Effect of grain size on mobility of dry granular flows of angular rock fragments: An experimental determination

Rapid mass movements of rock fragments are among the most hazardous natural phenomena. They are too dangerous to be approached and studied directly and, for this reason, experiments are necessary to understand their flow mechanisms. Here, we present the results of experiments on granular flow mobility obtained releasing batches of angular rock fragments down a curved chute. Our experiments demonstrate that the finer the grain size of the flow, the larger its mobility. This is accompanied by a decrease of fragments' agitation when grain size decreases as shown by particle image velocimetry measurements of high-speed video camera images. The smaller the agitation, the smaller is also the energy dissipated by flows per unit of travel distance. This explains the effect of finer grain size on flow mobility. These results are an important step towards the prediction of runout distances as a function of the features that characterize the different geophysical flows.