Development of a Three-Dimensional Groundwater Flow Model
Creators: name: Mayer, Alex S.
Publisher: name: The University of North Carolina at Chapel Hill University Libraries
Year: 1987

description: A three-dimensional model for the simulation of transient groundwater flow is developed. The model is called REGFED for REGional flow using Finite Elements and Difference methods. A review of groundwater flow and contaminant transport concepts and theory reveals that three-dimensional representation of groundwater systems is essential for realistic simulation of flow and transport. From an analysis of currently available groundwater flow models and algorithms, it is apparent that a mixed numerical method consisting of finite-elements and finite differences is a suitable method for solving the groundwater flow equation in three dimensions. An algorithm known as ALALS (Alternate sublayer And Line Sweep) is selected for the basic model algorithm. Finite elements are applied to areal components, and flnite differences are applied to vertical components of flow. The model accomodates both conflned and unconfined groundwater flow problems and is also capable of handling the draining and refilling of individual elements or entire layers. Because of the model's efficient algorithm, it can accomodate thousands of nodal unknowns with minimal computer storage and CPU time. Quasilinear unconfined groundwater flow problems are solved using a Picard iteration scheme. Entire confined layers are skipped in the iteration scheme in order to decrease the CPU time required to solve the problem. The model is validated under a wide assortment of conditions including confined flow, confined flow with partially screened wells, unconfined flow, combined confined/unconfined flow, and flow with drained and refilled layers. A heuristic error analysis shows that model results compare well with validation results. Mass-balance errors for various groundwater flow problems are minimal for most cases. The convergence speed and stability of the iteration scheme is evaluated for solution of unconfined groundwater flow problems. A benchmark comparison using sample groundwater flow problems was performed with the REGFED model and with the USGS McDonald-Harbaugh model. Example applications further demonstrate the flexibility of the model. A three-dimensional model for the simulation of transient groundwater flow is developed. The model is called REGFED for REGional flow using Finite Elements and Difference methods. A review of groundwater flow and contaminant transport concepts and theory reveals that three-dimensional representation of groundwater systems is essential for realistic simulation of flow and transport. From an analysis of currently available groundwater flow models and algorithms, it is apparent that a mixed numerical method consisting of finite-elements and finite differences is a suitable method for solving the groundwater flow equation in three dimensions. An algorithm known as ALALS (Alternate sublayer And Line Sweep) is selected for the basic model algorithm. Finite elements are applied to areal components, and flnite differences are applied to vertical components of flow. The model accomodates both conflned and unconfined groundwater flow problems and is also capable of handling the draining and refilling of individual elements or entire layers. Because of the model's efficient algorithm, it can accomodate thousands of nodal unknowns with minimal computer storage and CPU time. Quasilinear unconfined groundwater flow problems are solved using a Picard iteration scheme. Entire confined layers are skipped in the iteration scheme in order to decrease the CPU time required to solve the problem. The model is validated under a wide assortment of conditions including confined flow, confined flow with partially screened wells, unconfined flow, combined confined/unconfined flow, and flow with drained and refilled layers. A heuristic error analysis shows that model results compare well with validation results. Mass-balance errors for various groundwater flow problems are minimal for most cases. The convergence speed and stability of the iteration scheme is evaluated for solution of unconfined groundwater flow problems. A benchmark comparison using sample groundwater flow problems was performed with the REGFED model and with the USGS McDonald-Harbaugh model. Example applications further demonstrate the flexibility of the model.