Flow processes in the dry regime : the effect on capillary barrier performance Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/3197xp392

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  • Engineered capillary barriers typically consist of two layers of granular materials designed so that the contrast in sediment hydrologic properties and sloping interface retains infiltrating water in the upper layer. We report here on the results of two bench-top capillary barrier experiments, and associated modeling. These experiments were conducted to better understand the behavior of capillary barriers in particular with respect to an engineered barrier system proposed at the potential high-level nuclear waste site at Yucca Mountain, Nevada, where different engineered barriers have been under consideration throughout the site development process (Carter and Pigford 2005; Tidwell et al. 2003). We measured hydrologic parameters for both coarse materials using standard methods and found that the two materials had similar hydrologic properties despite being morphologically different (round, uniform vs. angular, non-uniform). The rounded 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, more than 93% of the infiltrating water was successfully diverted from the lower layer, but infiltration into the underlying layer was observed in both systems. Our experimental results show that capillary barriers designed based on standard hydrologic property measurements may result in invalid prediction of the system behavior. Moreover, our numerical simulations of these experiments show that the predicted capillary barrier performance was highly sensitive to physically realistic variability in model hydrologic parameter values. Based on this work, we believe that other non-continuum 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. By applying a theoretical film flow equation representative of sediment surface geometries we were able to show that infiltration into the underlying sediment layer can be dominated by water film flow, a characteristic that is typically not considered in mainstream numerical models.
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