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Trapping Efficiency of Non-Buoyant Microplastics by River Groynes
Summary
Flume experiments tested ten groyne configurations (river bank structures used for erosion control) for their ability to trap dense, non-floating microplastics, finding that all configurations retained at least 6% of particles and upstream-inclined groynes retained up to 20.7%. Rivers are major transport routes for microplastics moving from land to sea, and this study shows that existing river engineering structures can serve a secondary function as passive microplastic traps. Optimizing groyne design could offer a low-cost, passive strategy for intercepting microplastics before they reach the ocean.
Microplastics (MPs) have become an urgent environmental issue, driven by rapid industrial plastic use and inadequate waste management. As rivers act both as major transport pathways and temporary sinks, reducing MPs loads within them can effectively limit their transfer into the food chain. This study investigates the effectiveness of groynes, structures originally designed for flow and erosion control, in trapping non-buoyant MPs. Ten configurations formed from pairwise combinations of four common groyne geometries were tested using MPs (of equivalent diameter d_eq = 2.29 mm) of two densities (ρ_s = 1.08 and 1.11 g/cm³). Experiments were conducted in a recirculating flume using subcritical turbulent flow. A particle tracking model was employed to track and record particle trajectories around groyne fields, from which trapping efficiencies and retention zones were derived. It was observed that MPs entered the groyne field from the downstream end, forming a clockwise gyre. All scenarios retained > 6% of particles during the test period. When both groynes used in the experiments shared the same shape, upstream-facing inclined groynes exhibited the greatest trapping potential, achieving a peak retention rate of 20.7 ± 3.7% over extended durations. Despite their lower entrainment, straight and T-shaped groynes retained particles more effectively, with < 3% trapped particles leaving the field during the tested period. In cases where the upstream groyne was straight and the downstream geometry was varied, particle escape was negligible, with downstream repelling groynes consistently trapping more MPs than other configurations. Although particle density had little effect on particle entrainment, lighter particles showed lower escape and smaller retention zone, indicating MPs sensitivity to vertical flow dynamics within the groyne field.