Author(s): Angel Monsalve; Brandon Hilliard; Daniele Tonina

Linked Author(s):

Keywords: Hyporheic exchange; Permeability threshold; Computational fluid dynamic (CFD); Darcy-Forchheimer flow; Salmon redd hydraulics

Abstract: Hyporheic exchange in salmon redds governs embryo development, biogeochemical cycling, and thermal buffering in spawning gravels. Yet, the feedback between streambed permeability and surface–subsurface flow coupling remains poorly constrained. We investigate this feedback through a fully coupled Computational Fluid Dynamics (CFD) framework that resolves both turbulent surface flow and porous flow within the redd. The model couples the Detached Eddy Simulation (DES) and Volume-of-Fluid (VOF) approaches for t he free surface with a Darcy–Forchheimer formulation for the permeable domain. Model validation using refractive- index-matching, RIM, flume experiments and Stereo-Particle-Velocimetry/Planar-Laser-Induced-Florecense (SPIV/PLIF) imaging demonstrates strong agreement (R² > 0.9) in water-surface elevation, pressure distribution, and subsurface flow paths. Parametric simulations spanning five orders of magnitude in hydraulic conductivity reveal a distinct transition in hyporheic and surface hydraulic behavior. For conductivities below approximately K ≈ 0.2 m s⁻¹, pressure gradients dominate, and exchange is primarily pressure driven. In this regime, permeability exerts negligible influence on bulk hyporheic fluxes, and the pressure drop remains localized near the sediment –water interface. Above this threshold, advection within the redd intensifies, attenuating near-bed pressure gradients by up to

DOI: https://doi.org/10.64697/iahr.proc.ise2026.abs.144

Year: 2026