Post-flood macroplastic deposition in riparian vegetation and floodplains

Rivers store and transport large amounts of macroplastic and other macrolitter, and floods potentially drive their transport and deposition processes. Floods can have a major impact on the mobilization of plastic, yet factors determining the deposition remain unresolved. In this study, we sampled floodplains along two major Dutch rivers, following two flood events of different magnitude: the July 2021 extreme flood along the Meuse (> 100 year return period) and the January 2024 winter flood along the IJssel (3 year return period). Post-flood macroplastic and other macrolitter is found to locally accumulate in vegetation elements and debris piles, however it is not clear what this looks like at a river scale. We therefore decided to sample floodplains very detailed to analyze these deposition dynamics. We documented the specific location on the floodplain, and the element, e.g. debris pile or type of vegetation, in which each macrolitter item was found. Overall macrolitter accumulated mainly in vegetation elements along the Meuse river and within debris piles along the IJssel river. The average macrolitter concentration was lowest in grass (0.12 - 0.13 items/m²) and highest in debris piles (5.67 – 12.70 items/m²) for both rivers. There seem to be two ways macrolitter items are generally deposited on inundated riverbanks, above ground, and on the ground. Above ground, they can encounter an obstruction in their transport path and get entangled, e.g. in inundated tree branches, on the ground, they can be deposited where water and land surface meet, e.g. be left behind at the highest floodline. Macrolitter items deposited above ground in shrubs and trees, were larger compared to items deposited on the ground and in ground-covering vegetation. Macrolitter deposition following the flood in the Meuse showed distinct obstruction based deposition where the flood was most severe. Overall, along the Meuse, the items had a higher average mass and a higher mass concentration with 3.49 g/m², compared to the IJssel with 1.72 g/m². Deposition along the IJssel was debris pile dominated, with 69% of items deposited in debris < 2.5 cm. Debris piles along the IJssel constituted 2.2% of the sampled area but contained 58.3% of found macrolitter items and 32.3% of macrolitter mass. Identifying such hotspots and understanding how different floods drive the transport and deposition of macrolitter can contribute to guide improvements on monitoring and post-flood clean-up strategies, as well as prevention strategies, and impact assessment.