The influence of grain size and channelization on mobility of volcanic granular flows: insights from laboratory experiments

Laboratory experiments on granular flows using natural material were carried out under different conditions in order to investigate the behavior of fine to coarse-grained, channelized and non-channelized granular flows passing over a break in slope. Morphometric parameters of channelized and non-channelized experiments are compared for both fine-grained and coarse-grained grain size distributions. After normalization, morphometric data provide empirical relationships that highlight the major influence of grain size vs. channelization of the experimental flows. Normalized velocity data of the flow front show third-order polynomial fit for both coarse-grained and fine-grained experiments, as well as for non-channelized and channelized ones. This highlights similar complex behavior of the different experimental flows, which differs only for different partition of inertial and frictional forces at changing grain size of experimental mixtures. The runout of coarse-grained granular flows is always longer than that of fine-grained granular flows, irrespective if they are channelized or non-channelized. Finally, we discuss the applicability of the experimental results to natural granular flows, highlighting the change in physical behavior of fine-rich pyroclastic density currents and volcaniclastic flows depending on the effectiveness of fluidization processes in fine-grained natural materials.