Hazard of pyroclastic density currents at the Campi Flegrei Caldera (Southern Italy) as deduced from the combined use of facies architecture, physical modeling and statistics of the impact parameters

Abstract

Pyroclastic density currents of the recent eruptions at Campi Flegrei Caldera (CFC — Southern Italy) have been studied with the aim of assessing the potential impact of similar events in the future. Eruptions of different scales have been investigated by means of the combined use of facies architecture, laboratory analysis and physical modeling. Both in the small (Averno 2) and intermediate (Astroni) scales, facies analysis indicates that deposits result from the emplacement of pyroclastic density currents like base-surge, formed by multiple closely-timed impulses of phreatomagmatic origin. In the large-scale event (Agnano-Monte Spina), the facies architecture suggests that the currents started as concentrated flows near the vent, as originating from the collapse of a dense eruptive column, and evolved laterally into expanded flows by the propagation of the basal shear current. Laboratory analyses on samples from the main layers of deposits allowed obtaining the input data for the PYFLOW code, which was used for reconstructing the flow dynamic characteristics of the currents. The expected damage is discussed in terms of the probability density function of dynamic pressure and particle volumetric concentration. In this way, the range of potential impact that similar pyroclastic density currents could cause to buildings, infrastructures and population is defined.In the large-scale event, the dynamic pressure ranges from 9.38 to 1.00kPa (integrating the basal 10m of the current) at distances of 1.5 and 4.0km from the vent, respectively. The values are highly influenced by the local topography. In the intermediate-scale event, the dynamic pressure ranges from 2.43 to 0.26kPa at distances of 1.1 and 1.4km from the vent, respectively. In the small scale event, the dynamic pressure ranges from 1.49 to 0.39kPa at distances of 0.5 and 1.1km from the vent, respectively.The particle volumetric concentration at a height of 2m within the current is always lower than 0.01, but typically it is higher than 0.001 inside the caldera, and could have severe effects on the unsheltered population, also at locations (topographic highs) where velocity and dynamic pressures are low. In distal reaches outside the caldera the widespread fine ash originated by buoyancy from the currents, which was transported by lower atmosphere winds, is to be considered as a “fallout load” slowly accumulating on the ground surface (and building roofs) during the waning stage of the current.