Evaluating macroplastics transport thresholds for overland flow processes

Abstract

Macroplastic pollution is an emerging contaminant in urban landscapes, where it clogs storm drains, degrades water quality, harms wildlife, fragments into microplastics, and contributes to downstream pollutant loads. Yet the overland transport of macroplastics during storm events remains poorly quantified. This study investigates the impact of surface roughness, flow strength (stream power and shear stress), and plastic characteristics on both the initiation of movement and the extent to which macroplastics are transported across surfaces under overland flow processes. Controlled flume experiments were conducted with six bottle types across three roughness surfaces simulating roads, deteriorated pavement, and vegetated swales. Hydrodynamic equations were applied to calculate drag, friction, shear stress, and stream power, enabling mechanistic interpretation of transport dynamics. Results revealed a two-stage process, where plastics-initiated movement occurred at lower flows but required higher flows for complete transport. A nonlinear relationship emerged between flow strength and travel time, shaped by plastic-specific traits such as bulk density, shape, and cap orientation. For instance, low-density, streamlined bottles traveled longer distances under moderate discharges, while denser or bulkier bottles exhibited delayed or partial movement. These findings demonstrate that mass or discharge alone cannot predict macroplastic behavior. By quantifying transport thresholds, this study provides experimental benchmarks for contaminant fate models and highlights how urban roughness features can temporarily retain plastics until flushed by stronger flows. The results support stormwater monitoring and intervention strategies aimed at intercepting hazardous plastic debris before it reaches aquatic environments.