Pyroclastic flow dynamics and hazard in a caldera setting: application to Phlegrean Fields (Italy)

Numerical simulation of pyroclastic density currents has developed significantly in recent years and is
increasingly applied to volcanological research. Results from physical modeling are commonly taken into
account in volcanic hazard assessment and in the definition of hazard mitigation strategies. In this work,
we modeled pyroclastic density currents in the Phlegrean Fields caldera, where flows propagating along
the flat ground could be confined by the old crater rims that separate downtown Naples from the caldera.
The different eruptive scenarios (mass eruption rates, magma compositions, and water contents) were
based on available knowledge of this volcanic system, and appropriate vent conditions were calculated for
each scenario. Simulations were performed along different topographic profiles to evaluate the effects of
topographic barriers on flow propagation. Simulations highlighted interesting features associated with the
presence of obstacles such as the development of backflows. Complex interaction between outward
moving fronts and backflows can affect flow propagation; if backflows reach the vent, they can even
interfere with fountain dynamics and induce a more collapsing behavior. Results show that in the case of
large events ( 108 kg/s), obstacles affect flow propagation by reducing flow velocity and hence dynamic
pressure in distal regions, but they cannot stop the advancement of flows. Deadly conditions (in terms of
temperature and ash concentration) characterize the entire region invaded by pyroclastic flows. In the case
of small events (2.5 107 kg/s), flows are confined by distal topographic barriers which provide valuable
protection to the region beyond.

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