Short communication: Runout of rock avalanches limited by basal friction but controlled by fragmentation

Abstract. Rock avalanches display exceptionally long runouts, which are found to correlate with their volume and attributed to size dependent dynamic lowering of the effective basal friction. However, even for similar volumes, runouts are seen to span several orders of magnitude suggesting additional controlling factors. Here, we here analyse experiments with analogue models of rock avalanches aimed at testing the role of dynamic fragmentation. We show that for a fixed low basal friction, the runout of experimental rock avalanches varies over two orders of magnitude and is determined by their degree of fragmentation while the basal friction acts only as an upper limiter. We interpret the runout's dependence on fragmentation to be controlled by the competition between mobility enhancing spreading and energy consuming internal friction. We formalize this competition into a scaling law based on energy conservation which shows that variation in the degree of fragmentation can explain the large variation in runout of rock avalanches seen in nature.