Plastic litter has drastically increased since 1950s to around 359 million tons in 2018 (Plastic-the facts, 2019). Microplastic (MP) is an important part of plastic litter, which is present in marine waters and rivers. MP due to its small size (¡ 5 mm diameter) is more problematic than other plastic litter to the ecosystems and organisms. Therefore, extensive research is conducted to improve the understanding of MP effects, distribution, pathways, sinks and sources. However, quantitative studies on the MP-transport along rivers and estuaries are rare. Sediments are one of the major sinks of MP in rivers and marine waters. In addition, sediments could force the floating MP to settle and change their pathway and physical properties; especially when cohesive fine sediments at higher concentrations are involved (e.g. estuarine turbidity zone). MP found in bottom sediments, particularly fine MP of light polymers (e.g. PE, PP) pronounce the importance of further research on MP-sediment interaction. Above all, floating MP like PE and PP are the most frequently used polymers in everyday life, and therefore the main polymer-type of plastic litter in contact with suspended fine sediments. To understand the fate of MPs and associated transport mechanisms more accurately, a BMBF(Federal Ministry of Education and Research of Germany) supported research program about MP- contamination in the Weser river catchment and the German Wadden Sea (PLAWES) is currently under investigation. As a part of the PLAWES model chain, an Eulerian model approach for the overall estuarine MP-budget is set up and, among others, sediment-MP interaction is developed. All processes are implemented into a coupled hydro-morphodynamic model (using Delft3D) with water quality and ecology module (using Delft3D-WAQ). The presentation shows results of the numerical modelling of MP-transport with a focus on MP-fine sediment interaction and its effects on the fate of MP under estuarine dynamical conditions.