Transport and capture of particulate matter driven by coherent flow structures from in-stream obstructions.

The interaction between particles and coherent flow structures in streams is a fundamental mechanism affecting various environmental processes over a wide range of scales. In this study, we identify flow characteristics that create hotspots of particles near submerged obstacles, and analyze relevant particle scales that determine particle motion driven by such flow conditions. We conducted experiments on a closed-loop racetrack flume using two particle types: 1) neutrally buoyant particles with 1 mm diameter and 2) negatively buoyant particles with 4.8 mm diameter. We used Particle Tracking Velocimetry (PTV) to identify particle trajectories and preferred locations relative to the obstructions, and Particle Image Velocimetry (PIV) to characterize mean and turbulent conditions driving particle retention or redirection. Analyses of various obstacle configurations (i.e., varying spacing between neighboring obstacles, submergence ratio, and flow velocities) elucidates how coherent flow structures of different scales affect particle capture and transport as a function of particle size and density. Results of the study will allow for the development of targeted sampling, monitoring, and management strategies for particulate matter in streams, such as microplastics, drifting invertebrates, as well as fish eggs and larvae.