THREE-DIMENSIONAL MODELING OF HYPORHEIC FLOW THROUGH A HETEROGENEOUS STREAMBED

Flow through hyporheic zones controls the availability of nutrients and dissolved oxygen to hyporheic and riparian habitat and determines the overall quality of river water and shallow ground water. On large scales, hyporheic flow paths are determined by various combinations of river, floodplain, and aquifer geomorphology. On smaller scales, these flow paths are controlled by streambed and water surface topography and streambed hydraulic properties.

We present results from numerical modeling of hyporheic flow using MODFLOW. Our model domain is 45 m x 20 m x 1.2 m and is divided into 0.25 m x 0.25 m elements and 11 layers. The domain is characterized by a highly variable hydraulic conductivity (K) field that is based on hydraulic, geophysical, and sedimentological data collected along a meander of Prairie Creek in central Nebraska. We impose a no-flow boundary at the bottom and constant-head boundaries on all sides of the domain. The top boundary is prescribed as a spatially varying constant-head boundary. Variants of the top boundary include: (1) a uniform linear gradient in the downstream direction, (2) a sinusoidally varying head field (used as a proxy for bedform topography and/ or ripples on the water surface) superimposed on a linear gradient, (3) superposition of two perpendicular linear gradients (the across-stream gradient is used to simulate super-elevation of the water surface on the cutbank of river bends), and (4) a super-elevated water surface superimposed on a harmonic head field. These boundaries are imposed on a heterogeneous K field and an equivalent homogeneous media.

Our results show that consideration of heterogeneity even under simple boundary conditions, i. e., a uniform linear gradient, produces hyporheic zone structures that resemble other researchers’ observations. Thus, in addition to streambed and water surface topography, heterogeneity of streambed deposits is a key determining factor for flow structure in hyporheic zones.