Documentation of a computer program to simulate horizontal-flow barriers using the U.S. Geological Survey's modular three-dimensional finite-difference ground-water flow model

Abstract

As an addition to the U.S. Geological Survey’s Modular Three-Dimensional Finite-Difference Ground-Water Flow Model, the Horizontal-Flow-Barrier (HFB) Package simulates thin, vertical low-permeability geologic features that impede the horizontal flow of ground water. These geologic features are approximated as a series of horizontalflow barriers conceptually situated on the boundaries between pairs of adjacent cells in the finite-difference grid. The key assumption underlying the HFB Package is that the width of the barrier is negligibly small in comparison with the horizontal dimensions of the cells in the grid. Barrier width is not explicitly considered in the Package, but is included implicitly in a hydraulic characteristic defined as either (I) barrier transmissivity divided by barrier width, if the barrier is in a constant-transmissivity layer, or (2) barrier hydraulic conductivity divided by barrier width, if the barrier is in a variable-transmissivity layer. Furthermore, the barrier is assumed to have zero storage capacity. Its sole function is to lower the horizontal branch conductance between the two cells that it separates. Documentation of the HFB Package includes data-input instructions, narratives, flowcharts, program listings, and variables lists for the three primary modules and one submodule in the Package.