Impact of Spatial Permeability Distribution Characteristics on Hyporheic Flow Using a Physical System and Simulations

Abstract:

Permeability heterogeneity has been found to significantly affect the volume and speed with which water flows through the hyporheic zone. In this study we investigated how characteristics of spatial permeability distributions within a simple Tóthian system affected hyporheic flow both in physical and simulated domains. Our setup consisted of a 13x7x1 grid of two sediments with a divide bisecting the surface water and top middle cell creating two regions of constant head, which induced flow through the grid. Cells were filled with sand and sandy-gravel in a 2:1 ratio and positioned according to TProGS outputs. We ran a blue dye and salt solution through the system, recorded dye location using time-lapse photography, and measured the electrolytic conductivity as the water exited the system. We also calculated a grid of head values using MODFLOW and simulated flow through the system, yielding simulated dye-fronts, residence times, and exiting salt concentrations for the modeled system. We found strong agreement between the simulation and experimental procedure. We generated an additional 100 grids with the 2:1 sediment ratio for each of the transition probabilities 0.25, 0.50, and 1.0. We simulated these with 1, 2, and 3 order of magnitude differences in permeability values and used moving averages with varying window sizes to investigate the effect of the abruptness of transitions between sediment types. For these cases we compared cumulative residence time distributions, volumetric flux, and deviation from the normalized velocity field for homogenous sediment. We found that smoothing the transition between grid cells increased the volumetric flux, decreased the median residence times, and increased the deviation from a normalized homogenous velocity field. These effects were generally greater on grids created with larger transition probabilities and greater differences in K values.