Dune morphology control on redox conditions within the hyporheic zone

River morphology has a direct impact on stream nutrient removal and transport because of its influence on the hyporheic hydraulics. Microbially mediated transformations such as nitrification and denitrification processes that occur within the streambed sediment depend on the amount of reactants transported into the sediment. The interaction between stream flow and streambed topography regulates the hyporheic fluxes and thus affects the biogeochemical processes occurring in the sediment. Here we investigate the control by dune-like bed forms and flow on prevailing aerobic and anaerobic conditions within the streambed hyporheic zone. We apply a new Damköhler number, Da, which is defined as the ratio between the median hyporheic residence time and the time to consume dissolved oxygen to a prescribed threshold concentration below which anaerobic conditions are present, i.e. Da = τup,50/τlim. Values of Da larger than 1 indicate prevailing anaerobic conditions, whereas values smaller than 1 prevailing aerobic conditions, within the hyporheic sediment. Our results show a direct relationship between dune shape, ratio between dune amplitude and wavelength, streambed depth and flow velocity on Da and consequently on the volume of the hyporheic sediment in aerobic and anaerobic conditions.