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This paper presents experimental measurements of beaching times for buoyant microplastic particles released, both in the pre-breaking region and within the surf zone. The beaching times are used to quantify cross-shore Lagrangian transport velocities of the microplastics. Prior to breaking the particles travel onshore with a velocity close to the Lagrangian fluid particle velocity, regardless of particle characteristics. In the surf zone the Lagrangian velocities of the microplastics increase and become closer to the wave celerity. Furthermore, it is demonstrated that particles having low Dean numbers (dimensionless fall velocity) are transported at higher mean velocities, as they have a larger tendency to be at the free-surface relative to particles with higher Dean numbers. An empirical relation is formulated for predicting the cross-shore Lagrangian transport velocities of buoyant microplastic particles, valid for both non-breaking and breaking irregular waves. The expression matches the present experiments well, in addition to two prior studies. [Display omitted] • Experiments show that buoyant microplastic particles are transported onshore by waves until they are eventually beached. • Prior to breaking the microplastic transport velocity is near that of the fluid, regardless of particle characteristics. • In the surf zone the Lagrangian particle transport velocities increase, becoming closer to the breaking wave celerity. • Beneath breaking waves particles with high (low) rise velocities have larger (smaller) Lagrangian transport velocities. • An equation predicting wave-induced transport velocity of buoyant microplastics across the coastal profile is provided. [ABSTRACT FROM AUTHOR]

This paper presents experimental results on the cross-shore distribution of non-buoyant microplastic particles under irregular waves propagating, shoaling and breaking on live sediment sloping beds. Eighteen microplastic particle groups having various shapes, densities, and sizes are tested. The experiments consider two initial bottom configurations corresponding to a (i) plane bed and (ii) pre-developed singly-barred profile (more representative of field conditions). Four different microplastic accumulation hotspots are identified: offshore of the breaker bar, at the breaker bar, the plateau region between the breaker bar and beach, and the beach. It is found that the accumulation patterns primarily fall within three different particle Dean number regimes. The importance of plunger-type breaking waves for both on and offshore transport of microplastic particles is highlighted. [Display omitted] • The cross-shore transport of non-buoyant microplastic particles with various shapes, densities and sizes is investigated. • Irregular waves and two distinct initial bottom configurations are considered. • Transport and accumulation processes of microplastics are studied for O(100 hr) in model scale. • Four accumulation hotspots for microplastic particles are identified, falling primarily within three Dean number regimes. [ABSTRACT FROM AUTHOR]