Dynamics of microplastics bedload transport in turbulent open channel flows over smooth and rough beds

Microplastics (MPs) in aquatic environments pose a growing ecological threat, yet their bedload transport dynamics remain poorly quantified. This study introduces a novel formulation to predict the bedload layer thickness δ_MP and average bedload transport velocity u_MP of MPs under turbulent open-channel flows. Using 80 controlled flume experiments covering a wide range of microplastic types, bed roughness configurations, and flow conditions, further supported by data from existing literature, new empirical models are developed. The proposed new empirical formula for δ_MP based on the Shields number and relative density, reduces the prediction error by 75 % compared to the existing sediment-based formulas. Additionally, to explicitly quantify the u_MP in the bedload layer, an empirical formula incorporating the relative roughness has been established, with a 14.05 % error, substantially outperforming the existing approaches. These findings demonstrate the critical role of particle density and bed characteristics in shaping the MP bedload transport dynamics and offer a robust framework for improving MPs transport models in riverine systems.