Floating marine litter is transported by several mechanisms, including surface waves. In studies of marine litter transport, the wave-induced drift is set to be equal to the Stokes drift, corresponding to the Lagrangian-mean wave-induced drift of an infinitesimally small tracer. Large-scale experiments are used to show how the wave-induced drift of objects of finite size depends on their size, density, and shape. We observe increases in drift of 95% compared to Stokes drift for discs with diameters of 13% of the wavelength, up to 23% for spheres with diameters of 3% of the wavelength, whereas drift is reduced for objects that become submerged such as nets. We investigate what these findings may imply for the transport of plastic pollution in realistic wave conditions and we predict an increase in wave-induced drift for (very) large plastic pollution objects. The different extrapolation techniques we explore to make this prediction exhibit a large range of uncertainty.

The Ocean Cleanup Foundation is developing floating barrier systems to concentrate and extract buoyant plastics from the global accumulation regions located in the subtropical convergence zones in the world’s oceans. In this context, it is important to understand the transport and accumulation of these floating marine debris. Since marine litter transport is a multiscale problem, it is important to know the transport of these objects at their very own scale. For this end, two Discrete Particle Models have been implemented in the open-source two-phase Navier-Stokes flow solver Basilisk to study their wave-induced drift: i) using a simple linear interpolation and ii) using the high-order WENO interpolation scheme (Jiang and Wu [1999]) combined with reference frame intrinsic to the particle and its time-dependent submergence. It is shown that in longtime trajectory along the free surface, the two models differ drastically and the sophisticated one shows the best performance. A performance that is highlighted through a wave-induced drift factor defined as the ratio between the wavelength averaged plastic velocity and the wavelength averaged water particle Stokes drift.