A “sediment transport” perspective on microplastics movement: incipient motion

, Varrani Arianna.

Sediment transport in freshwater environments is generally divided into two modes: suspended load and bedload. In our anthropized rivers, both modes can include natural sediments (like sand and gravel) and artificial particles (such as macro- and microplastics-MP). The understanding of how these particles behave in water needs experimental evidence of their transport dynamics, which may take the form of bedload, especially for those MP which have a specific density higher than water. For this reason, mobile-bed laboratory experiments have been performed in a recirculating flume to evaluate the hydrodynamics conditions, namely the discharge, leading to initiation of motion (i.e. incipient motion) of single bed grains. The bed layer was composed by MP grains with a diameter around 3 mm. In sediment transport studies, the definition of incipient motion conditions is a knotty point since the fundamental studies made by Shields in 1936, which identified the threshold conditions for the transport of natural materials. The preliminary results show that, for PBT ellipsoidal particles and PVC cylindrical grains, the threshold for motion is comparable with lighter materials investigated by Shields. To confirm these initial findings, more laboratory investigations are needed, varying the flow conditions, the MP material and the bed composition.

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