Motion characteristics of floating debris under regular waves in nearshore shallow water

Floating marine debris poses growing risks in nearshore waters, where it threatens coastal ecosystems, obstructs navigation and fisheries, and can clog the cooling-water intakes of coastal and nuclear power plants. This study investigates the motion characteristics of floating debris under regular waves in a flat-bottom wave flume through physical model experiments. A systematic asymmetry in horizontal drift velocity was identified within each wave cycle: the slope k u = 0 , defined by connecting the two zero-velocity positions near the equilibrium level, is consistently positive across all tested conditions. This asymmetry originates from the horizontal drift component — hydrodynamic drag causes the horizontal displacement extrema of debris to lead those of water particles in phase, shortening both the first and third debris drift intervals. The slope k u = 0 correlates strongly with relative depth d / L 0 , which is identified as the dominant wave parameter governing drift asymmetry. Debris size has a notable effect on k u = 0 : larger dimensions produce steeper values, whereas the influence of density diminishes as size increases. Density primarily governs the per-cycle net drift, and its effect on size-sensitivity weakens as density approaches that of water. These results clarify the transport behavior of floating debris in nearshore regions and offer a parametric basis for improving predictive drift models.