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

Abstract

Rock avalanches produce exceptionally long run-outs that correlate with their rock volume. Thisrelationship has been attributed to the size-dependent dynamic lowering of the effective basal friction. However,it has also been observed that run-outs of rock avalanches with similar volumes can span several orders ofmagnitude, suggesting additional controlling factors. Here, we analyse analogue models of rock avalanches,with the experiments designed to test the role of dynamic fragmentation. We show that for a fixed low basalfriction, the run-out of experimental rock avalanches varies over 2 orders of magnitude and is determined bytheir degree of fragmentation, while the basal friction acts only as an upper limit on run-out. We interpret therun-out’s dependence on fragmentation as being controlled by the competition between mobility enhancingspreading and energy-consuming fragmentation limited by basal friction. We formalize this competition into ascaling law based on energy conservation, which shows that the variation in the degree of fragmentation cancontribute to the large variation in run-out of rock avalanches seen in nature