The influence of gas pore pressure in dense granular flows: numerical simulations versus experiments and implications for pyroclastic density currents

Abstract

We investigate the influence of gas pore pressure in granular flows through numerical simulations on horizontal and low-angle inclined surfaces. We present a two-phase formulation that allows description of dam-break experiments considering high-aspect-ratio collapsing columns and depth-dependent variations of flow properties. The model is confirmed by comparing its results with data of analogue experiments. The results suggest that a constant, effective pore pressure diffusion coefficient can be determined in order to reproduce reasonably well the dynamics of the studied dam-break experiments, with values of the diffusion coefficient consistent with experimental estimates from defluidizing static columns. The discrepancies between simulations performed using different effective pore pressure diffusion coefficients are mainly observed during the early acceleration stage, while the final deceleration rate, once pore pressure has been dissipated, is similar in all the studied numerical experiments.However, these short-lasting discrepancies in the acceleration stage can be manifested in large differences in the resulting run-out distance. We also analyze the pore pressure at different distances along the channel. Although our model is not able to simulate the under-pressure phase generated by the sliding head of the flows in experiments and measured beneath the flowsubstrate interface, the spatio-temporal characteristics of the subsequent over-pressure phase are compatible with experimental data. Additionally, we studied the deposition dynamics of the granular material, showing that the timescale of deposition is much smaller than that of the granular flow, while the time of the deposition onset varies as a function of the distance from the reservoir, being strongly controlled by the surface slope angle. The simulations reveal that an increment of the surface slope angle from 0° to 10° is able to increase significantly the flow run-out distance (by a factor between 2.05 and 2.25, depending on the fluidization conditions). This has major implications for pyroclastic density currents, which typically propagate at such gentle slope angles.

Fluidization by gas pore pressure of dense granular flows: numerical simulations versus experiments

“…The main characteristics (mass profile, runout distance, heigth, front velocity, deposition mechanics) of the collapse process are well reproduced (see [1] and [2]). The internal dynamics, consisting of a basal deposit overlain by a thin layer of moving particles whose base migrates upwards during the collapse, is well captured by the model (see [2] and [3]). Unlike the non-fluidized model, which is known to have instabilities in the sense that pressure small-scale oscillations appear and depend on the mesh size, the fluidized one has a regularizing effect.…”

Assessing minimum pyroclastic density current mass to impact critical infrastructures: example from Aso caldera (Japan)

“…Several recent papers have focused on pyroclastic flow emplacement by more sophisticated modeling; e.g., thermodynamics modeling of cooling effects and air entrainment effects (e.g., Bursik and Woods, 1996;Fauria et al, 2016), two layer systems (Burgisser and Bergantz, 2002;Doyle et al, 2007;Kelfoun, 2017;Valentine, 2020), the development of dense and dilute regimes within the same flow (Esposti Ongaro et al, 2011Kelfoun and Gueugneau, 2022;Neri et al, 2022), the effects of pore pressure on basal friction (Roche et al, 2021;Aravena et al, 2021), and the buildup of coherent turbulent structures and gravity waves (Lube et al, 2020;Brosch et al, 2021). However, since more advanced models require more detailed data to fit additional input parameters, simplified models are suitable for probabilistic modeling in uncertain frameworks, like the present Aso-4 case study, especially for capturing extreme probabilities.…”

Comment on nhess-2022-100

“…Several recent papers have focused on pyroclastic flow emplacement by more sophisticated modelling; e.g. thermodynamics modelling of cooling effects and air entrainment effects (e.g., Bursik and Woods, 1996;Fauria et al, 2016), two layer systems (Burgisser and Bergantz, 2002;Doyle et al 2007;Kelfoun et al, 2017;Valentine, 2020), the development of dense and dilute regimes within the same flow (Esposti Ongaro et al, 2011;2020;Kelfoun and Gueugneau, 2022;, the effects of 190 pore pressure on basal friction (Roche et al 2021;Aravena et al, 2021), and the build-up of coherent turbulent structures and gravity waves (Lube et al, 2020;Brosch et al, 2021). However, since more advanced models require more detailed data to fit additional input parameters, simplified models are suitable for probabilistic modelling in uncertain frameworks, like the present Aso-4 case study, especially for capturing extreme probabilities.…”