Mobility of Pyroclastic Flows and Rock Avalanches

Abstract

Rapid mass movements of rock fragments are among the most hazardous natural phenomena. The ability to foresee their mobility is important when assessing natural hazards in volcanic regions. The dynamics of granular flows is however a challenging multivariate problem. Among the variables that affect their mobility we can include grain size and flow volume. Unfortunately, there are no generally accepted scaling laws describing these phenomena with the certainty to have taken into consideration all important aspects of nature. There are also different ways to assess mobility. Some authors, for example, adopt the distance travelled by the flow front or other arbitrary distances which are inappropriate for energy budget considerations because these flows deform during motion and deposition. Because of the difficulties inherent in direct field observations of these catastrophic events, we resort to laboratory experiments where granular material is released down a chute whose shape is similar to the profile of Mayon Volcano in the Philippines. Our experiments show that in flows of angular rock fragments, the smaller the grain size (all the other things equal), the larger is mobility. Importantly, this mobility is assessed measuring the distance travelled by centres of mass. Particle image velocimetry analysis of high speed video camera images shows also that the smaller the grain size, the smaller is the agitation of the fragments. This can explain the increase of runout distance as grain size decreases because fragments that are less agitated dissipate less energy. This should also explain why larger flow volumes are known to be more mobile. The larger the volume, the relatively smaller is the mass of a fragment with respect to the total mass of the flow so that fragments of larger flows are less agitated and for this reason dissipate less energy.