Modified Stochastic Model for Settling and Rising Microplastic Transport in Open Channel Flows

Over the past two decades, microplastics (MPs) pollution has been recognized as a significant risk to public health and to a wide range of environments, particularly riverine, estuarine, and oceanic systems. However, much of the existing research on MPs has focused primarily on large-scale transport behavior in ocean zones using deterministic approaches. Consequently, many of the underlying fundamental principles governing the transport mechanisms of MPs and their fate in open channel flows remain poorly understood. Unlike sediments, which generally settle downward, MPs exhibit far greater variability in physical properties, including material composition, shape, size, drag, and density. Some MPs are even lighter than water, leading to upward or buoyant motion during transport and introducing additional complexity to the governing hydrodynamics.

To account for the geometric irregularity of particles, this study employs a stochastic diffusion particle tracking model (SD-PTM) that incorporates a modified vertical velocity formula to better represent the effects of inertial and viscous drag forces on MPs. In this model, the movement of suspended MPs is modeled as a stochastic process composed of a drift term and a random term, to represent particle transport in open channel flow. In addition, the genetic algorithm (GA) is applied to optimize the drag coefficients, thereby enhancing model robustness under data-limited conditions.

Compared with traditional models without consideration of MPs’ physical properties, the proposed modified stochastic model investigates not only the settling motion of MPs, but also extends, for the first time, stochastic modeling approaches to buoyant particles. The model results are compared with the experimental data provided by Born et al. (2023) across a range of flow conditions to calibrate the model coefficients. This study offers a new perspective on both rising and settling MP motion, thereby advancing the understanding of microplastic fate and transport in open channel flows.