Modeling the impact of fine-scale sediments on the hydrodynamics and chemical reactions at streambed using OpenFOAM

Hydrological exchange flows (HEFs) occurring at streambeds are the primary control of water quality, nutrient dynamics, and ecosystem health in dynamic river corridor systems. Previous work suggests that a few centimeters of sediments above the riverbed may dominate the overall exchange fluxes and significantly enhance riverbed chemical reactions. However, this assumption has not been examined due to limited measurements of HEFs at actual riverbeds. To this end, we present results of how fine-scale sediments (mm – cm) affect the dynamic pressure, bed shear stress, the mass exchange, and concentration fluxes at the riverbed using the hyporheicFoam, a newly developed surface-subsurface fully coupled model in OpenFOAM, and synthetic riverbeds with realistic fine-scale sediment geometric structure. Specifically, the hyporheicFoam solves the Reynolds averaged Naiver-Stokes equation for surface flow and Richards equation for subsurface flow. Chemical reactions are solved for both surface and subsurface flows. The coupling between surface water and subsurface water is realized through direct mapping of mass flux and concentration flux at the riverbed. Four synthetic riverbeds are generated with the first three used for the hyporheicFoam and the last one used for a single-phase solver in OpenFOAM. The first three riverbeds are created as follows: a realistic roughness field is firstly measured using the structure-from-motion photogrammetry, which is used to represent the mm-cm scale sediment structure in natural rivers. Three synthetic riverbeds are then generated to represent typical river spanwise shapes such as rectangular, V, and M shapes as shown in a LiDAR measured river bathymetry. Finally, we incorporate the measured roughness field to the synthetic riverbeds to create three synthetic riverbeds with both realistic fine-scale sediment structure and spanwise geometric features. The fourth riverbed is created by adding a layer of randomly packed cobbles/pebbles/boulders beneath a flat riverbed to further illustrate the impact of subsurface pore-structure on the riverbed hydrodynamics and chemical reactions.