Laboratory Measurements of the Wave‐Induced Motion of Plastic Particles: Influence of Wave Period, Plastic Size and Plastic Density

The transport of plastic particles from inland sources to the oceans garbage patches occurs trough coastal regions where the transport processes depend highly on wave‐induced motions. In this study, experimental measurements of the plastic particles wave‐induced Lagrangian drift in intermediate water depth are presented investigating the influence of the wave conditions, particle size and density on the motion of relatively large plastic particles. A large influence of the particle density is observed causing particles to float or sink for relative densities lower and larger than water respectively. The measured net drift of the floating particles correlates well with theoretical solutions for particle Stokes drift, where the net drift is proportional to the square of the wave steepness. Floating particles remain at the free water surface because of buoyancy and no evidence of any other influence of particle inertia on the net drift is observed. Nonfloating particles move close to the bed with lower velocity magnitudes than the floating particles’ motion at the free surface. The drift of nonfloating particles reduces with decreasing wave number, and therefore wave steepness. Marine plastic pollution attracts significant attention from scientists and the general public. Most focus is on the floating plastic that accumulates in the centers of the ocean basin. However, for plastic originating from land to end up in these open‐ocean regions, it first needs to be transported through the coastal zone. Little is known how coastal waves transport plastic. Here, we use a 16 m‐long wave‐flume to measure how waves transport both floating and nonfloating plastic particles in the laboratory. The floating particles move with the waves as predicted by the so‐called Stokes drift, while nonfloating particles feel the effect of the bottom shear. These results improve our understanding of how plastics move from the coast to the open ocean and vice versa, thereby supporting improvements in our modeling capacity of the transport of marine plastic litter. A 16 m wave flume is used to measure wave transport of floating and non‐floating plasticFloating plastic transport correlates well with Stokes drift and buoyancy is the only non‐inertial effectNon‐floating particles experience shear forces. Transport varies with particle size and density A 16 m wave flume is used to measure wave transport of floating and non‐floating plastic Floating plastic transport correlates well with Stokes drift and buoyancy is the only non‐inertial effect Non‐floating particles experience shear forces. Transport varies with particle size and density