A two-point three-phase resolved coupling framework for modeling boulder-laden debris flows

Stony debris flows, characterized by coarse boulders embedded in a sediment-laden matrix, greatly amplify destructive potential by altering flow dynamics and impact forces. Conventional single-phase particle-fluid mixture models often struggle to capture the complexities introduced by coarse boulders and multi-phase interactions, while strong-coupling methods can be computationally prohibitive for practical hazard assessments. In this study, we propose a semi-hybrid, fully resolved coupling numerical framework for modeling boulder-laden debris flows. This framework conceptualizes debris flows as a composite system comprising a continuous viscous fluid phase (including fine sediments) and a discrete phase of arbitrarily shaped coarse particles. The continuous phase is treated as a generalized nonlinear Coulomb-viscoplastic fluid using the smoothed particle hydrodynamics (SPH) method, while coarse particles are modeled via the distributed contact discrete element method (DCDEM). These two phases are coupled through an efficient two-way resolved scheme, ensuring accurate simulation of flow-boulder interactions within a unified timeframe. We validate the proposed method against two physical experiments: (1) gravity-driven concrete flows and (2) debris flow interacting with slit-type barriers. Results confirm the method's robustness in accurately capturing fluid-solid-structure interactions and deposition processes. Its capabilities are further showcased through the simulation of a stony debris-flow event in Wenchuan County, China, highlighting its promise for real-world engineering applications and validating the effectiveness of the existing cascade dam system in mitigating debris-flow impact and energy dissipation.