Lahar events in the last 2,000 years from Vesuvius eruptions. Part 2: Formulation and validation of a computational model based on a shallow layer approach

Abstract. In this paper we present a new model for the simulation of lahars, based on the depth-averaged code IMEX-SfloW2D with new governing and constitutive equations introduced to better describe the dynamics of lahars. A thorough sensitivity analysis is carried out to identify the critical processes (such as erosion and deposition) and parameters (both numerical and physical) controlling lahar runout, using both synthetic and real cases topographies. In particular, an application of the model to a syn-eruptive lahar from a reference-size eruption from Somma-Vesuvius, affecting the Campanian Plain (Southern Italy), described in Di Vito et al. (this issue), is used in this work for the sensitivity analysis. Effects of erosion and deposition are investigated by comparing simulations with and without these processes. By comparing flow thickness and area covered by the flow and their evolution with time, we show that the modelling of both the processes is important to properly simulate the effects of the bulking and debulking and the associated changes in rheology. From a computational point of view, the comparison of simulations obtained for different numerical grids (from 25 m to 100 m), scheme order, and grain size discretization were useful to find a good compromise between resolution and computational speed. The companion paper by Sandri et al. (this issue) shows an application of the presented model for probabilistic volcanic hazard assessment for lahars from Vesuvius deposits in the Neapolitan area.