Investigation of the Sheltering Effects on the Mobilization of Microplastics in Open-Channel Flow

Microplastics (MPs) can be sheltered by bed grains, resultingina higher threshold of mobilization compared with those moving on asmooth bed. Only one formula considering the sheltering effects hasbeen proposed for the critical shear stress (& tau; ( c )) of MP mobilization by including thedensities and sizes of MPs and bed grains, but it is inaccurate forMPs of certain materials such as PS and shows limitations in understandingthe sheltering effects. Additionally, no method exists for the criticaldepth-averaged velocity (U ( c )). In this study, experiments were conducted measuring both & tau; ( c ) and U ( c ) of MPs (made of PA, PVC, and PET) depositedon rough beds of different roughnesses, with a focus on the shelteringeffects. A power law relationship between & tau; ( c ) and the density and size of MPs wasfound, and a new formula for estimating & tau; ( c ) was proposed for MPs, which reduced theerrors by approximately 40% compared with that of the previous formula(27.8%). To explicitly quantify the sheltering effects in MP mobilizations,hiddenness (& UDelta;Z), exposure (& UDelta;H), and longitudinal exposure (& UDelta;X) were introduced,all of which were observed to follow a normal distribution. A semiempiricalmethod for U ( c ) was thenproposed incorporating the new measures, which reduced the errorsby around 70% compared to the methods developed for estimating the U ( c ) values of sediments. Previous research on the flow thresholdfor MP mobilizationshas been limited. This study presents a novel formula for quantifyingthe sheltering effects and proposes two new prediction methods forthe critical flow threshold of MPs.