Significance of Unsaturated Flow and Seepage Faces in the Simulation of Steady‐State Subsurface Flow

Abstract

AbstractA fully saturated, free‐surface ground water flow model and a variably saturated model are applied to the solution of the free‐surface ground water flow problem in order to determine the influence of the unsaturated flow component on the accuracy of the saturated flow solution and the water balance for homogeneous systems. The significance of the seepage face that must exist to provide a transition between a phreatic surface and a downstream surface water body is also evaluated. A series of simple generic cross‐sectional test cases are used as the basis for the numerical investigations. The fully saturated and variably saturated models used in this study are two‐dimensional Galerkin finite‐element models that incorporate exit seepage face options. The fully saturated solution is found to be almost identical to the variably saturated solution below the water table, the only significant difference being that the variably saturated model predicts a slightly lower water table due to horizontal flow components in the capillary fringe. This difference is negligible for large‐scale systems, snowing that for homogeneous systems, unsaturated flow can be neglected and steady‐state flow including seepage face discharge can be realistically simulated by a fully saturated free‐surface model. For aquifer‐scale systems where discretization limitations may prevent the explicit representation of seepage faces, the seepage face can be omitted because of its negligible influence on the overall water balance at the larger scale.