Salmon spawning activities alter streambed morphology, forming a dune-shaped egg nest called a redd. The spawning process increases redd sediment hydraulic conductivity, KD, and congregates large sediment grains to form an egg pocket, such that egg pocket hydraulic conductivity, KEP, may be higher than KD. Salmon females may create one or more egg pockets within a single redd. Although the impact of redd shape and KD on redd-induced hyporheic fluxes has been studied, the effects of streambed roughness, R, egg pocket permeability, and egg pocket location on egg pocket hyporheic fluxes have not yet been quantified. This study investigates this knowledge gap with a set of numerical simulations supported by flume experiments. We simulated hyporheic flows for five egg pocket locations, five KEP values from 0.0025 to 0.02 m/s, and 12 rough streambed surfaces. Surface roughness was scaled from a measured streambed surface in two ways - only vertically (R1) and both vertically and horizontally (R2) - with scaling coefficients ranging from 0.5 to 3. The measured streambed surface had a median diameter, D50, of 1 cm and a standard deviation of 0.77 cm. The results indicated that the dimensionless flux into the egg pocket increases noticeably with the downstream distance of egg pockets from the redd pit, and less strongly with KEP*=KEP/KD. The near-surface downwelling fluxes significantly increase with R1, but only negligibly with R2, and for deeper egg pockets, flux into the egg pocket is minimally impacted by surface roughness.

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.