Hyporheic Exchange Driven by Submerged Rigid Vegetation: A Modeling Study

Flow‐vegetation interaction affects fluid flow hydraulics and associated material transport in river corridors. Concomitant changes in pressure within the flow field due to the presence of vegetation may act as a driver for the formation of hyporheic flow across the sediment‐water interface. This potentially important process, however, has yet to be studied. In order to investigate vegetation‐induced hyporheic exchange, a series of numerical models of interlinked surface‐subsurface flow modified by plant stems was conducted. Periodically staggered plant stem arrays on a flat sediment bed were considered within a coupled multiphysics computational fluid dynamics approach. Plants were idealized as rigid cylinders and arranged in different streamwise and spanwise spacing distances. Each vegetation array was then subjected to a broad range of flow Reynolds Numbers (Re). The results showed that hyporheic flow occurs in all conditions with the presence of vegetation. The vegetation‐induced hyporheic flux is found to be a function of Re via a power law. The flux increases with interstem space until the space reaches the distance that rigid stems no longer affect the flow structures in the vicinity of each other. Larger intervegetation distances lead to a larger hyporheic zone. A direct comparison with bedform‐induced hyporheic flow showed that vegetation can induce higher hyporheic flux through relatively shallower exchange zones. The results of all the simulations were synthesized into predictive models for hyporheic flux, bulk residence time and exchange depth based on drag coefficient, vegetation density, and Reynolds Number. Plain Language Summary Rivers have complex beds and banks. When rivers flow over complex surfaces and through irregular paths, fluid pressure differences arise. The differences in fluid pressure drive river water to exchange with the permeable and porous riverbed by infiltration of river water into the bed where pressure is high and interstitial water exfiltration where the pressure is low. This hyporheic exchange is an important river function, ecologically vital, and impacts water quality. One common feature that adds complexity to rivers is aquatic vegetation. We hypothesized that aquatic vegetation, as river water flows through them, can drive hyporheic exchange. We tested this idea through computational simulations of river flow through vegetation and then linked these with simulations of hyporheic flow through sediment. We found tha