Augusto Neri

Pyroclastic Density Current Hazards and Risk

Abstract Pyroclastic density currents (PDCs) represent a formidable challenge for volcanology. This is true in terms of scientific understanding of their dynamics, and also in terms of assessment of their hazard and risk. These phenomena occurred in the famous, lethal 79 AD Pompeii eruption of Mt. Vesuvius, although recognition in “modern” times dates from observations made in 1902 at Montagne Pelee. Major progress in understanding has developed in the last few decades, due both to direct observations and improved modeling capability. Currently, PDC generation and propagation mechanisms are deeply investigated by field reconstructions, and also via detailed experimental and modeling studies. In this chapter we review the current status of knowledge on the dynamics of PDCs, and highlight the key processes and impacts as revealed by recent eruptive events. Some current approaches and modeling studies aimed at PDC hazard assessment are then illustrated, with specific reference to short-to-medium term hazard assessments carried out at the Soufriere Hills volcano at Montserrat (West Indies, UK), and to long-term assessments at the high-risk Vesuvius volcano and Campi Flegrei caldera (Italy). We conclude this review with remarks on the current and evolving state of knowledge of this dangerous volcanic phenomenon, and implications on the challenges for hazards mitigation. Significant limitations exist in our current ability to accurately forecast zones at risk from some PDCs, and in practice, hazard evaluations require cautious good judgment and not over-reliance on computations based on oversimplified algorithms and inputs.

Large-eddy simulation of pyroclastic density currents

We investigate the dynamics of turbulent pyroclastic density currents (PDCs) by adopting a 3D, Eulerian-Eulerian multiphase flow model, in which solid particles are treated as a continuum and the grain-size distribution is simplified by assuming two particulate phases. The turbulent sub-grid stress of the gas phase is modelled within the framework of Large-Eddy Simulation (LES) by means of a eddy-viscosity model together with a wall closure. Despite the significant numerical diffusion associated to the upwind method adopted for the Finite-Volume discretization, numerical simulations demonstrate the need of adopting a Sub-Grid Scale (SGS) model, while revealing the complex interplay between the grid and the SGS filter sizes. We also analyse the relationship between the averaged flow dynamic pressure and the action exerted by the PDC on a cubic obstacle, to evaluate the impact of a PDC on a building. Numerical results suggest that the average flow dynamic pressure can be used as a proxy for the force per unit surface acting on the building envelope (Fig. 5), even for such steeply stratified flows. However, it is not possible to express such proportionality as a constant coefficient such as the drag coefficient in a steady-state current. The present results indeed indicate that the large epistemic and aleatory uncertainty on initial and boundary conditions has an impact on the numerical predictions which is comparable to that of grid resolution.