Incipient motion conditions for 57 regular (spheres, cylinders, disks, square plates, cubes, square prisms, rectangular prisms, tetrahedrons, and fibers) and eight irregular microplastic particle groups, having various sizes and densities, are investigated in a circular flume. The present data set is combined with additional data from the literature and systematically analyzed. A new framework is developed for predicting incipient motion conditions for foreign particles, accounting for variations in static friction, hydraulic roughness, and hiding-exposure effects. Via this framework, incipient motion conditions for microplastic particles lying on a sediment bed are, for the first time, reconciled with the classical Shields diagram.
Keywords: Shields parameter; critical shear stress; hiding-exposure; hydraulic roughness; nonbuoyant; particle threshold.