Density-Driven Effects on Marine Plastic Beaching: Observations from Laboratory Flume Experiments
L.M.J. Swuste (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
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.