Research

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

Salmonids bury their eggs in hyporheic streambed gravel, forming an egg nest, called a redd, characterized by a pit and a hump topography resembling a dune. Embryos’ survival depends on downwelling oxygen-rich stream water fluxes, whose magnitudes are expected to depend on the interactions among redd shape, stream hydraulics, and the hydraulic conductivity of the streambed sediment. Here, we hypothesize that downwelling fluxes increase with stream discharge and redd aspect ratio, and such fluxes can be predicted using a set of dimensionless numbers, which include the stream flow Reynolds and Froude numbers, the redd aspect ratio, and the redd relative submergence. 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 flume experiments. We apply the modeling approach to three redd sizes, which span the observed range in the field (from ~1 to ~4 m long), and by increasing discharge from shallow (0.1 m) and slow (0.15 m/s) to deep (8m) and fast (3.3 m/s). Results support our hypothesis of downwelling fluxes increasing with discharge and redd aspect ratio due to the increased near-bed head gradient over the redd. The derived equation may help evaluate the effect of regulated flow (e.g., hydroelectric and flood control dams) on redd-induced hyporheic flows. 

The role of egg pocket location, hydraulic conductivities, and bed roughness on salmonid redd-induced hyporheic flows

Salmon spawning activities alter streambed morphology, forming a dune shape egg nest, called redd, increase redd sediment permeability, place large particles in the egg pocket and coarsen surficial grain size distribution. Salmon females may lay one or multiple egg pockets within the same redd. Whereas the role of redd shape and its permeability is well recognized to enhance flow through redds, we do not know the importance of streambed roughness and egg pocket permeability, distribution and location on downwelling flows to the developing embryos. This study investigates this knowledge gap with a set of numerical modeling. We simulated groundwater flow, utilizing a range of egg pocket permeabilities spanning from 2.566·10-11 m2 to 2.05·10-9 m2, along with varying streambed roughness scaled vertically (R1) and with the overall roughness size (R2) values up to 3 cm. Additionally, we investigated the impact of different egg pocket locations, including those receiving flows from both downwelling and upwelling regions. The results showed that flux (q*ep) into the egg pocket increases significantly with egg pocket hydraulic conductivity by ~42% with egg pocket K* from 1 to 8 . The position of the egg pocket also has a significant effect, with flux varying linearly with the placement of egg pockets located downstream in the redd. Furthermore, it was found that downwelling flux through the redd is higher for rough streambed, and this flux increases linearly with R1. However, the flux through the redd did not significantly vary with R2. The impact of each rough streambeds on the egg pocket was diverse, with an increase of ~ 51% in the flux entering the egg pocket for R1 and ~40 % increase for R2, for the 3 cm roughness, compared to a smooth streambed.

Evaluation of RANS Turbulence Models in Open Channel Flow over Salmon Redds

This study evaluates computational fluid dynamics (CFD) turbulence closures for Reynolds-Averaged Navier-Stokes equations against experimental data to model complex open channel flows, like those occurring over dune-shaped salmon spawning nests called “redds”. Open channel flow complexity, characterized by near-bed turbulence, adverse pressure, and free surfaces, requires suitable turbulence closure capable of capturing the flow structure between streambed and water surface. We evaluated three RANS models: standard k–ω, SST k–ω, and realizable k–ε, along with four wall treatments for the realizable k–ε: standard, and scalable wall functions, enhanced wall treatment, and an unconventional closure combining standard wall function with near-wall mesh resolving the viscous sublayer. Despite all models generally capturing the bulk flow characteristics, considerable discrepancies were evident in their ability to predict specific flow features, such as flow detachments. The realizable k-ε model, with standard wall function and mesh resolving viscous sublayer, outperformed other closures in predicting near-wall flow separations, velocity fields, and free surface elevation. This realizable k-ε model with a log-layer resolved mesh predicted the free surface elevation equally well but lacked precision for near-wall flows. The SST k-ω model outperformed in predicting turbulent kinetic energy and provided better predictions of the near- boundary velocity distributions than realizable k-ε closure with any of the conventional wall treatments but overestimated the separation vortex magnitude. The standard k–ω model also overestimated near-wall separation. This study highlights the variability in accuracy among turbulence models, underlining the need for careful model selection based on specific prediction regions.