Flow Processes in the Dry Regime: The Effect on Capillary Barrier Performance

Abstract

Engineered capillary barriers typically consist of two layers of granular materials designed so that the contrast in material hydraulic properties and sloping interface retain infiltrating water in the upper layer. We conducted two benchtop capillary barrier experiments, followed by interpretation and numerical modeling. The hydraulic parameters for two coarse materials were measured using standard methods, and we found that the materials had similar hydraulic properties despite being morphologically different (round vs. angular). The round sand provided a better functioning capillary barrier than the angular sand, but neither experiment could be characterized as a perfectly working capillary barrier. In both cases, >93% of the infiltrating water was successfully diverted from the lower layer; however, infiltration into the underlying layer was observed in both systems. Based on this work, we believe that noncontinuum processes such as vapor diffusion and film flow contribute to the observed phenomena and are important aspects to consider with respect to capillary barrier design as well as dry vadose zone processes in general. Using a theoretical film flow equation that incorporates the surface geometry of the porous material, we found that infiltration into the coarse underlying sand layer appeared to be dominated by water film flow. The NUFT (Nonisothermal Unsaturated–Saturated Flow and Transport) model was used for qualitative comparison simulations. We were able to reproduce the barrier breach observed in the experiments using targeted parameter adjustment, by which pseudo‐film flow was successfully simulated.