Eulerian–Lagrangian simulation of wave-induced microplastic dispersion in nearshore zones : processes and implications

Abstract

Microplastics are a pervasive marine pollutant, threatening coastal ecosystems and biodiversity. Understanding nearshore transport and dispersion is crucial for predicting microplastics’ fate and mitigating impacts. This study employs an Eulerian–Lagrangian model to simulate spherical microplastics, accounting for inertia and buoyancy, under regular and irregular waves in the nearshore region. Using realistic size–density distributions, the simulated particles range from non-buoyant to buoyant and from weakly to highly inertial. The shear layer, formed by Stokes drift and undertow, drives buoyant microplastics shoreward, controlling non-buoyant trajectories. The breaking region acts as a natural barrier, with few low-density, inertial particles passing through and potentially settling above the swash zone, while non-passing particles accumulate in the shoaling zone. Particle–wave characteristics are linked through dimensionless parameters, and we propose applicable dimensionless diffusive coefficients of K<inf>h</inf>(1)T/L<inf>0</inf>2≈O(10−4) and K<inf>v</inf>(1)T/L<inf>0</inf>2≈O(10−5). Dispersion regimes transit from initial ballistic to subdiffusive within 10 wave periods. The subsequent superdiffusive regime is governed primarily by wave steepness and weakly by depth, particle density, and size. However, the absence of a sustained diffusive regime indicates diffusion coefficients may misestimate nearshore microplastic concentrations.