Onset of motion of compact-shaped microplastics in open-channel flows

Microplastics (MPs) can be found at the bed of rivers and streams, and riverine transport of MPs has still many open questions, regarding its dynamics. The present work aims at establishing a basis for the understanding of compact-shaped MP incipient motion and remobilization from river beds, simulating in a laboratory flume the onset of motion of plastic particles able to settle, and, therefore, are considered to move bedload alike. The main aims of the research are: (i) setup a method to define threshold conditions for incipient motion of microplastics on homogeneous bed, accounting for State-of-Art developments and tools, (ii) evaluate the effects of the bed composition on plastic particles’ remobilisation threshold, (iii) check if MPs surface concentration plays a role in the definition of the remobilisation threshold. Laboratory experiments were performed at the Hydrodynamics Model Laboratory of IG-PAS in Warsaw to quantify the threshold conditions for the chosen MPs, over different types of flat-beds: (i) homogeneous bed of microplastic grains, (ii) gravel bed, (iii) sand bed. To characterise the near-bed velocity field, Doppler velocimetry was used, while to estimate the plastic particles’ motion, an image analysis approach was developed and used. The results show that compact-shaped MPs, which are the closest to modelling natural sediments and have been widely used in the past to scale sediments in laboratory settings, get remobilised on natural beds at bed shear stresses lower than on homogeneous plastic beds. This depends, among others, on their shape, on their exposure on the bed and on their submergence. At the same time, it is concluded that the surface concentration does not play a significant role, at least in the range tested.