Pore-resolved direct numerical simulations of hyporheic exchange induced by bedforms and bed roughness

In aquatic environments, bedform features of the sediment induce exchange of water and solutes across the interface, which play a significant role in controlling biogeochemical processes. Our recent pore-resolved simulations of flat-bed hyporheic exchange revealed that, grain-scale bed roughness leads to significant volumetric flux into the sediment and deep subsurface flow paths that yield heavy-tail residence time distributions similar to those observed in the presence of bedforms. Here, we explore the synergistic effects of bed roughness and bedforms on the exchange flux, based on direct numerical simulations of turbulent open-channels at a friction Reynolds number of 1580 over a sand bed with immobile porous dunes. Two different arrangements of the uppermost-layer sediment grains at the interface are evaluated: regular and random. Results show that bed roughness plays an important role in affecting time-mean pressure and velocity patterns both above and below the interface. Substantially longer residence times are observed for the regular roughness than the random, while the random roughness results in stronger mixing at the interface and larger volumetric fluxes. Results suggest that the effect of small-scale bed features on the hyporheic exchange cannot be ignored.