Particle clustering and dispersion in multi-scale Langmuir turbulence and regional flow convergence.

Concentrations of pollutants, such as microplastics, can be used to predict exposure and assess risk to organisms. In estuaries, the clustering and dispersion of materials is influenced by flows ranging from small-scale wind, wave, and tidal turbulence to regional-scale circulations. Axial convergence, paired regional-scale roll cells formed via cross-estuary density gradients from tidal jets in channel estuaries, collect surface particles in convergent bands over length scales of O(100 m-10 km). Turbulent roll cells, such as Langmuir cells, instead collect surface and bottom particles over scales of O(10-100 m). Here, the dispersion and clustering of suspended particles along-estuary and cross-estuary in an estuary with multi-scale turbulent-axial convergent flow is examined to diagnose the combined influence on particle concentrations and pollution risks. A turbulence-resolving shallow-water large eddy simulation is modified with cross-estuary varying tidal flow and along-estuary density gradients to drive axial roll cells. Wind stress and Stokes drift drive shear and Langmuir turbulence (LT). In axially converging estuaries, tidal jet shear dominates particle dispersion, acting most efficiently among neutrally buoyant particles; dispersion of buoyantly rising particles is enhanced by vertical mixing from LT. With LT, particle clouds show increased clustering compared to model runs without LT, dividing into more numerous, narrower, yet denser band-like clusters within windrows. Langmuir turbulence during slack tides alters the regional dispersion and clustering of particles by axial convergence.