Determination of Microplastics’ Vertical Concentration Transport (Rouse) Profiles in Flumes

The transport behavior of microplastics (MPs) in the fluvial environment is scarcely researched. Besides settling velocities and critical shear stress for erosion, only a few investigations aim at MPs’ vertical concentration profile and the underlying theory required. Therefore, this paper’s experiments investigate vertical concentration profiles of approximately spherical MP particles (d = 1–3 mm) with densities close to water (0.91–1.13 g/cm3) in flow channels, coupling them with fundamental theory for the first time. The experiments were conducted in a tiling flume (slope of 0–2.4%) at 67 and 80 mm water depth, with a turbulent flow, velocities ranging from 0.4 to 1.8 m/s, and turbulence kinetic energy from 0.002 to 0.08 m2/s2. The measured profiles confirm the assumption that the concentration profile shapes of settling plastics are similar to those of sediments and running reversed for buoyant plastics. Furthermore, the hypothesis of the Rouse formula’s applicability for floating and sinking plastics could be confirmed for approximately uniform flows. Future studies tying in with this research should increase particle properties and hydraulic parameter variation.