IMEX_SfloW2D v2: a depth-averaged numerical flow model for volcanic gas-particle flows over complex topographies and water

Abstract. We present developments to the physical model and the open source numerical code IMEX_SfloW2D (de' Michieli Vitturi et al., Geosci. Mod. Devel., 2019). These developments consist of a generalization of the depth-averaged (shallow-water) fluid equations to describe a polydisperse fluid-solid mixture, including terms for sedimentation and entrainment, transport equations for solid particles of different sizes, transport equations for different components of the carrier phase, and an equation for temperature/energy. Of relevance for the simulation of volcanic mass flow, vaporization and entrainment of water are implemented in the new model. The model can be easily adapted to simulate a wide range of volcanic mass flows (pyroclastic avalanches, lahars, pyroclastic surges) and here we present its application to transient dilute pyroclastic density currents (PDCs). The numerical algorithm and the code have been improved to allow for simulation of sub- to supercritical regimes and to simplify the setting of initial and boundary conditions. The code is open-source. The results of synthetic numerical benchmarks demonstrate the robustness of the numerical code in simulating trans-critical flows interacting with the topography. Moreover, they highlight the importance of simulating transient in comparison to steady-state flows, and flows in 2D versus 1D. Finally, simulation of the Krakatau 1883 eruption demonstrates the capability of the numerical model to face a complex natural case involving the propagation of PDCs over the sea surface and across topographic obstacles, showing the relevance, at the large scale, of non-linear fluid dynamic features, such as hydraulic jumps and Van Karman vortexes to flow conditions such as velocity and runout.