Abstract
Lahars (volcanic debris flows) are natural phenomena that can generate severe damage and wreak havoc in densely populated urban areas. The evaluation of the forces and pressures generated by these mass flows on constructions (e.g., buildings, bridges and other infrastructure) is crucial for civil protection, assessment of physical vulnerability and risk management. The current tools developed to model the spread of flows at large scale in densely populated urban areas remain inaccurate in the evaluation of mechanical efforts. Here, we developed a discrete numerical model for evaluating debris-flow (DF) impact forces at the local scale of one structure (pillar or column) like a building, a bridge and other infrastructure. In this model, the large-sized solid particles that damage infrastructures and edifices are explicitly modelled using Distinct Element Method (DEM). We considered the fluid and fine-grained solid particles not only in the frame of the pressure exerted on structures, but also through their effects on the movement of particles, i.e. buoyancy and drag. The fluid velocity field and the fluid free surface obtained from Computational Fluid Dynamics (CFD) calculation based on Navier–Stokes equations are imported in the DEM simulation. This model is able to reproduce a range of magnitudes of DFs in terms of volumes, velocities and flow heights. Finally, the model provides insights on impact forces generated by particles on structures and on hydrostatic and/or dynamic pressure due to the combined effect of fluid and solid phases. The model provides results consistent with existing empirical models.
Highlights
Lahars are fast moving mixtures of sediment and water, other than normal streamflow, originating from volcanoes (Doyle et al 2010; Manville et al 2013)
debris flows (DFs) generate three major mechanical effects (Zanchetta et al 2004): (1) hydrostatic pressure depending on fluid height and density, (2) dynamic pressure depending on fluid velocity and density, and (3) collision of particles acting as missiles and depending on their mass and velocity (Thouret et al 2020)
Most in situ experiments published to date have measured the density, velocity, and the height of natural debris flows but impact pressures are often considered proportional to the hydrodynamic pressure of a fluid or to the sum of hydrostatic and hydrodynamic pressures
Summary
Lahars (volcanic debris flows) are fast moving mixtures of sediment and water, other than normal streamflow, originating from volcanoes (Doyle et al 2010; Manville et al 2013). DFs generate three major mechanical effects (Zanchetta et al 2004): (1) hydrostatic pressure depending on fluid height and density, (2) dynamic pressure depending on fluid velocity and density, and (3) collision of particles acting as missiles and depending on their mass and velocity (Thouret et al 2020) These effects may vary in space and time, since debris flows usually break down into three segments, which are head, body and tail, each with different characteristics (Vallance 2000; Cui et al 2015; Vallance and Iverson 2015). Measuring the impact of DF on structures aims to use the quantitative data obtained from in situ measurements or laboratory experiments (flow velocity and height, impact pressure, and collision) to help engineers design structures that can mitigate the damage caused by DFs (Zanchetta et al 2004; Federico and Amoruso 2008; Hu et al 2011; Bugnion et al 2012)
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