Submergence ratio and spacing between in-stream obstructions determine capture and accumulation of drifting particles in rivers

• Flow penetration increases with gap length and submergence ratio, and converges once coherent flow structures emerge. • Particles follow coherent eddies to enter a gap, but enhanced turbulence increases particles’ deviation from flow and deter particle entry. • Increased particle size and density retard the particle response to flow, creating high particle concentration at low-velocity regions. This study investigates the effect of submergence ratio on the transport of exogenous particles in streams, targeting particles with density and diameter representative of microplastic and eggs of invasive species, which are of major concern in management of aquatic environments. Transport of two types of surrogate particles, with mean diameters of 1 and 4.8 mm and specific gravities of 1.00 and 1.0025, respectively, was assessed through experiments in a laboratory flume. Submerged obstacles with simplified geometries were mounted on the bed of a flume to represent in-stream obstructions. Image processing techniques, Particle Image Velocimetry (PIV) and Lagrangian Particle Tracking, were used to obtain flow velocity fields and particle trajectories. Angular momentum theorem was used to quantify the emergence of coherent eddies, which increase particle entry and timespans between submerged obstacles. Two indices are introduced: particle entry ratio and timespan of particles, which depend on particle characteristics, submergence ratio, and gap length. The study provides insights into the fundamental physics of particle transport, offering practical implications for aquatic debris and invasive species management, including effective monitoring locations and trap designs.