Laboratory Study of Non-buoyant Microplastic Transport Beneath Breaking Irregular Waves on a Live Sediment Bed

Wave flume experiments investigating the cross-shore transport and accumulation patterns of non-buoyant microplastic particles under irregular waves propagating, shoaling and breaking on a live sediment bed are considered. Eighteen microplastic particle groups having variable shape, density and size are tested. The experiments considered a pre-developed singly-barred profile, reasonably representative of field conditions. Four different microplastic accumulation hotspots are identified: (1) the offshore toe of the breaker bar, (2) at the breaker bar, (3) the plateau region between the breaker bar and beach, and (4) the beach. The accumulation patterns primarily fall within three different particle Dean number regimes (ratio of the characteristic wave height to the product of the settling velocity and characteristic wave period). For the parameter space tested, the dominant transport direction generally depends on the importance of offshore-driving gravitational effects and onshore-driving effects associated with nonlinear wave shapes. Particle position relative to the jet of plunging breaking waves likewise plays a key role in determining the net transport direction, especially near the breaker bar. These results help to improve understanding of the transport mechanisms and accumulation patterns of microplastic particles in e.g. nearshore coastal environments is required, as a prerequisite to any successful management or mitigation strategies.