A Shape‐Based Model for Drag and Terminal Velocity of Non‐Spherical Plastic Particles

Abstract Plastic pollution in rivers is governed by how individual particles move among the bed, the water column, and the free surface, processes that depend primarily on drag and terminal velocity. Although plastic particles span a much wider range of shapes than natural sediments, many existing drag and terminal‐velocity formulations still rely on natural‐sediment theory or introduce empirical correction factors calibrated to specific data sets. In this work, we develop a physically based framework to predict drag coefficients and terminal velocities of rigid, non‐spherical plastics using two shape‐sensitive length scales: the disk‐area equivalent diameter and a composite length that combines particle volume and projected area. Using these scales, we re‐analyse a large data set of settling and rising measurements for non‐spherical plastics, and show that their drag behavior collapses closely onto the standard drag curve for spheres. Based on this normalization, we propose a drag law in which the Stokes and Newton correction factors depend explicitly on particle shape, providing a compact description of drag across the Stokes, transitional, and Newton regimes. Based on dimensional analysis, we also derive a simplified relation for the terminal velocity that smoothly bridges viscous and inertial limits. Comparisons with widely used non‐spherical drag laws and plastic‐specific settling formulas show that our framework reduces the scatter in both drag and terminal velocity. Taken together, our developments provide a physically interpretable description of non‐spherical particle drag that extends beyond fluvial transport simulations.