Simulating microplastic dynamics in fluvial networks: A case study of the Irwell-Mersey river system.

Microplastics are increasingly recognised as persistent contaminants in fluvial sediments, yet the processes governing their storage and redistribution remain poorly constrained. Here, we use a numerical sediment-transport model to investigate the accumulation, mobilisation, and redistribution of microplastics in two contrasting rivers within the Irwell-Mersey catchment in Greater Manchester, United Kingdom. Simulations are initialised with a uniform sedimentary microplastic distribution and exclude ongoing external inputs, allowing the role of internal river processes to be isolated. Despite these simplifying assumptions, the model generates pronounced spatial heterogeneity in bed-sediment contamination and large event-driven reductions following extreme floods, consistent with patterns observed between 2015 and 2016. Model results demonstrate that high-flow events exert primary control on microplastic mobility, driving bed scour, downstream transfer, and reorganisation of particle concentrations. Particle density strongly modulates these responses: low-density microplastics are readily mobilised during moderate floods, whereas higher-density particles require larger discharges for significant entrainment. Differences between the Irwell and Upper Mersey reflect contrasts in hydraulic forcing rather than mean channel slope, with higher peak discharges and shear stresses in the Irwell promoting localised accumulation in mild reaches, and stronger slopes in the Upper Mersey favouring more effective flushing, with accumulation mainly in the lower reach. By explicitly resolving flow, sediment transport, and particle properties, the model provides a process-based explanation for observed spatial and temporal variability in sedimentary microplastic contamination. The results show how event magnitude, particle density, and channel morphology interact to control redistribution and retention, offering a quantitative framework for interpreting monitoring data and for assessing the sensitivity of sedimentary microplastic inventories to hydrological extremes.