Paper:

# Numerical Simulations of Dome-Collapse Pyroclastic Density Currents Using faSavageHutterFOAM: Application to the 3 June 1991 Eruption of Unzen Volcano, Japan

## Hiroyuki A. Shimizu

National Research Institute for Earth Science and Disaster Resilience

3-1 Tennodai, Tsukuba, Ibaraki 305-0006, Japan

Corresponding author

Pyroclastic density currents (PDCs) are one of the most dangerous but least understood phenomena of volcanic eruptions. An open-source numerical depth-averaged model of dense granular currents controlled by physical processes such as energy dissipation, basal deposition, and erosion (faSavageHutterFOAM) was applied to investigate the basal concentrated region of a dome-collapse PDC generated on June 3, 1991 at Unzen volcano (Japan) to assess the effects of the physical processes (and their interplay) on the flow dynamics and run-out area of the PDC. Numerical simulations show that energy dissipation process decreases the flow velocity and increases the basal deposition rate, which reduces the run-out distance. The simulations also reveal that erosion process during flow propagation decreases the flow velocity and increases the run-out distance. The numerical results are sensitive to the parameters of energy dissipation (dry friction coefficient μ and collisional or turbulent friction coefficient χ) and erosion (specific erosion energy e_{b}). The results are fitted to field data for run-out distance and flow velocity when μ is between 0.01 and 0.1 with χ∼10^{3} m^{-1} s^{-2} (or when χ is between 10^{4} and 10^{5} m^{-1} s^{-2} with μ∼0.2) and e_{b}∼10^{2} m^{2} s^{-2}. The estimated value of e_{b} suggests that re-entrainment of deposit mass played an important role in controlling the flow dynamics and run-out area of the PDC. The estimated values of μ and χ are correlated, but the estimation of these parameters might be improved by further constraints from field data. The presented results serve as a basis to make further quantitative estimations of the model parameters (μ, χ, and e_{b}) for applying the faSavageHutterFOAM model to hazard assessments of PDCs.

*J. Disaster Res.*, Vol.17, No.5, pp. 768-778, 2022.

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