Stratification-Induced Porosity Variations in Granular Packings—Part II: A Step Closer to Natural Sediments via DEM

This study investigates the influence of stratification—the vertical layering of particles with different sizes—on porosity in granular sediment packings. Conventional porosity models are typically formulated for homogeneous, well-mixed grain assemblies; however, natural riverbed sediments often exhibit stratification, leading to deviations from these idealized conditions. Part I established empirical relationships describing transition layer geometry and porosity in systems composed of low-friction glass beads. Building on this foundation, Part II extends the analysis by incorporating the higher inter-particle friction characteristic of natural sediments, using discrete element method (DEM) simulations to quantify its effect on packing structure and porosity. A refined method is used to extract porosity and density distributions from simulated packings, enabling accurate identification of transition layers. Empirical formulas are developed to predict key transition-layer parameters (thickness, average porosity, and minimum porosity) as functions of the grain-size ratio. A density-based porosity prediction model is introduced and coupled with an existing model for well-mixed sediments, allowing for a quantitative comparison between stratified and homogeneous packing scenarios. Results show that stratification can increase porosity by 44–57% relative to well-mixed samples of an identical grain-size composition. These findings highlight the importance of considering sediment stratification when modeling riverbed porosity and pave the way for improved sediment transport and hydraulic predictions.