Simulating the effects of a capillary barrier using the two-dimensional variably saturated flow model SWMS-2D/HYDRUS-2D

approved: John S. Selker Engineering applications of a capillary barrier system an arrangement of fine material overlaying a coarser material at a sloping interface to protect e.g. waste or low radioactive deposits are considered to be an alternative to the typically used low permeability layer as a protector against water percolation into deposits. Ross (1990) developed equations that used material and site-geometric properties as well as infiltration rate to calculate the diversion capacity, Qa,a, total amount of water diverted along the sloping interface, and the diversion length, L, length along the interface before water is percolating into the coarse layer. While Ross' equation strongly depends upon the value of a (exponent in Gardner's conductivity function), Steenhuis et al. (1990) developed a similar equation with water (h,) and air entry pressure (ha) as most critical parameters. Steenhuis et al. (1991) combined Ross' and Steenhuis' solution to obtain a more accurate prediction of the diversion length. Stormont (1995) modified Ross' solution to account for constant anisotropy within the fine layer. Redacted for Privacy Billiotte et al. (1988), Fayer et al. (1992), and Oldenburg and Pruess (1993) performed numerical simulation to test the effect of a capillary barrier. However, none of these articles provided comprehensive tests of the validity of above mentioned equations. In this project the SWMS-2D/HYDRUS-2D code of the U.S. Salinity Laboratory, Riverside, California was used to simulate the water flow in a two-dimensional variably saturated capillary barrier system. The program allowed to incorporate changes in dip angle, infiltration rate, material properties, anisotropy, defects along the interface as well as to show the effects of single-layered and double-layered straight and curved interfaces. A sensitivity and plausibility analysis was performed to test the outputs of the simulation and a comparison with a finite difference method are outlined in this paper. The results of the simulation showed a much more complicated flow pattern in the vicinity of a capillary barrier than predicted by the simple analytical approaches. While the analytical solutions predict limited leakage into the coarse material for a down-dip distance smaller than L, the simulation outputs showed insignificant percolation in that region. A parabolic curved interface, compared to a straight interface of same drop height between up-dip and down-dip end, was shown to provide a smaller breakthrough. Defects, both protending upward and downward, at the interface lead to a penetration of the barrier far before that predicted by the analytical solution. Simulating the Effects of a Capillary Barrier using the Two Dimensional Variably Saturated Flow Model SWMS-2D / HYDRUS-2D by Thilo Stefan Heiberger A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Presented May 20, 1996 Commencement June 1997 Master of Sciences thesis of Thilo Stefan Heiberger presented on May 7, 1996