Microplastic Retention Due to Gravel Obstacles in Combined Wave - Current Flows

Abstract

This paper presents experiments involving microplastic (MP) transport and retention by marine obstacles such as gravel in (non-breaking) irregular waves combined with currents. Seven different non-buoyant MP groups having different shapes, dimensions and relative densities in the range 1.06 - 1.36 are considered, both in the presence and absence of gravel patches having different coverage densities and lengths. The results show that the retention and transport behavior of MP particles are significantly influenced by both particle Dean number and shape. Particles with higher Dean numbers (i.e. lower settling velocities) have higher mobility and lower retention tendency. Moreover, increasing the gravel obstacle coverage density and length substantially enhances MP retention efficiency, particularly for particles with lower Dean numbers. In wave±current conditions, the retention efficiency was generally reduced compared to pure wave scenarios. In addition, extended gravel patches with high density effectively retained most particle groups. These findings emphasize the role of benthic structures in influencing MP accumulation and suggest that small-scale marine obstacles can act as significant MP sinks under certain hydrodynamic conditions.