The role of riverine bed roughness, egg pocket location, and egg pocket
permeability on salmonid redd-induced hyporheic flows
Abstract
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.