A pore-scale study of flow and transport across the sediment–water interface: From dispersive to turbulent regimes

Understanding the mass transfer between surface and subsurface water is essential in the fluvial environment. Transport across the sediment–water interface (SWI) can be strongly influenced by pore-scale vortices and coherent turbulent structures (sweep and ejection) near the interface. However, the functions of these hydrodynamic structures over the exchange at the interface and pore water movement in the bed are still unclear. In this paper, the pore-scale flow and transport across the SWI under dispersive and turbulent regimes were studied. The results show that pore-scale vortices are the main hydrodynamic form at the SWI in dispersive regimes, creating a transition layer and impeding interfacial transport. Periodic sweep and ejection prevail in turbulent regimes, which alter the structures of pore scale vortices, enhance interfacial transport, generate periodic pressure eruptions at the interface, and pulse the pore water to move vertically and periodically in the bed. Different forms of the dominant hydrodynamics structures near the interface will affect the residence time and trajectories of solutes in the bed. In dispersive regimes, the residence time of solutes exhibits a positive correlation with the Reynolds number due to the increasing penetration of solutes, while, in turbulent regimes, this correlation turns negative because of the unchanged penetration depth of solutes. The mechanism demonstration from numerical simulations clarifies the functions of pore-scale vortices and coherent turbulent structures on mass transfer between the surface and subsurface water, which can enhance the understanding of the role of sediments in the aquatic ecosystem.