THREE-DIMENSIONAL MODELING OF HYPORHEIC FLOW THROUGH A HETEROGENEOUS STREAMBED
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.