Hyporheic Exchange Driven by Three-Dimensional Sandy Bed Forms: Sensitivity to and Prediction from Bed Form Geometry

Bed form-driven hyporheic exchange is vital to biogeochemical processes occurring within aquatic sediment. The integrated effects of hyporheic processes in a bed form ultimately impact watershed-scale water quality. However, much of what is understood regarding bed hyporheic exchange is based on idealized two-dimensional bed forms despite the prevalence of potentially complex three-dimensional (3-D) bed forms in sandy riverbeds. We thus examined the impact of bed form three-dimensionality on hyporheic exchange. Bed form three-dimensionality was represented by two groups of geometric parameters: (1) crest planform curvature and (2) transverse and longitudinal wavelength and amplitude. A wide variety of synthetic bed forms was generated based on the geometric parameters. Then, surface flow over and hyporheic flow through each bed form was calculated using a 3-D multiphysics computational fluid dynamics model implemented across a range of Reynolds Number. We found that hyporheic exchange is sensitive to both types of parameters that determine the bed form three-dimensionality. Hyporheic exchange is dominated however by the three-dimensionality caused by out-of-phase superimposed sinusoidal surfaces. The results of the complex flow models were synthesized into simple equations for predicting hyporheic flux and bulk residence time based on bed form longitudinal and transverse wavelengths, bed form height, and Reynolds Number.