资源环境科技发展态势分析平台

Undular flows over riffles (standing waves with underlying open‐channel flows) facilitate a quality environmental setting for riverine habitats. Understanding their link with hyporheic exchange flow (HEF) is important from eco‐hydro‐geomorphic perspectives. We conduct refractive‐index‐matched scanning experiments to resolve the exchange between surface and subsurface flows across a coarse granular bed. We show that the HEF includes wave pumping (WP) and grain‐scale pumping (GP) components. Mechanistically, the dynamic‐pressure gradient best explains the HEF on both wave and grain scales. However, due to the underlying open‐channel flow, the result exhibits a phase lag compared to the theoretical wave pumping, suggesting a necessary shift from the existing water‐surface‐based model to a pressure‐gradient‐based approach. Quantitatively, with the present experimental setting, the GP flux is one order of magnitude smaller than the WP flux. The relative significance of WP to GP is further increased at higher discharges, indicating potential utility of WP in habitat vitalization.