An erodible substrate and a sharp slope break affect the dynamics and deposition of long runout landslides. We study the flow evolution of a granular mass (1.5-5.1 litres of sand or gravel) released on a bilinear chute, i.e., an incline (between 35° and 66°) followed by a horizontal sector, either sand-free or covered (1-2 cm thick sand layer). Monitoring the time evolution of the falling mass profiled at 120 Hz, the impact dynamics, erosion of the basal layer, and modes of deposition are studied. The frontal deposition is followed by a backward propagating shock wave at low slope angles (<45°), or by a forward prograding flow at greater angles. Experiments with colored sand layers show a complex sequence of dilation, folding and thrusting within both the collapsing sand flow and the substrate. Experimental results are compared with real rock avalanche data and nearly vertical collapses. The observed increase of the drop height divided by the runout (H/L or Heim's ratio) with both chute slope angle and thickness of the erodible substrate, is explained as an effect of vertical momentum loss at the slope break. Data suggest a complex evolution, different from that of a thin flow basal shear flow.To provide an approximate explanation of the dynamics, three analytical models are proposed. Erosion of a 1 cm-thick substrate is equivalent to 8-12% increase of the apparent friction coefficient. We simulate the deposition and emplacement over an erodible layer with a FEM arbitrary Lagrangian Eulerian code, and find a remarkable similarity with the time evolution observed in the experiments. 2D models evidence the internal deformation with time, 3D models simulate deposition. Capabilities and limitations of an elasto-plastic Mohr Coulomb approach for material modeling are discussed.

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