Effects of flow volume and grain size on mobility of dry granular flows of angular rock fragments: A functional relationship of scaling parameters

Abstract

.[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,

Combined effects of grain size, flow volume and channel width on geophysical flow mobility: three-dimensional discrete element modeling of dry and dense flows of angular rock fragments

“…Importantly, we demonstrate that finer grain size flows are intrinsically less energetically dissipative than coarser grain size flows, irrespective of the presence of an interstitial fluid (either gaseous or liquid). Our new set of simulations also confirms that the larger the flow volume (all the other features being the same), the less mobile the center of mass of the granular flows (Okura et al, 2000;Cagnoli and Romano, 2012a;Cagnoli and Piersanti, 2015). The mobility of the center of mass is different from the mobility of the flow front, which becomes more distal as flow volume increases (Scheidegger, 1973) because the larger the flow volume, the longer the longitudinal spreading of the flow and its deposit (Davies, 1982;D'Agostino et al, 2010).…”

“…The new set of numerical simulations illustrated in this paper confirms that the finer the grain size (all the other features being the same), the more mobile the center of mass of the granular flow (Cagnoli and Romano, 2012a;Cagnoli and Piersanti, 2015). For example, we expect that in nature the fragmentation during motion of rock clasts with different hardness can generate flows with different grain size.…”

“…These polyhedrons represent an equant, an oblate and a prolate particle, respectively. Nonspherical particles are preferred because when interacting amongst themselves and with the boundary surfaces, their energy dissipation mechanism (due to collisions and attrition) is comparable with that of natural fragments since they are both angular (Cagnoli and Romano, 2012a;Mead and Cleary, 2015). The proportion of each particle shape is always the same in all flows irrespective of grain size, flow mass or channel features: the equant particles are 38 %, the oblate particles 22 % and the prolate particles 40 % of the flow mass.…”

“…In the present paper, we illustrate the combined effect of grain size, flow volume and channel width on granular flow mobility. This result is built on our previous laboratory experiments Romano, 2010, 2012a) and numerical simulations (Cagnoli and Piersanti, 2015) where we focused on the quantities that enter the numerator of a scaling parameter that the apparent coefficient of friction µ A (i.e., the reciprocal of mobility) is proportional to. The main original contribution of our paper here consists of assessing the effect of the channel width on flow mobility.…”

“…In our new three-dimensional numerical simulations, we vary the width w of the concave-upward channel we used in our previous laboratory experiments (Cagnoli and Romano, 2012a) and numerical simulations (Cagnoli and Piersanti, 2015). These virtual channels were generated by means of a CAD software (Rhinoceros).…”