Seepage Modeling with SEEP/W - GeoStudio 2007 version 7.22

Chapter 2: Numerical Modeling
SEEP/W
It is unrealistic to dump all your information into a numerical model at the start of an analysis project and
magically obtain beautiful, logical and reasonable solutions. It is vitally important to not start with this
expectation. You will likely have a very unhappy modeling experience if you follow this approach.
Do numerical experiments
Interpreting the results of numerical models sometimes requires doing numerical experiments. This is
particularly true if you are uncertain as to whether the results are reasonable. This approach also helps
with understanding and learning how a particular feature operates. The idea is to set up a simple problem
for which you can create a hand calculated solution.
Consider the following example. You are uncertain about the results from a flux section or the meaning of
a computed boundary flux. To help satisfy this lack of understanding, you could do a numerical
experiment on a simple 1D case as shown in Figure 2-18. The total head difference is 1 m and the
conductivity is 1 m/day. The gradient under steady state conditions is the head difference divided by the
length, making the gradient 0.1. The resulting total flow through the system is the cross sectional area
times the gradient which should be 0.3 m 3 /day. The flux section that goes through the entire section
confirms this result. There are flux sections through Elements 16 and 18. The flow through each element
is 0.1 m 3 /day, which is correct since each element represents one-third of the area.
Another way to check the computed results is to look at the node information. When a head is specified,
SEEP/W computes the corresponding nodal flux. In SEEP/W these are referred to as boundary flux
values. The computed boundary nodal flux for the same experiment shown in Figure 2-18 on the left at
the top and bottom nodes is 0.05. For the two intermediate nodes, the nodal boundary flux is 0.1 per node.
The total is 0.3, the same as computed by the flux section. Also, the quantities are positive, indicating
flow into the system. The nodal boundary values on the right are the same as on the left, but negative. The
negative sign means flow out of the system.
3
2
6
5
9
8
12
11
15
3.0000e-001
14
18
17
1.0000e-001
21
20
24
23
27
26
30
29
1
4
7
10
13
16
19
22
25
28
1.0000e-001
Figure 2-18 Horizontal flow through three element section
A simple numerical experiment takes only minutes to set up and run, but can be invaluable in confirming
to you how the software works and in helping you interpret the results. There are many benefits: the most
obvious is that it demonstrates the software is functioning properly. You can also see the difference
between a flux section that goes through the entire problem versus a flux section that goes through a
single element. You can see how the boundary nodal fluxes are related to the flux sections. It verifies for
you the meaning of the sign on the boundary nodal fluxes. Fully understanding and comprehending the
results of a simple example like this greatly helps increase your confidence in the interpretation of results
from more complex problems.
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