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    TWODAN Description

    Features

    System Requirements

TWODAN Description

The analytic method described by Strack (1989, Groundwater Mechanics, Prentice-Hall) is the mathematical basis for TWODAN. The principle advantages of this method over conventional numerical methods are its simple input, accuracy, speed, and lack of a fixed grid. With the analytic method, only the boundaries of the domain are discretized, not the domain itself as in finite-difference and finite-element methods.

You can model a huge area and still retain great accuracy in small regions of the model. 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 on the aquifer. The amount of required inputs is minimal compared to numerical methods.

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Features

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

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.

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).

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.

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.

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.

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

TWODAN 5.0 has a seamless, Windows-standard user interface. It has been designed to be 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 spreadsheet-like grids. 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.

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.

Your plots can include any of the following:

• Contours of heat, potential, or stream function

• Pathlines can be traced upstream or downstream from single points, a series of points along a line, or a series of points around a circle (useful for defining the capture zone of a well). Arrows along pathlines are spaced at user-defined time intervals.

• The DXF basemap

• A layout of model elements

TWODAN plots can be output to a huge array of devices -- all those supported by the Windows operating system. Graphic plots may be 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 now much easier 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 (1 inch=500 feet, for example.). 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.

TWODAN labels contours automatically. You elect 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.

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.

Like many models, TWODAN will compile a list of target heads, model-calculated heads, and differences between these. In addition, TWODAN 5.0 has a very useful feature that plots the spatial distribution of the differences. TWODAN also calculates these summary calibration statistics:

• Mean error

• Sum of the squared differences

Some other interface features deserve at least a brief mention:

• Report-ready ASCII text output file summarizing the model inputs

• Sum the aquifer discharge across a polyline. Digitize a multi-segment polyline, and TWODAN will calculate the discharge across this line. Handy for remediation design

• Sum well or linesink discharges. Quickly sum the discharge of a wellfield or of a surface water defined by constant-head linesinks

• Examine the analytic solution at a point. You digitize a point, and TWODAN calculates head, gradient, transmissivity, aquifer discharge, potential, and stream function at a point

• Create a hydrograph of head vs. time at a point. Useful for models using transient well solutions. The output file is a comma-delimited list of head, time that can be imported into spreadsheet and other graphics programs for nice plotting

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System Requirements

• 32-bit Windows operating system (Windows 95 or Windows NT)

• 4Mb of hard disk space

• 8Mb of memory

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