Three‐dimensional versus two‐dimensional bed form‐induced hyporheic exchange

The hyporheic zone is often a critical component of river systems. Hyporheic exchange is generally forced by variation in riverbed topography such as due to bed forms. Most previous research on bed form‐driven hyporheic flow has focused on two‐dimensional (2‐D) dunes and ripples, while little has been done on their three‐dimensional (3‐D) counterparts. Here we compared hyporheic exchange and associated metrics for a previously studied pair of corresponding 2‐D and 3‐D bed forms. To accomplish this, a series of multiphysics computational fluid dynamics models were conducted both in 2‐D and 3‐D with similar open channel Reynolds numbers ( Re ). Results show that the pressure gradient along the sediment‐water interface is highly sensitive to the spatial structure of bed forms, which consequently determines hyporheic flow dynamics. Hyporheic flux is a function of Re for both 2‐D and 3‐D dunes via a power law; however, the equivalent 3‐D dunes have a higher flux since the 3‐D form induces more drag. The hyporheic zone depths and volumes are only slightly different with the 3‐D case having a larger volume. The mean fluid residence times for both cases are related to Re by an inverse power law relationship, with the 3‐D dune having smaller residence times at moderate to high Re . The effects of increasing flux on residence time in 3‐D dunes are partly modulated by a slightly increasing hyporheic volume. Our results suggest that a 2‐D idealization is a reasonable approximation for the more complex 3‐D situation if local details are unimportant but that development of predictive models for mean fluxes and residence times, which are critical for biogeochemical processes, based on 2‐D models may be insufficient.