A Numerical Investigation of Soil-Bedrock Interface Flow Impedance

It is common for subsurface flow to transition between porous and fractured media. Two examples include vertical infiltration of precipitation through a permeable soil overlying a fractured bedrock, and horizontal ground water flow through a combination of porous and fractured geologic units. A reduction in flow or flow impedance may occur across these interfaces caused by head losses associated with flow path reorganization, as flow at the pore-scale must transition to open fractures. A two-dimensional numerical modeling framework – that simulates saturated, vertical flow through permeable soil overlying fractured bedrock – is utilized to investigate the hydraulic influence of porous and fractured media flow transitions, given centimeter-scale grid resolution with localized grid refinement that captures micron-scale fractures, various configurations of soil-bedrock thickness and ranges in hydraulic properties. Results indicate that flow across soil-bedrock interfaces can decline by nearly two orders of magnitude, depending on soil thickness, fracture spacing and hydraulic properties of the soil and fractured bedrock. Fracture halfspacing and soil depth define the curvature or horizontal deviation of flow paths from vertical, whereas the ratio between soil and fractured bedrock define relative permeability contrasts. These results indicate that watershed process models used to provide estimates of net infiltration and ground water recharge need to account for flow impedance at soil-bedrock interfaces. This can be accomplished by decreasing equivalent hydraulic conductivity values assigned to the fractured bedrock.