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

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 a 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 from 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 flow-substrate 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.