Geology 724
Problem Set #4
Hubbertville and The Green Swamp - Steady State Analysis
This is the first part of a two part problem set involving the Hubberville/Green Swamp problem. In
part 2 (Problem Set 5) we will solve a transient version of the problem.
In Problem Set #4, we will use MODFLOW with Groundwater Vistas to solve problems 4.1a, 4.1c,
4.3 and 4.6 on p. 143-145 in Anderson and Woessner (A&W).
Recommended Option
It is a good idea to become familiar with MODFLOW’s input file structure. Groundwater Vistas
(GWV) automatically generates the input files for you. For this problem, these include: bas, bcf,
rch, sip, oc and also wel when you include pumping.. Take a look at each of the input files that
GWV creates. You will find a description of the data in each of the input files in the input
instructions for MODFLOW on the course web page. It is recommended that you solve the
problem by running MODFLOW outside Groundwater Vistas in a dos box. You can import the
results of the run into GWV and use GWV to view the results.
General Tips for running GV
In GV there are 3 places you need to update to store your files properly:
-- Model>Modflow>packages: choose a root file name for your simulation.
-- File>save as: choose the name for the gwv file.
-- Model>paths to models: choose the directory where files created by GV will be stored.
Grid Design
Use a regular grid with nodal spacing equal to 500 m. This will give you a grid with 9 columns and
21 rows. (Remember that in a block-centered grid, flow boundaries are located at the edge of the
finite difference cell and specified head boundaries are located directly on the node.)
Tips on Model Design
Prob. 4.1a (no pumping). Set the steady state head closure criterion to 1E-5. Be sure to set the
head at the no flow cells in the basic package (HNOFO) equal to some number that does not fall
within the range of heads calculated for the active cells, i.e., set HNOFLO to something outside of
the range 900-1020. This is done so that GV will not color in one of your active cells as a no flow
cell.
Prob. 4.1c (include pumping). Add the well using the BC option in GV.
Prob. 4.3 (include pumping). Add the river package under BC in GV. Set the length of the river
to 500 m, which is the length within each of the 9 cells. Conductance is calculated for each cell;
you need to specify the width and length of the river in a cell. Include pumping. Be sure to check
the Modflow>packages window and make sure that non-zero unit numbers are entered for the bcf,
well and river packages in the "cell-by-cell flow" column.
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You will find that when you run this simulation with an error tolerance (HCLOSE) of 1E-5, GV
will say that the solution has not converged after 50 iterations and ask if you want to go ahead. Say
yes. The solution is actually correct even though it hasn't converged. You can check the water
budget and you will see that GV gives a good water balance error. Or, if you prefer, you can set
HCLOSE equal to 1E-4 in the Solver package.
Prob. 4.6 (include pumping): You can experiment with grid spacing (i.e., you can use
irregular grid spacing) in order to keep the problem domain 4500m wide. One way to modify grid
spacing is with grid>edit. Neither MODFLOW nor GV accounts for fluxes between specified
heads along a boundary. Normally we don't care about this. But in this problem there is flow out of
the first and 9th column into the swamp/marsh. This water is not accounted for in the mass balance
summary and if you look for this flux with the cursor it will register as zero. You should
compute it by hand using Darcy's law and add it into the amount of water flowing into the
swamp/marsh.
Tips on Analyzing Results
Prob. 4.1a. After you run this simulation, take a look at the MODFLOW output file you created.
Note the listing of head residuals for each iteration. Also look at the water budget at the end of the
output. There are two sets of columns. The first gives cumulative volumes and the second gives
rates. Normally, you want the second set (L**3/T). Now notice that there are two sets of numbers:
one listing the amount of water entering the model (IN) and the other listing the amount leaving the
model (OUT). For example, under the label “constant head boundary”, there will be two numbers:
the total going into all the constant head nodes (i.e. leaving the model) and the total leaving all the
constant heads nodes (i.e. entering the model). For Prob. 4.1a where both the swamp and the river
are represented as constant head nodes, the flows are lumped. However, if you set up your output
control file properly, you should find a listing of cell-by-cell flows for each constant head cell
printed out earlier in your output file.
In Groundwater Vistas, view the model’s water balance in the Plot>Mass balance>model window.
You can also use the cursor to view fluxes to the swamp on the screen. Click on BC>constant
head. Then move the cursor to a constant head cell and watch for the flux to appear in the lower
left hand corner of the screen.
N-S profiles. GV will graph the head profiles in the cross section that is displayed on the screen.
You can change the orientation of the cross section in the XSect window. Plot the cross section by
using the Plot>profile window. You may want to change the orientation of the plot on the page by
clicking on file>print>properties and changing from portrait to landscape orientation.
Looking at heads. If you want to see the point values of heads printed out, you can get them
printed out into the MODFLOW output file (*.out) by removing the check from the "disable
printing of heads" box that you will find in the MODFLOW>output control window. Also
be sure to select a good format for the printing. Use either 15F7.2 or 15F7.3, which will give you
either 2 or 3 numbers after the decimal point. Also you can export the heads to an ASCII file to use
in Surfer (or other software packages) by using File>export.
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Write-up. Your write-up should be brief but complete and should include the following sections.
Problem Description. State the modeling objective, which is to test the effect of boundary
conditions on the prediction of discharge to Green Swamp as it is affected by future pumping from
a proposed water supply well. The boundaries considered are: a) the internal groundwater divide
between the river and the swamp (prob. 4.1c); b) the river (prob. 4.3); c) the Slate Mountains (prob.
4.6). Describe the conceptual model. Show your grid and label the pumping well and the boundary
nodes. Present the mathematical model for the boundary conditions of 4.1a. Identify the code used
to solve the problem.
Results. Present the results of the steady-state analysis (Problem 4.1 a, c). Show your results in the
form of N-S profiles of head in a cross section that goes through the pumping node. Present a
water-table contour map and a table of the discharge rate to The Green Swamp.
Sensitivity Analysis. Tabulate the discharge to the swamp and the head at the pumping node for
problems 4.1c, 4.3a,b and 4.6b. Compare with the results from problem 4.1c. Discuss the effects
of the changed boundary conditions on the discharge to the swamp.
Discussion. Discuss the effect of pumping on discharge to the Green Swamp.
Conclusion. Make some conclusions about your modeling work as it relates to the modeling
objective.