faDebrisFOAM: debris-flow modeling with coupled terrain evolution in OpenFOAM

Abstract. Debris-flow numerical models are widely used for hazard assessment and engineering design, yet many models still treat bed evolution in a simplified or uncoupled manner. This study presents faDebrisFOAM, an open-source depth-averaged solver developed in OpenFOAM® that couples debris-flow motion, entrainment, deposition, and terrain evolution within a finite-area framework. The model represents the mixture as an equivalent fluid of variable density and incorporates mass exchanges with an erodible bed layer through entrainment and deposition laws. The Terrain Modification Algorithm (TMA), developed in this work, updates the computational grid at each time step, allowing topographic changes to feed back directly into the governing equations and simulated flow dynamics. The model is evaluated through three applications: validation against large-scale US Geological Survey (USGS) flume experiments, testing in an idealized parabolic geometry, and application to the 2015 debris-flow event in the La Mesilla catchment in northern Chile. The solver reproduces the measured evolution of flow thickness in the USGS flume experiments with good agreement. The idealized application demonstrates that terrain evolution is not a passive output but an active control on run-out, velocity, flow depth, and volume redistribution. The La Mesilla case further shows that coupled terrain modification promotes surge self-generation, self-channelization, and channel-plug formation, which are absent or weakened when the TMA is disabled. These results indicate that coupled terrain evolution can modify flow confinement, stress distribution, and preferential routing throughout the simulation. faDebrisFOAM therefore provides an open-source framework for investigating morphodynamic feedbacks in debris-flow propagation.