Ripple Effects: Bed Form Morphodynamics Cascading Into Hyporheic Zone Biogeochemistry

The water quality and ecosystem health of river corridors depend on the biogeochemical processes occurring in the hyporheic zones (HZs) of the beds and banks of rivers. HZs in riverbeds often form because of bed forms. Despite widespread and persistent variation in river flow, how the discharge‐ and grain size‐dependent geometry of bed forms and how bed form migration collectively and systematically affects hyporheic exchange flux, solute transport, and biogeochemical reaction rates are unknown. We investigated these linked processes through morphodynamically consistent multiphysics numerical simulation experiments. Several realistic ripple geometries based on bed form stability criteria using mean river flow velocity and median sediment grain size were designed. Ripple migration rates were estimated based primarily on the river velocity. The ripple geometries and migration rates were used to drive hyporheic flow and reactive transport models which quantified HZ nitrogen transformation. Results from fixed bed form simulations were compared with matching migrating bed form scenarios. We found that the turnover exchange due to ripple migration has a large impact on reactant supply and reaction rates. The nitrate removal efficiency increased asymptotically with Damköhler number for both mobile and immobile ripples, but the immobile ripple always had a higher nitrate removal efficiency. Since moving ripples remove less nitrogen, and may even be net nitrifying at times, consideration for bed form morphodynamics may therefore lead to reduction of model‐based estimates of denitrification. The connection between nitrate removal efficiency and Damköhler number can be integrated into frameworks for quantifying transient, network‐scale, HZ nitrate dynamics. Plain Language Summary Sandy riverbeds are very rarely flat. They are typically covered by ripples and dunes. Because of their topography, these ripples and dunes drive variations in water pressure across their surfaces due to deflection, acceleration, and deceleration of the river flow. These pressure variations drive river water to infiltrate into the porous and permeable sediment where pressure is high and exit from the sediment where it is low. This pressure‐driven flow, called hyporheic exchange, is critical to the water quality of rivers since it allows river water to undergo biogeochemical reactions that take place within the sediment. Ripples are highly dynamic however and respond readily to changes in riv