Slightly Negative Buoyant Particle Settling under Breaking Wave

Vertical transport of microplastics is controlled by rapidly decaying turbulence produced by breaking waves. This experimental work investigates 0.7 -1.3 mm spherical plastic particles with specific gravity of 1.03 settling under a two-dimensional plunging breaker in a 6 m wave flume. Particle Image Velocimetry (PIV) is utilized to characterize the spatiotemporal evolution of the breaker-generated turbulence, while Particle Tracking Velocimetry (PTV) quantifies the settling trajectories. The results show immediately after impact; the flow contains both a strong orbital roller embedded with patches of rapidly decaying turbulence. The background turbulence energy dissipation rate is ~1 m²s^-3 shortly after wave breaking and 10^-3 m²s^-3 60 s later. Across this decay, the millimeter-scale particles settle at only ~30 % of the corresponding quiescent terminal speed presumably due to the combined effects of the coherent large-scale motion and decaying turbulence. At early times, the settling is affected by the large-scale vortical motion, effectively “saltating” and inhibiting the net settling. As the flow homogenizes, particle settling correlates strongly with turbulence level indicating preferential sweeping and turbulence-induced drag fluctuations play a role. These findings show how roller and decaying turbulence jointly regulate particle settling. Incorporating such regime-based settling corrections will improve predictions of microplastic residence time and benthic deposition in wave-impacted zones.