Microplastic retention in marine vegetation canopies under breaking irregular waves.

The present study provides indications and underlying drivers of wave-induced transport and retention potential of microplastic particles (MP) in marine vegetation canopies having different densities. The anthropogenic occurrence of MP in coastal waters is well documented in the recent literature. It is acknowledged that coastal vegetation can serve as a sink for MP due to its energy dissipating features, which can mimic a novel ecosystem service. While the transport behavior of MP in vegetation has previously been investigated to some extent for stationary flow conditions, fundamental investigations for unsteady surf zone flow conditions under irregular waves are still lacking. Herein, we demonstrate by means of hydraulic model tests that a vegetation's retention potential of MP in waves increases with the vegetation shoot density, the MP settling velocity and decreasing wave energy. It is found that particles migrating by traction (predominantly in contact with the bed) are trapped in the wake regions around a canopy, whereas suspended particles are able to pass vegetated areas more easily. Very dense canopies can also promote the passage of MP with diameters larger than the plant spacing, as the canopies then show characteristics of a solid sill and avoid particle penetration. The particle migration ability through a marine vegetation canopy is quantified, and the key drivers are described by an empirical expression based on the particle settling velocity, the canopy length and density. The findings of this study may contribute to improved prediction and assessment of MP accumulation hotspots in vegetated coastal areas and, thus, may help in tracing MP sinks. Such knowledge can be considered a prerequisite to develope methods or new technologies to recover plastic pollutants and rehabilitate valuable coastal environments.

Shields Diagram and the Incipient Motion of Microplastic Particles.

Incipient motion conditions for 57 regular (spheres, cylinders, disks, square plates, cubes, square prisms, rectangular prisms, tetrahedrons, and fibers) and eight irregular microplastic particle groups, having various sizes and densities, are investigated in a circular flume. The present data set is combined with additional data from the literature and systematically analyzed. A new framework is developed for predicting incipient motion conditions for foreign particles, accounting for variations in static friction, hydraulic roughness, and hiding-exposure effects. Via this framework, incipient motion conditions for microplastic particles lying on a sediment bed are, for the first time, reconciled with the classical Shields diagram.

Settling velocity of microplastic particles having regular and irregular shapes.

The settling velocities of 66 microplastic particle groups, having both regular (58) and irregular (eight) shapes, are measured experimentally. Regular shapes considered include: spheres, cylinders, disks, square plates, cubes, other cuboids (square and rectangular prisms), tetrahedrons, and fibers. The experiments generally consider Reynolds numbers greater than 102, extending the predominant range covered by previous studies. The present data is combined with an extensive data set from the literature, and the settling velocities are systematically analyzed on a shape-by-shape basis. Novel parameterizations and predictive drag coefficient formulations are developed for both regular and irregular particle shapes, properly accounting for preferential settling orientation. These are shown to be more accurate than the best existing predictive formulation from the literature. The developed method for predicting the settling velocity of irregularly-shaped microplastic particles is demonstrated to be equally well suited for natural sediments in the Appendix.

Experimental investigation on the nearshore transport of buoyant microplastic particles.

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.

Experimental study of non-buoyant microplastic transport beneath breaking irregular waves on a live sediment bed.

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.