Adhesive particle transport in idealized vegetated hyporheic zones: Pore-scale simulation and stochastic upscaling

Vegetation is ubiquitous in stream channels and significantly affects hyporheic exchange, which refers to mass exchange at the interface between surface water and porous sediment regions within the channel benthic area. Despite its importance, a mechanistic understanding of how vegetation-derived hyporheic exchange influences particle transport remains incomplete. Investigating particle transport in vegetated hyporheic systems is challenging because particles often adhere to the surfaces of vegetation or solid grains within subsurface porous zones. Additionally, pore-scale simulations, which are crucial for obtaining a mechanistic understanding of vegetation effects on hyporheic exchange and subsequent particle transport and adhesion, are rarely conducted due to their high computational requirements. In this study, we investigate the effects of emergent rigid vegetation on hyporheic exchange and the transport of adhesive particles by employing an advanced immersed boundary method-based direct numerical simulation for flow and a Lagrangian particle tracking algorithm for adhesive particle transport. To isolate the effects of vegetation-induced hyporheic exchange, this study considers a hydraulically flat sediment bed composed of uniform spherical grains and examines vegetation-driven exchange under spatially uniform pressure forcing. Under this idealized, monodisperse porous media configuration, our results indicate that vegetation enhances particle exchange and adhesion in the hyporheic zone, with these effects becoming more pronounced as bulk flow velocity increases. Furthermore, the effects of particle density become more pronounced under low-flow conditions, leading to increased particle adhesion. We develop a stochastic upscaling framework based on a spatial Markov model, which successfully captures adhesive particle transport by incorporating a velocity-dependent adhesion probability.