Decoupling of pyroclastic currents: hazards assessments

Gravitational settling of fragments, gas elutriation and boundary layer processes cause concentration values to change within different parts of sediment gravity flows during movement. These processes result in flow transformations, density stratification and decoupling of single currents into separate sediment gravity flows. Density can change by an increase in solids concentration in basal zones of currents by downward fall of particles, or by incorporation of particles from the substrate, thereby leaving their upper parts more dilute. In some flows, upward-streaming gases and elutriation of fines cause dilute turbulent currents to develop at the top of a dense current. Increasing concentration dampens turbulent action, transforming the turbulent to the nonturbulent state, commonly at the currentʹs base. Conversely, dilute parts of currents become or remain turbulent. Currents consisting of turbulent and nonturbulent parts can decouple by shearing past one another because of differing flow properties, with each part taking separate paths. nner by which a pyroclastic flow moves affects its predicted runout distance, its distribution as continuous sheets or in isolated basins, and whether it can cross barriers such as high mountain ranges or water bodies. During the eruption of Unzen volcano on June 3, 1991, decoupling of a pyroclastic surge from the top of a pyroclastic flow caused the deaths of Harry Glicken, Maurice Krafft, Katia Krafft and 40 Japanese citizens. Mapping volcanic hazard zones requires that consideration be given to decoupling processes to help prevent similar tragedies in the future.