Reconstructing Pyroclastic Currents’ Source and Flow Parameters from Deposit Characteristics and Numerical Modelling: The Pozzolane Rosse Ignimbrite case study (Colli Albani, Italy)

Pyroclastic currents (PCs) are composed of hot mixtures of gas and pyroclastic particles, which travel at moderate to very high speed (tens to hundreds of m/s), under the effect of their density contrast with the surrounding atmosphere. They can be flowing over obstacles with ease but their pathway is often controlled by the topography they flow over. These characteristics make them one of the most dangerous and inaccessible to direct study, natural phenomena. For this reason, the use of numerical modeling could be one of the most useful tools to provide key quantitative information about their internal dynamics. In this study, we used the available data about Pozzolane Rosse ignimbrite (Colli Albani, Italy) caldera-forming, - 460 ka, 63 km³ DRE - to model source and flow dynamics with a depth-averaged model for inertial PCs. Numerical simulations allowed us to test the effects of 1) atmospheric air entrainment, by varying the Richardson number (), 2) the initial flow thickness, 3) initial flow velocity, 4) grain-size distribution, and 5) mixture density on PCs runout and thickness decay pattern. Model validation was performed by comparing i) model runout and field data; ii) the thickness of the deposit compared to the thickness of the model output with the distance; iii) the mass fractions of the different grain size classes for the actual deposit compared to the model output. Several simulations were carried out considering i) the influence of parameters h and v; ii) the density; iii) the temperature and iv) the topography. The results allowed us to understand and quantify the first-order variables that characterize flow propagation (runout) and thickness decay pattern, indicating that the depth-averaged model may be suitable to represent the dynamics of large PCs, such as those of the Pozzolane Rosse.