Benchmarking of Geophysical Mass Flow models used for concentrated pyroclastic currents: from synthetic to experimental approaches

Abstract:

Validation and benchmarking of numerical models for pyroclastic currents (PCs) is required to establish the limits and constraints on the use of these models for scientific and hazard assessment purposes. The aim of a benchmark is to compare models using different numerical approaches but aimed at simulating the same physical process upon common initial and boundary conditions. Here we present the first results of an ongoing benchmarking initiative built for evaluating Geophysical Mass Flow models (i.e; SHALTOP, TITAN2D, VolcFlow and IMEX_SfloW2D) commonly used by the community to assess concentrated PCs hazards. Because of the complex physics of PCs, such a benchmarking exercise cannot be directly establish using a natural case and must be based on simplified cases with reduced complexity first. A two steps procedure with increasing complexity is proposed to better evaluate the performance of such models:

In the first step, the benchmark procedure is based on four synthetic topographies that mimics a real volcanic valley: (1) a straight valley with an obstacle, (2) a straight valley with a break in slope, (3) a straight valley with a constriction, and (4) a valley with a sharp bend. Results given by the four aforementioned models in each scenario are evaluated in their capacity to reproduce: 1) the physical processes observed in natural PC cases (i.e., overbank/avulsion processes, non-uniform flow depth and velocity fields induced by topographic changes), and 2) flow deposit shape and distribution.

The second step of the benchmark is an experimental set up based on our newly built large-scale experimental apparatus (PyroCLAST for Pyroclastic Current Large Scale Apparatus using Synthetic Topographies), for which experiments are used as the referential scenario. The modular capacity of the apparatus allows to test the mobility of various granular mixtures under various topographic conditions, including the four synthetic scenarios used in the first step. Dimensions of the apparatus and initial conditions have been scaled according to those from natural concentrated PCs, so that the ability of models to reproduce the selected experiments can be correctly evaluated and quantified.

The overarching goal is to provide an interpretation framework for volcanic flow hazard assessment studies to the broader community.