hungr 5, I. Ionescu 6, N. Mangold 7 1 Equipe de Sismologie, Institut de Physique du Globe de Paris, Université Paris-Diderot 7, France 2 Laboratoire Magma et Volcans, Université Blaise Pascal, Clermont-Ferrand, France 3 Institut for Non Linear Science, University California San Diego, USA 4 Département de Mathématiques et Applications, ENS et CNRS, Paris, France 5 Earth and Ocean Sciences, University of British Columbia, Vancouver, Canada 6 Laboratoire des Propriétés Mécaniques et Thermodynamiques des Matériaux, Villetaneuse, France 7 Laboratoire de Planétologie et Géodynamique, Université de Nantes, France Understanding of the mechanisms that produce the observed high mobility of natural flows such as avalanches or pyroclastic flows remains a major challenge. A related task is to explain flow surges that can travel long distances down-slope without decelerating or losing momentum. Volcanic granular flows often propagate over slopes covered by the deposits of former events. We focus here on the effect of entrainment of material already present on the topography on avalanche mobility. Entrainment may decrease or significantly increase mobility depending on the slope angle or the nature of the material involved. We present here experimental results and numerical simulation involving dry granular flows over erodible bed with variable thickness, lying on a plane with different inclination angles. We show that, depending on the slope of the topography, the granular flow can be either insensitive or accelerated by the presence of an erodible bed and possibly turn to a surge propagating at constant velocity down the slope with dramatic impact on avalanche mobility. Field evidence of erosion processes The 3×10 5 m 3 Nomash River rock avalanche, Canada, 1999, eroded nearly the same volume of bed material when flowing over a colluvial apron [Hungr and Evans, 2004].  8 m The Ganges Chasma landslide, Valles Marineris, Mars, seems to be slowed down when it passes over a former landslide deposit, CTX image from MRO [Mangeney et al., 2008]. Erosion processes : increasing or decreasing the mobility of granular avalanches ? Volcanoes, mountain and valley slopes are generally covered by erodible material. As rock avalanches, debris and pyroclastic flows hurtle down, they entrain and drop off bed material. Quantification of erosion processes in the field is hardly achieved. On Lascar volcano, Chile, there is evidence of granular wear by pyroclastic flows into bedrock made of the deposits of former events [Calder et al., 2000]. flowing grains static grains Discrete element simulation Volfson et al., 2003  static contacts  contacts The order parameter characterizes the « state » of granular matter u Momentum and mass conservation (thin layer approximation without depth-averaging) Constitutive relation Constitutive relation for flowing and static grains No sliding at the base Theory of phase transition Theory of phase transition (Ginzburg-Landau equation) related to the Mohr-Coulomb criteria  free energy of the system When a granular avalanche flows over a static layer, some of the grains are sliding past each other, while others maintain prolonged static contacts with neighbors due to friction and jamming. No-flow and flowing zones not only coexist but exchange mass and momentum. The partial fluidization model describes explicitly the static/flowing transition through an order parameter related to the fraction of static contacts between the grains [Aranson and Tsimring, 2002; Aranson et al., 2008]. Numerical simulation of erosion processes : the partial fluidization model, Fluid stress : The dynamics of the order parameter is controlled by the source term f which has been calibrated based on discrete element simulations. Introduction where is the stress tensor, the pressure, the thickness, the mean downslope velocity and the slope angle. x y