Scales
Heterogeneity
Barrier Elements
Input
Plot Settings
Calibration
Demo

TWODAN Product Information

TWODAN (TWO-Dimensional ANalytic) is a popular groundwater flow model that combines advanced analytic elements with an easy-to-use interface. These powerful features make TWODAN an excellent tool for many remediation design, capture zone analysis and regional modeling problems.

TWODAN's advanced capabilities, interface quality and price make it a great value for two-dimensional groundwater flow modeling!

About the Analytic Method | Modeling Capabilities | User Interface
Documentation | Technical Support

About The Analytic Method

Summary

The analytic element method described by Strack (1989, Groundwater Mechanics, Prentice-Hall) is the mathematical basis for TWODAN. The principal advantages of this method over conventional numerical techniques are its simple input, speed, accuracy and lack of a fixed grid.

Compared to standard numerical (finite-difference and finite-element) methods, discretization and input requirements are much simpler and less time consuming with the analytic method. Only the boundaries of the domain are discretized, not the domain itself as in numerical methods. This distinction greatly simplifies input and allows you to model a large region while retaining great accuracy in very detailed areas of interest.

With analytic elements, there is no need to arbitrarily define the limits of the model--just model far enough from the area of interest to account for the real boundary conditions influencing the aquifer!

Click here for an illustration of the detail possible with a TWODAN model.

Details

In the analytic element method, large numbers of analytic solutions are superpositioned to solve complex groundwater flow problems. The superpositioned functions are associated with particular aquifer features. For example, some of the functions contain parameters that are unknown when the problem is posed (for example, the discharge of a head-specified linesink). These unknowns are determined by specifying boundary conditions at control points located on or near the aquifer features (for example, setting the head at the center of a head-specified linesink). The number of specified boundary conditions equals the number of unknown parameters, yielding a system of linear equations which is solved by standard methods. Once the unknown strengths are solved for, the resulting composite analytic solution satisfies the governing differential equation exactly except at singular points or lines associated with the analytic functions. The specified control point boundary conditions will be met exactly, and boundary conditions will be approximate between control points.

Modeling Capabilities

TWODAN has a comprehensive suite of advanced analytic modeling features that allow you to model everything from a single well in a uniform flow field up to complex remediation schemes with numerous wells, barriers, surface waters and heterogeneities.

Heterogeneous, Layered Aquifers

The aquifer modeled by TWODAN can consist of one or two hydraulically connected layers; it can be confined and/or unconfined; and it can be homogeneous or heterogeneous. Heterogeneities are input as closed polygon regions, each with a distinct set of aquifer properties (base elevation, lower layer K, lower layer thickness, upper layer K, and upper layer thickness). Heterogeneities can be nested inside one another and they can abut one another.

Click here to see a model of an aquifer with a variety of heterogeneities and mixed confined/unconfined conditions.

Impermeable and Resistant Boundaries

These features can have irregular shapes consisting of open or closed strings of line segments. The analytic implementation of these elements in TWODAN gives much greater accuracy than is possible with numerical methods. The discharge through resistant boundaries is proportional to a user-specified resistance (thickness/conductivity) and the head difference across the boundary. These elements offer an accurate way to model flow fields containing slurry walls, sheet-pile walls, etc. (see Fitts, C.R., Groundwater, 35(4), 1997).

Click here to see models using both the impermeable and resistant elements in remediation simulations.

Wells

Solutions for both steady-state and transient wells are available.  TWODAN is also capable of optimizing discharges of steady wells based on specified head and aquifer discharge conditions (see Fitts, C.R., Groundwater, 32(4), 1994.). When transient wells are used, you can write an ASCII file listing head vs. time at a specified location; this file can be imported to a spreadsheet for plotting hydrographs.

Linesinks

Both discharge- and head-specified linesinks are implemented.  Head-specified linesinks are typically used to represent constant-head boundaries. Discharge-specified linesinks can be used to model infiltration or pumping trenches.

Infiltration/Leakage

Vertical infiltration or leakage to or from the aquifer can be modeled as uniform or as locally variable. Locally variable infiltration/leakage is modeled using solutions for circular area sources.

Uniform Regional Flow

A uniform cross-flow in the aquifer can be input at any angle and discharge rate.

User Interface

TWODAN has a familiar Windows user interface that is very simple and fast to use. Most common modeling operations are executed at the push of a button on the main screen.

Instead of using tedious data-entry forms, model input data and plot settings data are accessed directly in spreadsheets. You can quickly edit all aspects of the model input in the model input screen and all the plot settings from the plot settings screen. The input data for a model is stored in one file while the settings for a particular plot are stored in another. This separation is efficient, allowing you to push one button to repeat a plot with the same contours, pathlines, window coordinates, etc.

Digitizing Features

Digitize using the mouse and a DXF basemap overlay. You can also digitize over the top of the previous plot showing contours, pathlines, etc. You can zoom or pan to a different view during the middle of a digitizing operation. You can continuously digitize multiple points, lines, or circles, quickly defining long barrier, heterogeneity, and constant-head boundaries. The digitized coordinates are temporarily stored in the Windows Clipboard. From there, the coordinates can be pasted into the TWODAN data grid or into other editor or spreadsheet files. As a bonus, you can use TWODAN as a general-purpose digitizing program with DXF basemaps.

Graphic Output Options

Your plots can include any of the following:

TWODAN plots can be output to a huge array of devices--all those supported by the Windows operating system. Graphic plots may directed to the screen, Windows printer devices, bitmap (*.bmp) files, the clipboard, Surfer GRD files, or to DXF files. With all these options, it is easy to incorporate TWODAN graphic output into your reports.

Printer plots may be scaled automatically to fit the page, or manually scaled to a specific scale (e.g., 1 inch = 500 feet). TWODAN automatically centers the plot on the page.  You may print a plot with a landscape or portrait orientation, and you may add a border box and up to three lines of title text.

Automatic Contour Labeling

TWODAN labels contours automatically. You select which contours to label (each contour, every other one, every fifth one, or every tenth one), and how frequently the labels occur along a given contour. Set these parameters once in the plot settings screen, and they will be used each time you press a button to make a plot.

On-Line Help Systems

The on-line help is extensive and well designed.  The following forms of help are available: tips that automatically display when you pause over a control, context-sensitive help (F1 key), and Windows-standard detailed help. The detailed help is indexed, searchable, printable, and it contains embedded jumps to related topics.

Graphic Plotting of Calibration Results

Like many models, TWODAN will compile a list of target heads, model-calculated heads, and differences between these. In addition, TWODAN has a very useful feature that plots the spatial distribution of the differences and calculates the following summary calibration statistics:

Click here to see an example of the graphic plotting of calibration results.

Other Nifty Utilities

Some other interface features deserve at least a brief mention:

Documentation

Most of the information about how to use the software is in the on-line help system. The TWODAN manual is a 7x9 inch spiral bound booklet containing the following sections:

Technical Support

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