Effect of Surface Hydraulics and Salmon Redd Size on Redd-Induced Hyporheic Exchange

Salmonids create an egg nest, called a redd, in hyporheic streambed gravel to bury their eggs, characterized by a pit and a hump topography, resembling a dune. Embryos' survival depends on downwelling oxygen-rich stream water fluxes, influenced by the redd shape, stream hydraulics, and the hydraulic conductivity of the redd sediment, K. We hypothesize that downwelling fluxes increase with stream discharge and redd aspect ratio (A(R) = A/L, with A, the redd amplitude and L, its length), and they can be predicted using a set of dimensionless numbers, which include the stream flow Reynolds (Re) and Froude (Fr) numbers, A(R), and the redd relative submergence (A/Y-0, with Y-0, the water depth). We address our goal by simulating the surface and subsurface flows with numerical hydraulic models linked through the near-bed pressure distribution quantified with a two-phase (air-water) two-dimensional surface water computational fluid dynamics model, validated with experiments. We apply the modeling approach to five redd sizes, which span the field observed range (from & SIM;1 to & SIM;4 m long), and by increasing discharge from shallow (0.1 m) and slow (0.15 m/s) to deep (8 m) and fast (3.3 m/s). Results confirm that downwelling fluxes increase with discharge and redd aspect ratio due to the increased near-bed head gradient over the redd. Averaged downwelling fluxes are a function of Re, Fr, A(R), and hydraulic conductivity of the redd and not of the undisturbed streambed material. The derived regression equation may help evaluate regulated and unregulated flow impacts on hyporheic flows during embryo incubation.