IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

1A2_3G-O7

Room A4

Date/Time: July 21 10:45-11:15

Assessing pyroclastic density currents hazard by means of complex multiphase flow
modelsTomaso Esposti Ongaro1 , Augusto Neri1 , Simone Orsucci2
1Istituto Nazionale di Geofisica e Vulcanologia - Sezione di Pisa, Italy, 2 Università degli Studi di Pisa Dipartimento di Fisica, Italy
E-mail: ongaro@pi.ingv.it

Pyroclastic density currents (PDC) hazard assessment at active volcanoes represents a complicated problem and ascientific challenge, since the fundamental mechanisms of PDC propagation and interaction with topography are notfully understood and initial conditions cannot be predicted on a deterministic basis. On the other hand, probabilistic
approaches integrating different sources of uncertainty (including those associated to the target eruptive scenario)need rapid execution of numerical models with variable input parameter distributions and control on errors anduncertainty. Such conditions are seldom simultaneously fulfilled by multidimensional multiphase flow models, which
are needed to describe the complex fluid dynamics of high-temperature particulate currents.
To provide a useful tool for probabilistic hazard assessment, while retaining parts of the information supplied bycomplex multiphase flow models, we have undertaken a systematic study on the propagation of gravity-driven PDCs
aimed at deriving a parameterization of the radial trend of hazard variables (dynamic pressure and temperature)and their associated uncertainty. In particular, we here focus on the propagation of PDCs in the absence of asignificant topographic mean slope (as significant for caldera settings). To evaluate PDC hazards in a caldera, weneed to understand the mechanism and the controlling factors of current sedimentation and deposition during its
propagation, since these control PDCs stratification and their capability of overcoming topographic obstacles andeventually inundate the region outside the caldera margins.
To this aim, we have analyzed the dynamics of finite-volume pyroclastic currents on a flat surface by means of
two- and three-dimensional numerical simulations. Initial conditions have been simplified in order to reduce theinput parameter space to a few variables (initial mass and buoyancy and mixture temperature), in respect to which
scaling properties and uncertainty have been evaluated. By adopting a multiphase flow model for a polydisperse
gas-pyroclast mixture and new constitutive equations based on kinetic theory of particulate flows and laboratory
experiments, we have simulated the mechanism of flow stratification and the influence of the flow rheology on
the large-scale dynamics. In particular, the front propagation velocity, the momentum dissipation rate and thekinetic energy decay have been quantified. A semi-empirical law expressing the decay of dynamic pressure andtemperature defined on this basis will be tested in the framework of probabilistic PDC hazard mapping at the Campi
Flegrei caldera.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

944