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)