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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]

This paper presents numerical simulations of a bichromatic wave group propagating and breaking over a fixed breaker bar. The simulations are performed using a newly stabilized Reynolds-averaged Navier–Stokes (RANS) two-equation turbulence closure, which solves the longstanding problem of over-production of turbulence beneath surface waves in the nearly potential flow region prior to breaking. This model has previously been tested on small-scale spilling breaking regular waves, whereas in this work focus is on full (rather than model) scale application, wave groups (rather than regular waves) and plunging (rather than spilling) breakers. Additionally this paper has novel emphasis on bottom boundary layer dynamics which are very important for cross-shore sediment transport predictions. The model is validated by comparing with results from a previous experimental campaign. The model is shown to predict the surface elevations, velocities and turbulence well in the shoaling and outer surf-zone, avoiding turbulence over-production and incorrect undertow structure typical of standard turbulence closures. Comparison with detailed boundary layer measurements in the shoaling position reveals that the model is able to accurately capture the temporal dynamics of the entire wave boundary layer, including evolution of the boundary layer thickness, velocity overshoot and phase-shifts. Comparison in the surf zone additionally reveals that the model is able to accurately capture the transport of breaking-induced turbulence into the wave boundary layer. The performance of the model indicates that it can be used directly in the simulation of cross-shore sediment transport and morphology and also be used to study important hydrodynamic processes, which can help improve the predictive skill of morphodynamic profile models applied in coastal engineering. • Showed that the model handles propagation and breaking of bichromatic waves over a fixed breaker bar. • Demonstrated that only the stabilized model can predict the undertow profile going from shoaling to surf zone. • Demonstrated that stabilized model predicts well boundary layer characteristics. • Showed that the model accurately predicts breaking-induced turbulence entering the boundary layer. [ABSTRACT FROM AUTHOR]