Structure-Induced Enhancement of Oxygen Penetration in Coarsened Sediment Beds: Insights from Large-Eddy Simulations

This study employed large-eddy simulations (LES) to investigate how local bed coarsening influence near-bed vertical velocity perturbations and scalar transport. Six cases were configured with varying degrees of sediment coarsening at a fixed Reynolds number of 10,000. Coarsening was quantified by the coverage ratio (A_c/A_t) of coarse particles (A_c) on the bed surface (A_t), ranging from 0% to 100%. To isolate the effects of heterogeneous permeability, the crest elevations of both non-coarsened (d) and coarsened particles (D, where d = 0.5D) were kept equal, effectively eliminating variations in bed elevation.

Results show that A_c/A_t = 64% induced the strongest perturbations: (i) Sediment coarsening enhances near-bed vertical velocity and turbulence, with increases of 10.0 and 3.0 times at 64%, and 3.5 and 1.5 times under full coarsening, relative to the non‑coarsened case. (ii) Bed coarsening strengthens downward advective and turbulent fluxes, peaking at 14.1 and 1.7 times the non‑coarsened values at 64%, and remaining elevated at 11.6 and 1.4 times under full coarsening; (iii) Coarsening increases scalar penetration, shortens residence time (RT), and enhances transfer coefficients on both water and sediment sides. Under non- and fully coarsened beds, penetration depths are limited to d and D, respectively, while partial coarsening (A_c/A_t = 16–64%) allows penetration to the bed bottom. RT drops from 4.49 s at 0% to 4.21 s at 64%, then slightly rises to 4.24 s under full coarsening. At A_c/A_t = 64%, transfer coefficients rise to 2.4 times (water side) and 1.8 times (sediment side) those of the non-coarsened case, and to 1.6 and 1.4 times under full coarsening.

The primary mechanism driving the intensification of vertical scalar transport is the enhancement of vertical instantaneous velocities, which subsequently leads to increased advective and turbulent fluxes. Consequently, near-bed scalar concentrations increased by 37.1% at A_c/A_t = 64% and by 65.4% under full coarsening compared to the non-coarsened case. The results offer new insights into how bed heterogeneity influences hyporheic exchange, biogeochemical coupling, and solute retention in permeable sediments.