Influence of Wave Steepness on the Transport of Marine Plastic Debris in the Nearshore Environment: Insights from Laboratory Flume Experiments
C. Cocozza (TU Delft - Civil Engineering & Geosciences)
RW Hut – Mentor (TU Delft - Water Systems Monitoring & Modelling)
A.W. Baar – Mentor (TU Delft - Surface and Groundwater Hydrology)
Ton van den Bremer – Mentor (TU Delft - Environmental Fluid Mechanics)
Erik van Sebille – Graduation committee member (Imperial College London)
Marc Schneider – Graduation committee member (Universiteit Utrecht)
MA de Schipper – Graduation committee member (TU Delft - Coastal Engineering)
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Abstract
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.