Impact behavior and long-term retention mechanism of binary-size debris flows against a slit dam

Debris flows are catastrophic multi-phase hazards whose impact intensity is strongly amplified by boulder entrainment. While slit dams are widely used as mitigation structures, the specific role of large-particle concentration in governing impact dynamics and retention efficiency across varying slopes remains insufficiently quantified. This study employs a coupled VOF-DEM framework to investigate binary-size debris flows with different large-particle contents impacting a slit dam under various channel inclinations. The coarse-grain phase is resolved using the Discrete Element Method, while the muddy fluid phase is represented by a Herschel-Bulkley non-Newtonian fluid. Results reveal that debris-flow kinetic energy and impact forces escalate significantly with large-particle content on steep slopes, whereas non-monotonic variations are observed on gentle slopes. These non-monotonic behaviors may originate from the compensation between the gravitational effect, interparticle friction, and formation of granular arches, leading to a complex energy profile that differs from the inertia-dominated behavior seen on steep slopes. Remarkably, the study reveals a non-intuitive long-term retention mechanism: while higher boulder concentrations promote stable granular arching near the slit, they simultaneously reduce overall trapping efficiency at the recommended 3.6 relative slit-size threshold. This reduction is driven by intensified size segregation, enhanced run-up heights, and the formation of a porous skeleton that facilitates rapid muddy fluid drainage and overflow, resulting in a reduction of the fluid elevation. Analysis of force-chain networks and drainage patterns reveals the mesoscale mechanisms governing debris retention and discharge. These findings provide a physics-based understanding of debris-flow behavior and offer an essential reference for designing structural countermeasures in mountainous regions.