Lab 3. Flownet Modeling

Purpose: Use computer software FLOWNET to model 2-D steady state saturated groundwater flow in homogeneous and heterogeneous media. We will also examine how the relief of the undulating (wavy) topography and water tables may affect subsurface groundwater flow.

Reading Assignment

Fetter, p. 275-287.


Open the Computer software "flownetd"

First open the executable program "flownetd" from DOS. You need exit Windows in order to use MS-DOS. Choose the MS-DOS Prompt icon from the Main group. By doing this, when you return to Windows, your previously open windows and files remain unchanged.

As you are using MS-DOS, after the command prompt C:\>, type

cd \flownet

this will bring you to the 'flownet' directory, you will see the following

C:\flownet>

Now type

graphics

so you can print out the screen display, as we will do later.

Now run the program, type

flownetd

Once the program is running, the instructions are on the screen. (The computer software can be downloaded from the floppy disk included in Fetter's textbook, see Appendix 15 on p. 632 for information of loading and running three programs - FLOWNET, QUICKFLOW, and AQTESOLV).

The main program control keys are PG UP and PG DOWN. The program is, like a booklet, subdivided into several (7) pages. PG DN moves program control to the next page from 1 to 7. PG UP moves program control on page back. Page 1 is introduction. Pages 2 through 6 use alphanumerical keys for data entry to define model parameters, hydraulic head, hydraulic conductivity, drawing parameters, and calculation and drawing of the flownet .

Exercise 1 (homogeneous media):

We will first model the regional flow system in an area of sloping linear topography and water table (Fig. 8.3 of Fetter), presented by Toth (1962). If you simply page through all seven pages of the program (by PG DN key) without changing any parameters, the program will calculate the Toth model (this is a test case). The default model has closed boundaries on all sides but the top. The fixed hydraulic heads on the top are displayed at a relative scale along the top open boundary.

Once the flownet is on the screen, print the flownet by pressing Shift and Print Screen keys (at the same time). We can also do a simple "animation" to trace the "water molecule" movement along the flow paths. First press the PG UP key once; then use the Down-Arrow key to move the little arrow to the line marked "Time steps are not drawn". Press the Right-arrow key, and the line will change to "Time steps are drawn". Now press the PG DN key, and the flow net will reappear with little dots on the flow lines. Press the space bar and the little dots will begin to move. Use the "+" key to make them move faster and the "-" key to make them move slowly.


Exercise 2 (heterogeneous media):

Based on the default (Toth model) solution, place a horizontal sand lens with horizontal conductivity (Kh) and vertical conductivity (Kv) equal to 1000 and effective porosity equal to 0.3. The sand lens is located in the center of the cross-section, and the size of the lens is 6 (columns) _ 2 (rows).

Conductivity values are entered on page 5. The 'matrix' (shown as x) on this page represents rows and columns. There is a blinking cursor that can be moved with the Arrow keys. To put in a layer of different hydraulic conductivity, move the cursor to the point where the layer starts (upper left corner of the layer). Then type any letter except (lowercase) x, for example, b. You then enter the new horizontal and vertical hydraulic conductivity values (1000 for this exercise) and porosity for the new symbol, using the Down-arrow key to move from one value to the next. After enter the porosity and press the Down-arrow key, the program will return to the hydraulic conductivity matrix. The blinking cursor is at the same place. Type b to access the newly defined symbol. Move the cursor with the Arrow keys to encompass the area covered by the different conductivity layer (by doing these, you can avoid typing the same conductivity values for all node points included in the layer). This can be done more than one row thick. When you have entered the position of the layer, press x to return the cursor to the default value. Using the method you can enter several layers with different conductivity values. Once the conductivity values are entered, press PG DN twice to redraw the flownet. Print the flownet and comment on the flow system. (5 pts)


Exercise 3 (Uneven topography)

Next we will examine how the relief of the undulating (wavy) topography and water tables may affect subsurface groundwater flow. In some basins, both local and regional flow systems may exist, while in other basin with a similar depth/length ratio but with a more pronounced water-table relief, only (deep) local flow systems develop (see Figure 8.5 of Fetter for illustration).

Run the program "flownetd" and reload the default file "test" (on page 3 you can load or save a model data by press F1 function key). On page 3, change "number of column" to 20 and "number of row" to 10. Move the cursor down and change "model length (m) is" to 1000 and "model height (m)" to 200.

Next we need to specify hydraulic heads along the top boundary. The hydraulic head of columns/rows can be a value, or functions dependent of the column/row number x. Press PG Dn key to move to page 4. You will see current values of head along the top boundary. Press F1 and the control will move to the function entry page. Specify "Value or function of a formula is" to "sinx - x/3". This function will created an "wavy" water table configuration along the top boundary ("imagine" this configuration before drawing the flownet). Note that "starts at column/row nr. x = " is set to 1 and "ends at column/row nr. x = " is set to 20. Don't change these two indices. These two indices determine the formula range (which will cover the entire top boundary). Now press F1 again the program will go back to page 4 and automatically update the new hydraulic head data along the top boundary (notice the head data has been updated).

Press PG Dn three times to redraw the flownet. Initiate an "animation" to trace the "water molecule" movement along the flow paths. Print the flownet and Comment on the flow system. Do both local and regional flow systems exist? (2 pts).

Now we increase the amplitude of the undulations of the water table. Go back to page 4 and specify a new function "3*sinx-x/3" (Remember to press F1 again). Redraw the flownet. Initiate an "animation" to trace the "water molecule" movement along the flow paths. Print the flownet and comment on the flow system. Do both local and regional flow systems exist? (3 pts)