.[1] Flows of angular rock fragments are released down a concave upward chute in the laboratory to study their mobility. This mobility is measured as the reciprocal of the apparent coefficient of friction that is equal to the vertical drop of the center of mass of the granular material divided by its horizontal distance of travel. Our experiments show that the finer the grain size (with all the other features the same), the larger the mobility of the center of mass. We believe this to be due to the fact that in finer grain size flows there are less agitated particles per unit of flow mass, so that these flows dissipate less energy per unit of travel distance. Our experiments show also that the larger the volume (with all the other features the same), the larger the apparent coefficient of friction. We believe this to be so because the frontal portion of a flow reaches the less steep part of a curved slope and stops before the rear portion, preventing the rear portion and the center of mass from traveling farther downhill. This phenomenon (which is more prominent in larger-volume flows, whose rear and frontal ends are more distant) counteracts the decrease of energy dissipation per unit of flow mass because of the decrease of particle agitation per unit of flow mass that is expected when the volume of a flow increases (with all the other features the same). Our analysis generates a functional relationship between the dimensionless apparent coefficient of friction and a scaling parameter whose numerator is equal to the mean grain size multiplied by the cube root of the deposit volume and whose denominator is the square of the channel width. The purpose of our experiments is to understand the dynamics of rock avalanches and dense pyroclastic flows.Citation: Cagnoli, B., and G. P. Romano (2012), Effects of flow volume and grain size on mobility of dry granular flows of angular rock fragments: A functional relationship of scaling parameters,

[1] Granular mass flows of rock fragments are studied in the lab by means of a high-speed video camera at 2000 frames per second. These granular flows are generated using beds of pumice fragments positioned on a rough rotating disk, whose angular velocity is controlled by a motor. The experimental apparatus allows an understanding of the arrangement of the particles in granular mass flows with relatively small and relatively large values of the Savage number (the Savage number represents the ratio between grain collision stresses and gravitational grain contact stresses). In particular, these flows develop a basal layer of agitated and colliding particles underneath a relatively rigid upper layer. Our experimental results suggest the validity, on average, of the Coulomb's relationship between shear and normal forces at the base of granular mass flows irrespective of their Savage number value. In Coulomb's equation the shear stresses do not depend on the shear rate. We expect the Coulomb friction law to be valid also in moving pyroclastic flows. Our experiments suggest that the collisions and subsequent comminution of pumice fragments in moving pyroclastic flows could provide ash for the overriding ash clouds. In our experiments the amount of ash generated by particle-particle and particle-boundary interactions increases as the value of the Savage number increases. In nature, part of this ash may also simply move toward the base of the flows because of kinetic sieving.

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