Hyporheic Flow and Residence Time Distributions in Heterogeneous Cross-Bedded Sediment

The permeability structure of streambed sediment influences hyporheic zone depth, flux, and residence time. We numerically simulate bedform-induced hyporheic exchange using turbulent open channel flow simulations coupled to groundwater flow simulations that consider high-resolution permeability datasets. The permeability fields are from the Massillon Sandstone and modern deposits of the Brazos River in Texas; both deposits exhibit cross-bedding. In both cases, permeability heterogeneity decreases average hyporheic flux rates and increases average residence times relative to equivalent homogeneous sediment. The impact of heterogeneity on hyporheic exchange depth varies with the orientation of high- and low-permeability zones with respect to bedform geometry. Since high-resolution permeability data are difficult to obtain, effective anisotropic permeability tensors are traditionally assigned to cross-bedded sediment. This approximation may adequately predict the hyporheic exchange area but underestimates average residence times. Our results suggest that a homogeneous, isotropic permeability model may not be appropriate for characterizing surface water-groundwater exchange in cases where streambed sediment has strong permeability structure at the spatial scale of the bedform. Because the permeability heterogeneity generally reduces hyporheic fluxes and increases residence times, it may also influence the distributions of dissolved solutes and temperatures within shallow river sediment. However, in typical streambed sediment, the effect of the permeability structure on hyporheic exchange is secondary to the influence of current-topography interaction and channel hydraulics.