Pyroclastic flow hazard assessment at Vesuvius (Italy) by using numerical modelling. II. Analysis of flow variables

Abstract

The numerical simulations of pyroclastic flow propagation along the Vesuvius slopes, described in a companion paper (Todesco et al. 2002, this volume), are herein analyzed in terms of the local variables of the flow with the aim of assessing quantitatively pyroclastic flow hazard in the Vesuvian area. The analysis was carried out by determining the timewise behavior of select- ed hazard variables of the flow – such as density, velocity, temperature, dynamic pressure, and pressure perturbation – at various distances from the crater and in the proximity of ground level. Density, velocity, temperature, and pressure of the flow were directly computed by the model during a period of about 15 min. These vari- ables were then used to estimate two additional vari- ables: the dynamic pressure of the flow and the isotropic pressure variation with respect to the undisturbed atmospheric pressure. These two variables may be both critical for the assessment of damage produced by the flow on engulfed buildings and people indoors. Dynamic pressure is associated with the motion of the flow, whereas the isotropic pressure variation accounts for the large-scale dynamics of the process, including the effect of the weight of the flow and the propagation of pressure waves into the atmosphere. A post-processing of the time series produced led to the definition of peak curves of the main hazard variables as a function of distance from the vent. The results indicate that pyroclastic flow propagation is an unsteady process producing strongly transient stresses on the structures encountered. Mass flow-rate per unit angle of propagation of the flow is confirmed as the main parameter controlling the flow dispersal and, therefore, the intensity of dynamic pressure and isotropic pressure variation. For the worst case considered, i.e., a mass flow-rate of 5×107 kg s–1 spread- ing over a sector of 90°, the two contributions have a similar magnitude, with typical values ranging between 3 and 1 kPa in the medium (4 km) and distal (7.5 km) regions of the flow, respectively. For this case, no significant variations are predicted between the southern and northern axisymmetric profiles assumed. Production of missiles transported by the flow is expected to represent an additional significant hazard associated with the flows.