Can Continuum Extensions to Multiphase Flow Models Describe Preferential Flow?

Gravity-driven preferential flow paths (fingers) have been shown to be the result of nonmonotonic saturation profiles observed during constant-flux, one-dimensional infiltration. These nonmonotonic profiles, also known as saturation overshoot, cannot be described by traditional unsaturated continuum flow models, such as the Richards equation. Instead, continuum models require additional extensions, higher order in eithertime or space, which are still open to much question. In this study, an extensive data set was obtained on preferential flow and overshoot behavior in well-sorted sands. The overshoot behavior is the strongest evidence of dynamic effects in porous media because other reported experimental evidence can be the result of varying measuring volumes or media heterogeneities. Thus the extensive overshoot data set provides the best testing ground for the various proposed extensions, and the data set was compared with recently proposed continuum extensions to unsaturated flow models.