Synthetic microfibers driven by turbidity currents: Transition from smooth bed to macro-roughness

Recent research confirmed the significant role turbidity currents play in transporting microplastics (MPs) over long distances in aquatic environments. However, only a limited number of studies have specifically addressed the transport dynamics of microfibers (MFs). These synthetic fibers present unique challenges due to their distinctive physical characteristics, such as high aspect ratios, flexibility, and low densities compared to natural sediments, which influence their behavior, settling patterns, and environmental fate. Using lock-exchange flume experiments, this work examines how MFs of differing shapes and sizes are transported by turbidity currents traveling over different bedforms. Experiments were conducted in a flume under a wide range of bed layer conditions (from micro to macro-roughness) and water depths, to assess how these factors influence MF transport and deposition. Results indicate that both bed roughness and water depth significantly affect the inertial velocity of turbidity currents, thereby affecting the accumulation of suspended MFs as they propagate along the flume. Despite their tendency to settle, MFs were transported over greater distances within the flow, likely due to their alignment with the streamlines, particularly in the case of smaller diameters. For the first time, a non-dimensional model was developed to predict MF transport as a function of bed granulometry, MF settling velocity, water column height, and MF shape (characterized by the Corey Shape Factor). The findings provide valuable insights into the complex behavior of MFs in natural environments and lay the groundwork for further research on the fate of plastic pollution in aquatic systems.