Erosion effects on the size and mobility of granular avalanches and landslides

The research field dealing with dynamic analysis of gravitational mass flows is rapidly expanding. One of its ultimate goals is to produce tools for prediction of velocity and deposit extent of rapid landslides. Of special interest are experimental, theoretical and modeling developments that can help explain the occurrence of rapid motion over long distances. Despite the great amount of work devoted to the study of landslide and avalanches, there is no consensus about the physical or mechanical processes at the origin of the high mobility of avalanches or of the occurrence of surges that can propagate along the slope without decelerating. We show here that erosion of granular material already present on the bed can significantly increase the size and mobility of the flow and possibly generate surges. Laboratory experiments of granular material flowing over an inclined plane covered by an erodible bed are presented, designed to mimic erosion processes of natural flows traveling over deposits built up by earlier events. Two controlling parameters are the inclination of the plane and the thickness of the erodible layer. We show that erosion processes increases by up to 40 % the flow mobility (i. e. runout) over slopes with inclination close to the repose angle of the grains even for very thin erodible beds. Erosion efficiency is shown to strongly depend on the slope of the topography. Entrainment begins to affect the flow at inclination angles exceeding a critical angle, almost equal to half of the repose angle. Runout distance increases almost linearly as a function of the thickness of the erodible bed suggesting that erosion is mainly supply-dependent. Two regimes are observed during granular collapse: a first spreading phase with high velocity followed by a slow thin flow, if either the slope or the thickness of the erodible bed is high enough. Surprisingly, erosion affects the flow mostly during this slow regime. The avalanche excavates the erodible layer immediately at the flow front. Waves that help removing grains from the erodible bed are observed behind the front. Triangular shaped frontal surges are seen at high inclination angles over rigid or erodible bed. Finally, simple scaling laws are proposed making it possible to obtain a first estimate of the size of the deposit and of the dynamics of granular collapse over rigid or erodible sloping bedrock.