Lahars as repeated hazardous phenomenon around Vesuvius volcano (Italy) during and after impacting explosive eruptions

Lahars represent some of the most dangerous phenomena in volcanic areas for their destructive power, causing dramatic changes in the landscape with no premonitory signs and impacting on population and infrastructures. In this regard, the Campanian Plain turns out to be very prone to the development of these phenomena, since the slopes of the Somma-Vesuvius and Campi Flegrei volcanoes, as well as the nearby Apennine reliefs, are mantled by pyroclastic deposits that can be easily remobilized, especially after intense and/or prolonged rainfall. Our recent studies focus on the analysis of the pyroclastic fall and flow deposits, and of the syn- and post-eruptive lahar deposits related to two sub-Plinian eruptions of Vesuvius, 472 CE (Pollena) and 1631. Historical and field data from the existing literature and from hundreds of outcrops were collected and organized into a database. Stratigraphic, sedimentological, and archaeological analyses were carried out, in addition to rock magnetic investigations and impact parameter estimations. The field data analyses show that in both eruptions the dispersal area of the primary pyroclastic deposits is wider than previously known. Such distribution of the deposits directly affects the one of the lahar deposits, even because a significant remobilization took place during and after the studied eruptions, involving the distal phreatomagmatic ash. From these analyses, it was possible to constrain the timing of the deposition, and to estimate the thicknesses, velocities and dynamic pressures (impact parameters) of the lahars. A new shallow layer model based on depth-averaged variables, named IMEX-SfloW2D, was developed for the simulation of lahar dynamics. A thorough sensitivity analysis was carried out to identify the critical processes (erosion and deposition) and parameters (numerical and physical) controlling lahar runout, using both synthetic and real cases topographies. Effects of erosion and deposition were investigated by comparing field data with the output of simulations including vs. excluding these processes. By comparing observed and simulated flow thickness and area covered by the flows, and their evolution over time, it can be shown that the inclusion of erosion and deposition is important to properly simulate the impact parameters of lahars, particularly on uneven terrain. Lastly, a novel workflow for Probabilistic Volcanic Hazard Assessment (PVHA) for lahars was developed and applied to the Vesuvius case study. Such a workflow explores the effect of uncertainty of the flow initial conditions on the impact parameters of lahars on the target area, by sampling coherent sets of values for the input model parameters and running thousand simulations. The simulation outputs were processed to produce hazard curves, hazard maps, and probability maps for the maximum flow thickness, and hazard surface and probability maps for joint thresholds in flow thickness and dynamic pressure. It is believed that the latter hazard products represent a novel product in PVHA for lahars around Vesuvius volcano and can be applied worldwide. The multidisciplinary approach adopted in this work shows how it is crucial to assess the impact of lahars in densely populated areas, even at distances of several to tens of km from active volcanoes like Vesuvius.