Wave-induced incipient motion of non-buoyant plastic particles: Laboratory experiments

The incipient motion of nine non-buoyant plastic particles, having different shapes and densities, beneath surface waves is systematically investigated on both smooth and rough beds in a 2D flume. In each test, ten identical particles are placed on the bed, and the threshold for motion is defined when at least half of the particles move during each wave cycle. Experimental results are first compared with the classical Shields curve, originally developed for natural sediments under steady flow. To enhance predictive accuracy, the present dataset was combined with additional data sets from the literature acquired under steady flows, and the consolidated data were subjected to a systematic analysis. As has been established for steady flows, the effects of static friction and hiding-exposure need to be accounted for. However, it is found that these alone are not sufficient to achieve reconciliation with the classical Shields diagram for parameterizing incipient motion conditions. To ensure consistency with classical formulations based on steady flows, the shear velocity near the bed was estimated under wave forcing by applying a phase-averaging method. The peak near-bed velocity during the wave crest phase was extracted and used to compute the corresponding friction velocity. The novelty of this study is that an additional function, depending on the ratio of boundary layer thickness to particle size, has been incorporated to parameterize incipient motion beneath unsteady (oscillatory) wave-induced flows to account for the partial submergence of particles within the boundary layer. After accounting for this additional dependence, reconciliation with the Shields diagram is achieved, with remaining scatters being of the same order of magnitude for all the considered datasets. The proposed framework improves predictions of plastic debris mobility under both steady and wave-driven flow conditions.