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

Abstract

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.