Microplastics (MPs, ≤ 5 mm) exhibit diverse settling behaviors in aquatic environments, influenced by their distinct physical and chemical properties. Though there has been a growing interest in investigating the settling velocity of MPs in still water, none of these investigations have taken into account the hydrodynamics of MPs in turbulent flow conditions. This study investigated the settling velocity of different sizes and types of MPs in a turbulent open channel flume under different flow conditions. Spherical-shaped MPs of three different materials were analyzed: polystyrene, cellulose acetate, and acrylic, with diameters ranging from 1 mm to 5 mm. A particle tracking technique was employed to record and analyze the MPs motion within the ambient flow while an acoustic Doppler velocimeter (ADV) was used to measure the velocity time series. For a total of 20 scenarios, the mean settling velocity of MPs was found to range from 0.026 to 0.157 m/s which is lower than the settling velocity calculated in quiescent water. In this study, a drag model was proposed to estimate the settling movement of MPs as well as the trajectories of MPs in the riverine system. Understanding the mechanism of MPs settling behaviors in the turbulent flow and the observed data, and relationships of MPs settlement from this study contributes to the development and validation of a 1D or 2D to characterize and quantify microplastics in a riverine.
