  Flexible barriers are increasingly used for the protection from debris flow
in mountainous terrain due to their low cost and environmental impact. However,
a numerical tool for rational design of such structures is still missing. In
this work, a hybrid computational framework is presented, using a total
Lagrangian formulation of the Finite Element Method (FEM) to represent a
flexible barrier. The actions exerted on the structure by a debris flow are
obtained from simultaneous simulations of the flow of a fluid-grain mixture,
using two conveniently coupled solvers: the Discrete Element Method (DEM)
governs the motion of the grains, while the free-surface non-Newtonian fluid
phase is solved using the Lattice-Boltzmann Method (LBM). Simulations on
realistic geometries show the dependence of the momentum transfer on the
barrier on the composition of the debris flow, challenging typical assumptions
made during the design process today. In particular, we demonstrate that both
grains and fluid contribute in a non-negligible way to the momentum transfer.
Moreover, we show how the flexibility of the barrier reduces its vulnerability
to structural collapse, and how the stress is distributed on its fabric,
highlighting potential weak points.
