SWICHA is a three-dimensional finite element code for analyzing seawater intrusion in coastal aquifers. The model simulates variable density fluid flow and solute transport processes in fully-saturated porous media. It can solve the flow and transport equations independently or concurrently in the same computer run. Transport mechanisms considered include: advection, hydrodynamic dispersion, absorption, and first-order decay.

The flow and transport equations are discretized using the finite element method. Simple rectangular and triangular prism elements are used. The combination of such elements enables flow regions of complex geometry to be modeled accurately. In addition, element matrices can be computed efficiently without having to perform numerical integration. Matrix assembly is performed slice by slice, and the matrix solution is achieved by using a slice successive over-relaxation (SSOR) scheme. This scheme permits a fairly large number of unknowns to be handled cost effectively. For a coupled flow and transport problem, the nonlinearity is handled by Picard iterations. At the user's option, artificial dispersion can be added to the transport equation stiffness matrix to prevent exceeding a critical Peclet number.

SWICHA is capable of performing several types of analysis, performed in an areal plane, vertical cross-section, axi-symmetric configuration, or a fully three-dimensional mode. Because of its special design features, SWICHA is capable of handling a wide range of complex three-dimensional, steady-state or transient field problems.

Several Test Problems have been used to verify the code and check the accuracy of the numerical solution and the solution algorithms. These problems range from simple one-dimensional to complex three-dimension, coupled flow and transport.

The SWICHA code may be used in many types of practical situations. These include: 1) ground-water resource evaluations; 2) assessments of well performance and pumping test analysis; 3) ground-water contamination investigations; 4) hazardous waste subsurface storage programs; and 5) seawater intrusion studies.

SWICHA runs in batch mode. Example data sets are provided, which can be copied and edited with a text editor for individual problems. Results are saved in text files. SWICHA is distributed on DOS-formatted disks containing source code, executable image, and example data sets. The user's manual contains installation instructions and program documentation.

SYSTEM REQUIREMENTS

• Intel 80386/80486 based computer
• 4 Mb RAM
• hard drive with at least 5 Mb free disk space
• DOS 3.0 or higher
• math coprocessor

Author: B. Lester (GeoTrans, Inc.)