Groundings of drifters in the Wadden Sea inform the transport of floating macroplastic

Abstract. We study floating marine macro plastic pollution with a dedicated campaign where we released and tracked 24 shallow surface drifters in the Dutch Wadden Sea, an intertidal region where water dynamics are driven by semi-diurnal tides and wind. The campaign captured water surface transport patterns, and around 150 observations of the drifters interacting with the coast and mudflats, where groundings of varying durations occurred. We identify these transitions in the recorded trajectories from both manual inspection and by formulating and evaluating a statistical approach for automatic detection. We then characterize the transitions by relating them to the coinciding wind conditions and tides, and with the duration of the groundings. The results of this analysis generally agree with the intuitions that wind towards the coast increases the frequency of interactions and that the drifters are rarely re-floated during falling tides. Furthermore, we find that groundings tend to be shorter duration when they occur shorter before the next high tide. We present multiple visualizations that characterize the groundings in more detail. The results provide a statistical characterization of coastal interactions that can be integrated in models and used for the planning of beach-cleanups. They may be improved by considering the specific environmental conditions of the campaign, and by analyzing the influence of the weight and size of the drifters, which are both larger than typical floating macroplastic.