Particle Sorting in Scree Slopes: Characterization and Interpretation From the Micromechanical Perspective

• Published in: Journal of geophysical research. Earth surface Vol. 127; no. 5
• Main Authors: Dai, Bei‐Bing, Wu, Fan‐Yu, Zhong, Wei‐Tao, Shi, Yi‐Hao, Qin, Jin‐Tao, Yang, Jia‐Jun, Yang, Jun
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-05-2022
• Subjects: discrete element method; Distance; Energy dissipation; Energy exchange; Friction; Kinetic energy; Kinetic energy dissipation; Mass flow; Mathematical models; Model testing; Numerical simulations; particle property; Particle size; Particle sorting; scree slope; Segregation; Shape; Shape factor; Simulation; Slope; Slopes; sorting phenomenon; Tracer particles; Tracers
• ISSN: 2169-9003; 2169-9011
• DOI: 10.1029/2021JF006372

Particle sorting is one of the most interesting phenomena observed in the formation of scree slopes. We present a study of this phenomenon through physical model tests and numerical simulations at the grain scale, with a focus on the link between the degree of sorting and particle attributes, as well as the underlying mechanisms. By characterizing the degree of sorting with the graphic segregation index, we show that the degree of sorting increases as the difference in particle size and shape between the component materials of the deposited mixture increases. We propose two factors, the relative size factor (SFr) and relative shape factor (ORr), and show that the degree of sorting increases with the absolute values of these two factors (|SFr| and |ORr|). By monitoring the motions of tracer particles in the numerical simulations, we found that particle migration proceeds in a periodical mode of "collision/friction ⇌ regaining ⇌ collision/friction." Also, we performed energy analyses, revealing that relatively large and rounded particles tend to have a lower rate of kinetic energy dissipation, migrate at a higher velocity over a longer period of time, and have a larger migration distance. In addition, we define a relative kinetic energy dissipation rate |Dk_r| and find that it is positively correlated with |SFr| and |ORr|; the discrepancy in particle size and shape between the component materials is responsible for the difference in their kinetic energy dissipation rates, and it is the latter that determines their migration distance difference and sorting features.
Plain Language Summary
A scree slope is an inclined accumulation of the rock fragments and sand grains sourced from a weathered and failed cliff. Particle sorting is an interesting phenomenon observed in scree slopes, which refers to the special arrangement of particles in terms of particle size and shape along the longitudinal direction of slopes, and affects the overall motion of granular mass flows during the formation and failure of scree slopes. Through physical model tests and numerical simulations at a particulate scale, we attempt to understand the sorting phenomenon at a fundamental level. We found that the degree of sorting in a scree slope increases with the increasing discrepancy in particle size and shape between the component materials. We identified a positive correlation between the sorting index and the relative particle size and shape factors. We reveal that the difference in particle size and shape is associated with the difference in the dissipation rate of kinetic energy of particles, and the latter determines the difference in the migration distance of particles and the sorting feature. Understanding the sorting phenomenon is important for the evaluation of the influence of geophysical mass flows in scree slopes and for the prevention of related hazards.
Key Points
We conducted physical model tests and numerical simulations to study particle sorting in scree slopes from a micromechanical perspective
Relative particle size and shape factors are proposed to interpret the sorting feature described by a novel segregation index
The dissipation rate of kinetic energy is a vital index relating to the particle migration distance, which determines the sorting feature