Abstract
Numerical results of a two-layer depth-averaged model of pyroclastic density currents (PDCs) were compared with an experimental PDC generated at the international eruption simulator facility (the Pyroclastic flow Eruption Large-scale Experiment (PELE)) to establish a minimal dynamical model of PDCs with stratification of particle concentrations. In the present two-layer model, the stratification in PDCs is modeled as a voluminous suspended-load layer with low particle volume fractions (lesssim {10}^{-3}) and a thin basal bed-load layer with higher particle volume fractions (sim {10}^{-2}) on the basis of the source condition in the experiment. Numerical results for the suspended load quantitatively reproduce the time evolutions of the front position and flow thickness in the experimental PDC. The numerical results of the bed-load and deposit thicknesses depend on an assumed value of settling speed at the bottom of the bed load ({W}_{mathrm{sH}}). We show that the thicknesses of bed load and deposit in the simulations agree well with the experimental data, when {W}_{mathrm{sH}} is set to about 1.25times {10}^{-2} m/s. This value of the settling speed is two orders of magnitude smaller than that predicted by a hindered-settling model. The small value of {W}_{mathrm{sH}} is considered to result from decreasing in the effective deposition speed due to the erosion process accompanied by saltating/rolling of particles at the bottom of the bed load.
Highlights
Pyroclastic density currents (PDCs) are a frequent and hazardous process during volcanic eruptions
The lower region has various characteristics depending on the source conditions; the lower region behaves as a dense gas-pore pressure-modified granular flow with very high particle volume fractions ( ∼ 0.4 ; referred to as a “dense underflow”; e.g., Breard et al 2016; Roche et al 2016; Lube et al 2019) or as a flow of saltating/rolling particles with relatively low particle volume fractions ( ∼ 10−2 ; referred to as a “bed load”; e.g., Valentine 1987; Dufek and Bergantz 2007; Brosch and Lube 2020)
The results of the suspended loads in the numerical simulations are almost unaffected by the characteristics of the bed load regardless of WsH, because the bed loads have a negligible effect on the dynamics of the suspended load
Summary
Pyroclastic density currents (PDCs) are a frequent and hazardous process during volcanic eruptions They occur when a hot mixture of volcanic particles and gas is ejected. The flow dynamics of the lower region is controlled mainly by gas-particle interaction, particle–particle collision, frictional interaction between the current and the ground, and deposition/erosion processes at the base (e.g., Roche et al 2008; Girolami et al 2010; Lube et al 2019; Brosch and Lube 2020). The behavior of the whole stratified current is determined by the dynamics of both the upper and lower regions and the interactions between them (i.e., transfers of mass, momentum, and energy from one to the other) The effects of these physical processes on the flow dynamics depend on the source conditions and topography. The lower region has various characteristics depending on the source conditions (for instance, the particle concentration at the source; e.g., Lube et al 2015; Breard et al 2018; Valentine 2020); the lower region behaves as a dense gas-pore pressure-modified (i.e., fluidized) granular flow with very high particle volume fractions ( ∼ 0.4 ; referred to as a “dense underflow”; e.g., Breard et al 2016; Roche et al 2016; Lube et al 2019) or as a flow of saltating/rolling particles with relatively low particle volume fractions ( ∼ 10−2 ; referred to as a “bed load”; e.g., Valentine 1987; Dufek and Bergantz 2007; Brosch and Lube 2020)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
