Wave-induced transport of non-buoyant microplastic particles: Phase-resolved experiments and excess-Shields scaling

The present study investigates the bed-load dynamics of non-buoyant microplastic particles subjected to wave forcing, with emphasis on the phase-resolved relationship between particle motion and near-bed hydrodynamics. Specifically, five groups of PLA particles, differing in shape and density, were tested in a wave flume under five regular wave conditions representative of intermediate-depth coastal environments. Based on high quality video analysis, particle mobilisation occurred when the modified Shields parameter for plastics exceeded the incipient-motion threshold derived from previous work. The experiments highlighted a residual onshore drift that depends on the particle characteristics and increases with wave steepness, resulting from asymmetries in the near-bed velocity field. For the first time, a link has been established between the phase-resolved particle velocity, normalised by the near-bed flow velocity, and the ratio of the modified Shields number for plastics to the incipient motion threshold, revealing a clear non-linear dependence between particle mobility and the applied force.