Abstract
Hyporheic exchange across salmonid redds plays a critical role in embryo development and biogeochemical cycling. Using fully coupled numerical models validated against measurements from non-invasive high-resolution optical techniques in large-scale open-channel laboratory experiments, we examine how streambed permeability governs the transition from pressure- to advection-driven hyporheic flows. Our results show that hydraulic conductivity K = 0.2 m/s (order of 10(-1) m/s) represents a critical threshold: below this value, sequential surface-subsurface models adequately capture bulk hyporheic fluxes, while above it, fully coupled modeling becomes essential due to significant momentum exchange between flow domains. The experimental validation at K= 0.17 m/s, achieved through simultaneous calibration against both surface flow patterns and subsurface velocity measurements, provides direct evidence of flow regime transition mechanisms near this threshold. At higher permeabilities present in highly porous formations, feedback between surface and subsurface flows becomes significant, reducing near-bed pressure gradients by up to 80%, altering surface hydraulics, and substantially changing hyporheic exchange morphology. We propose a correction factor to extend existing predictive models to high-permeability regimes, accounting for the transition from pressure-driven to advection-dominated flow conditions. These findings highlight the importance of dynamically coupled modelling for assessing ecohydraulic processes in permeable streambeds and inform habitat assessments, restoration strategies, and evaluations of fish-built structures as ecosystem engineers.