Marine plastic debris has become an established concern as a threat to marine and coastal ecosystems. Despite progress in understanding plastic transport dynamics under deep-water conditions, the characterisation of these processes in the nearshore environment remains incomplete. This poses significant challenges in their parametrisation, essential for the accurate representation of coastal transport dynamics in predictive models.

In this study, experimental measurements of the plastic particles wave-induced transport in intermediate to shallow water depths are presented. The focus is put on the influence of wave steepness as a key parameter affecting the transport of marine plastic debris in the transition from deep water to the shoreline. Its potential as a predictive parameter is investigated through controlled laboratory experiments involving the generation of seven regular breaking wave conditions, characterised by varying offshore steepness, propagating in shallow water depth over a sloped bathymetry.

The results reveal a consistent increase in particle drift speed with increasing offshore wave steepness. While the exact functional nature of the observed positive relationship could not be definitively concluded, the trend appears more likely linear than quadratic, aligning with previous findings for particles deviating from perfect tracers undergoing deep water breaking conditions. Furthermore, wave breaking was observed to play an important role in enhancing particle drift speed. Finally, particle drift speeds were consistently underestimated by the Stokes drift and only partially captured by the wave crest speed estimates, progressively diverging from the former and approaching the latter as offshore steepness increased, though remaining consistently lower than crest speeds. This trend was most recognisable in the breaking zone across all the tested wave conditions.

Overall, the findings suggest offshore wave steepness as a robust predictor for marine plastic debris transport in the nearshore environment, proving its value as a classification parameter for future modelling efforts. By investigating how plastic particles respond to changing wave conditions in the nearshore environment, this study aims to contribute to a better understanding of their dynamics.

Floating marine plastic debris has emerged as a major global environmental threat in recent years due to its persistence, long-distance transport, and harmful impacts on marine ecosystems. Understanding the key processes affecting plastic beaching is essential for accurately modelling plastic transport and predicting accumulation zones in nearshore marine environments. So far, research on plastic transport in shallow, nearshore waters is limited compared to deep ocean studies, resulting in significant uncertainties about wave-driven transport in these zones. While it is established that the impact of plastic density on the movement of floating plastic debris varies across wave zones, the dynamics in shallow water remain poorly understood. This research investigates how the density of finite-sized plastic particles influences the beaching dynamics under controlled, regular wave conditions in a laboratory flume simulating a nearshore environment using a sloped bathymetry. The densities relative to water of idealized spherical particles were systematically varied ranging from 0.09 to 0.93. Particles were released in the shoaling zone and tracked through the wave flume until beaching, allowing drift speeds to be analysed across different wave zones. It is observed that prior to breaking, in the shoaling zone, particles travel onshore with a speed close to the locally estimated Stokes drift regardless of the particles' relative density. In the breaking zone, density significantly affects particle drift speed: low-density particles accelerate strongly, nearing crest and phase speeds, while higher-density particles show only modest acceleration. Extending these findings to real-world coastal environments indicates that low density plastics tend to beach quickly, while denser particles remain suspended longer and thus may be affected more by lateral currents. While further research is needed to fully understand the role of density in plastic transport near the shore, this study clearly demonstrates that density significantly influences beaching dynamics—underscoring its importance in accurately modelling plastic transport in the nearshore environment.