A model for generation of ash clouds by pyroclastic flows, with application to the 1980 eruptions at Mount St. Helens, Washington

Abstract

Pyroclastic flows are defined as consisting of a dense basal avalanche underlying a turbulent ash cloud. The generation of ash clouds by the basal avalanche is commonly assumed to be closely related either to fluidization mechanisms within the avalanche or to airflow/flow front interactions. These mechanisms carry ash upward from the basal avalanche into the ash cloud in currents of gas or air, but so far no mechanisms have been proposed to keep the ash in turbulent suspension in the ash cloud. Here I consider the ash cloud turbulence that is generated in two ways: by temperature differences between the basal avalanche and overriding air, and by the motions of the blocks, lapilli, and ash on the outer surface of the basal avalanche. This turbulence would carry ash from saltation into turbulent suspension at the base of the ash cloud, and this transition would simultaneously occur from the upper surface and front of the basal avalanche. This approach quantifies one type of airflow/flow front interaction. The analysis incorporates studies of heat and mass transfer from rough surfaces during high Reynolds number flow. Equations for conservation of mass, momentum, heat energy, and ash particle concentration are formulated and presented in integral form. Model results show that for typical avalanche velocities and temperatues, temperature differences between the basal avalanche and the surrounding air are insignificant in generating the initial turbulence within the ash cloud. Instead, the initial distribution of particle sizes in the ash cloud is controlled by the turbulence generated by the saltating/cascading particles in the basal avalanche. Study of the morphology of observed ash clouds suggests that thermally driven convection often becomes significant hundreds of meters or more behind the flow front, and this may result from deposition of significant quantities of ash from the ash cloud onto the basal avalanche. The variations in ash cloud heights, morphologies, and particle sizes that are predicted by this model are consistent with published observations of small pyroclastic flows at Mount St. Helens in 1980.