Effects of aspect ratio on the incipient motion of plastics in open-channel flows

Abstract: Over the last decade, plastic pollution in river systems has been identified as a major threat to flora and fauna, as well as to human health. As such, the adequate description of plastic transport processes is of great importance, but there is a knowledge gap in our understanding of the underpinning transport mechanics. This paper studies the influence of the aspect ratio of cylindrical plastic particles on their incipient motion through experiments performed in an open-channel flume with a smooth bed configuration. Our results provide insights into shape-dependent onset conditions of plastic motion, which have not been accounted for previously. We define the critical plastic Shields parameter, which can be interpreted as a dimensionless bed shear stress, to describe the interaction among the bed, flow, and plastic particles, further considering shape effects, plastic drag and lift coefficients, as well as the friction coefficient between the plastic particle and the bed. By maintaining a smooth bed as the boundary condition, the experiments focus on the effects of particle's aspect ratio on the critical onset conditions. The experiments adopt a generalized framework to determine the plastic Shields parameters, and our results show, that, for the case of incipient motion due to the sliding of the particle, the critical velocity for incipient motion can be described by an analytical solution, derived from a first principle approach. Keywords: Plastic Shields parameter, Incipient motion, Riverine plastic transport