Abstract Landslides composed of particles of varying sizes undergo particle‐size segregation during motion, forming heterogeneous dams that influence breaching processes and resultant flooding. To elucidate the mechanisms of such heterogeneity, this study develops a three‐dimensional Riemann‐based smoothed particle hydrodynamics framework to simulate bidisperse landslide dam formation by coupling a nonlocal granular fluidity model that accounts for local and nonlocal effects in flow behavior transitions with a segregation model governed by competing gravity‐driven percolation and diffusion. During high‐velocity motion, the landslide behaves as a fully mobilized granular flow with pronounced particle‐size segregation. After impacting the valley floor, it transitions to a quasi‐static regime with reduced segregation. Higher flow mobility in the fully mobilized flow state exerts a stronger influence on the enhancement of segregation and dam non‐uniformity than that in the quasi‐static state. The evolution of segregation mechanisms results in two stages: a percolation‐dominated stage, in which small particles migrate downward while large particles rise to form an inverse grading structure, followed by a percolation–diffusion coupled stage in which diffusion promotes particle mixing and forms a large–small particle transition zone. Increasing the particle size enhances percolation in the initial stage but strengthens diffusion more strongly in the latter stage, leading to reduced heterogeneity. The model is further applied to a field‐scale Hsiaolin heterogeneous landslide dam and reproduces the inverse grading patterns, consistent with field investigations. These findings enhance the understanding of the formation mechanisms of heterogeneous landslide dams and contribute to the prediction of dam breaching and disaster mitigation.
Peng et al. (Mon,) studied this question.