Effects of particle size distribution and moisture content on the motion and deposit characteristics of debris avalanches

Abstract

Debris avalanches characterized by rapid granular flow pose significant hazards, yet the joint influence of particle size distribution, moisture content, and compound topography on their dynamics remains poorly constrained. To address this, we performed multivariable flume experiments simulating the transformation of landslides into granular flows under varying gradations and moisture levels (0–4 %). Our results reveal a robust linear scaling law between deposit runout and width, establishing a stable geometric proportion governing planform spreading. Kinematic analysis demonstrates that moisture content shortens total motion duration primarily by accelerating the deposition stage, whereas mixed gradations exhibit regime-shifting behaviors due to particle segregation. Crucially, we introduce a planform spreading-angle framework that unifies these observations into a single back-calculated coefficient (η). This parameter quantifies lateral spreading capacity, increasing monotonically with particle size while remaining weakly sensitive to moisture for single-size groups. Furthermore, a volumetric-equivalent scale analysis indicates that moisture drives a three-dimensional redistribution of the deposit mass rather than uniform scaling. These findings offer a simplified, physically based approach to predict the runout and spreading of non-cohesive debris avalanches under complex terrain conditions.