Effects of Some Common Geological Features on Two-Dimensional Variably Saturated Flow
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EFFECTS OF SOME COMMON GEOLOGICAL FEATURES ON TWO-DIMENSIONAL VARIABLY SATURATED FLOW
Amvrossios C. Bagtzoglou, Rachid Ababou, Budhi Sagar, and M. Rashidul Islam Center for Nuclear Waste Regulatory Analyses - Southwest Research Institute 6220 Culebra Road - San Antonio, TX 78228-0510, U.S.A.
ABSTRACT
This paper presents results of unsaturated flow simulations undertaken as an auxiliary analysis for the Iterative Performance Assessment (IPA) project, one of the approaches adopted' by the U.S. NRC to develop repository license application review capabilities. The effects on flow of common geological features, such as nonhorizontal stratification and vertical or nearvertical fault zones intersecting the strata, in a two-dimensional (2D) domain are studied. Results indicate that the presence of layers and crosscutting fault zones tend to induce three-dimensional (3D) unstable flows in the unsaturated zone. The instability is manifested in our simulations by an oscillatory behavior of steady state. This numerical instability imposes extremely stringent criteria on the time step used in the simulation. Finally, once stable steady-state solutions are attained, the effect of the crossing point in the matrix-fault unsaturated hydraulic conductivity curve on groundwater flux vectors and moisture content distributions is studied. INTRODUCTION A numerical investigation of quasi-3D unsaturated flow in a vertical cross-section with dipping layers and a subvertical fault is presented. In this work, numerical analysis is performed for a deep (approximately 530 m) hard rock system. The BIGFLOW numerical code [2] is used in these simulations. Some of the data for the analysis were taken from the Yucca Mountain project reports, but were freely modified to enhance the effects under study. Therefore, conclusions regarding suitability of Yucca Mountain for the proposed HLW repository are not directly derivable from this analysis. Recognizing that there are no simple, natural initialboundary conditions that can be used for the more complex problems, a method of successive approximation is implemented. This method uses solutions of auxiliary flow problems to set up pressure boundary conditions for the more complex problems. This is necessary because, in practice, no natural boundaries exist (or are adequately defined), especially in the lateral directions. The investigation is limited to 3D simulations in a vertical, "thin slice" cross-section with dipping layers intersected by a subvertical fault zone. The simulations are performed in a transient mode to study the manner in which the solutions of the flow equation approach steady state. A "wet", in terms of perceived net infiltration rates, hydro-climatic condition corresponding to a net annual infiltration rate of 50 mm/year is modeled. A hypothetical test problem is developed to study the effects of bedding, presence of a subvertical fault, and inclination of the beds. The BIGFLOW simulation code accommodates 3D transient or steady flow in saturated, or partially saturated porous me
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