Long-term hazard of pyroclastic density currents at Vesuvius (Southern Italy) with maps of impact parameters

The hazard of pyroclastic density currents (PDCs) at Vesuvius is investigated by analysing deposits from past eruptions. No specific eruption was chosen as representative of the hazard of PDCs, and the analysis is extended to all the eruptions that left substantial deposits on the ground. Based on the stratigraphic evidence, we assume that at Vesuvius the currents are bipartite, with a highly concentrated basal part, which was fed from the collapse of the eruptive fountain on the ground, and an overlying part generated by the expulsion of gas and fine particulate matter that fed a dilute and turbulent shear flow. Dynamic pressure, particle volumetric concentration, temperature, and flow duration are hazardous characteristics of PDCs that can impact buildings and population and are defined here as impact parameters. They have been calculated by means of an implementation of the PYFLOW code, which uses the deposit particle characteristics as input. The software searches for the probability density function of impact parameters. The 84th percentile has been chosen as a safety value of the expected impact in the long term (50 years). There is no correlation between eruption size and impact parameters. Maps have been constructed by interpolation of the safety values calculated at various points over the dispersal area, and they show how impact parameters change as a function of distance from the volcano. The maps are compared with the red zone, which is the area that the National Department of the Italian Civil Protection has declared to be evacuated under conditions of an impending eruption. The capacity of currents to damage buildings and population is discussed for both the highly concentrated part and the diluted one.