Transport and retention characteristics of floating debris in the Han River Estuary

The Han River estuary is subject to strong tidal transformations due to its complex topography and shallow depth, making it a potential hotspot for the retention of floating marine debris. However, research in this region remains limited due to restricted access as a designated military protection zone. This study quantitatively analyzed the movement, retention, and seaward exit of GPS-tracked drifters in response to seasonal variations in freshwater discharge and tides in the coastal waters near Ganghwa Island. A total of 49 drifters were deployed at three stations over a six-month period, and their trajectories were assessed in relation to changes in discharge and tidal conditions. Most drifters remained confined within the narrow channels near Ganghwa for over a month due to tidal influence and repeated grounding-refloating cycles. Only about 12 % exited the estuary, primarily during periods of high freshwater discharge (top 5 %), through the northern Ganghwa channel. During lower discharge periods, drifters tended to stay near intertidal zones or became immobilized after grounding. Drifter movement was most active during spring tides or during transitions to neap tides, with current velocities nearly twice as high during spring tides. Notably, seaward exit also occurred under neap tide conditions when intertidal exposure was reduced. These findings suggest that the transport and retention of marine debris are governed by the combined effects of freshwater inflow, tidal cycles, and geomorphology. The results provide a scientific basis for modeling debris dynamics and inform environmental management strategies for the Han River estuary and surrounding areas.