Hyporheic exchange under undular flows over a coarse granular bed

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 WP, 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 1 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.