Continuous Near-Bed Movements of Microplastics in Open Channel Flows: Statistical Analysis

The ubiquitous distribution of microplastics (MPs) in aquatic environments is linked to their transport in rivers and streams. However, the specific mechanism of bedload microplastic (MP) transport, notably their stochastic behaviors, remains an underexplored area. To investigate this, particle tracking velocimetry was employed to examine the continuous near-bed movements of four types of MPs under nine setups with different experimental conditions in a laboratory flume, with an emphasis on their streamwise transport. It was found that the streamwise velocity of MPs follows a normal distribution, which can be characterized using the proposed equations to estimate the ensemble mean and standard deviation of MP streamwise velocity. The proposed equations show low relative errors of ∼5% when compared to experimental data. This study also revealed similarities in the continuous movement of MPs and sediments in the streamwise diffusion process. A superdiffusive regime was observed, with particle inertia identified as the primary source of this anomalous diffusion. These results indicate that adopting a probabilistic framework may provide a promising avenue for improving numerical models and enhancing the understanding of MP transport behavior.