Changes in granular textures of debris flow viewed from the unified grain size distribution

Grain composition of debris flow material (including sources, flow bodies, sediments, and deposits) satisfies the unified grain size distribution (GSD), P(D) ~ D–µexp(–D/Dc), thus the GSD function provides a pair of integrated parameters µ and Dc to describe grain mixing during debris flow evolution. It is observed that µ decreases and Dc increases from fluid to deposit, coinciding with the entrainment of fine grains and loss from deposit. This study explores the GSD variations through the analysis of data from field observations and simulation experiments. It is found that debris flow is “well-organized” in that the flow regimes and dynamical parameters are strongly correlated to the GSD. This implies that debris flows originating from various source materials are likely to find their own critical state defined by µ and Dc, i.e., Dc governs collisional regimes (via coarse-grain dominance) and µ controls viscous regimes (via fine-grain modulation). This reveals the underlying universality of diverse appearances of debris flows, and the findings are heuristic in understanding the granular effects in debris flows and formulating more realistic dynamic models.