Example Problem 4, A Water Supply Problem

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Example Problem 5, Landfill

Introduction

A fully worked-out version of Example 4 is in the "Examples\example4" directory. The files include a fully worked-out model for this example named "Watsup1.mmb",a map of the region named WatSup.DXF, and a two sets of Elevations that you can import into Argus ONE to define the top and bottom of the aquifer.They are named Elevation_Bottom_Unit1.exp and Elevation_Top_Unit1.exp.

In this example we will look at a site where a well test was performed near a river and a no-flow boundary (the edge of the aquifer). Because the boundaries were nearby, the assumptions underlying the usual methods for analyzing the pumping tests were not met. By using MODFLOW, we can calculate the expected drawdown at the observation wells and compare them to the observed drawdown. We can then modify our assumed hydraulic conductivity and specific storage to better match the observations.

This example will also illustrate the use of several other tools to prepare MODFLOW input files.


Model Characteristics

First, create a new MODFLOW project. On the Project tab select MODFLOW-96. On the Geology tab change the model to a 1-layer, confined aquifer. On the Packages|Stresses 1 tab, check "river" and "well". On the time tab change the length of the stress period to "129600 seconds" (1.5 days). Change the number of time steps to 10. Change the multiplier to 1.414 (square root of 2). Change to transient flow. On the output tab change the file root to WatSup and change the times to record overall volumetric budget to every time step. Finally on the Packages|Solvers tab, change RCLOSE to 0.01 and click on the OK button.


Importing DXF Files

Because the area to be modeled is relatively complex, we will need some sort of guide when designing the model. There is a DXF file of the model area that you can use as a guide. To import the DXF file, first Switch to "Maps". Then select "File | Import Maps | DXF File..." You will be prompted for a file name. The file is named WatSup.DXF. You will be prompted to change the scale to match the scale of the diagram. Accept the suggested scale.


Resizing the DXF Map

This particular DXF file is not scaled in real world units so you will have to scale it. First, select "Special | Scale and Units" and change the scale so that distances will be in feet. Notice that on the DXF figure there is a 500 ft. scale in the lower right corner. Zoom in on this and compare its length to the units shown on the ruler bars. To zoom in, select the Zoom button in the lower left corner. (Of the three buttons there, it is the longest one.) The cursor will change to a magnifying glass. Outline the scale with the magnifying glass and the view will zoom in to it. Make a rough guess how much the scale will have to be expanded. (It will probably be about 33x.)

You can scale the layer so that it will be closer to the 500 feet it should be. Select "Special | Rotate and Scale". Then change "Scale X" to 3300% or to your estimate of how much the DXF file needs to be expanded. Under "Scale" select "Current layer" or "Entire document". Click the OK button. Now the diagram is so much bigger that probably none of it is on the screen. To see it, zoom out. Most of it will be off the drawing area. You can change the drawing size to fit the diagram. I ended up with a horizontal extent of 10000 a vertical extent of 6000, a horizontal origin of -5000 and a vertical origin of -3000. You will need to zoom out some more to see the entire diagram. The "Navigation | Zoom to Fit" would be the easieset way to zoom out. At this point the diagram is probably close to the right size but may still need some adjustment. Zoom in on the ruler and scale the layer as before until you are satisfied that the scale is sufficiently accurate.

Once you are satisfied with the scale, zoom out. You'll see that it shows the position of a pumping well, three observation wells, a river, and three zones of varying aquifer thickness. The area surrounded by the light blue line was modeled as 40 ft thick, in the original model. (See the legend on the diagram if you are unsure which line this is.) In the slightly larger, dark-blue zone around it, the thickness was modeled as 20 feet and in the rest of the area the thickness was modeled as approximately 10 ft.

 

Determining Aquifer Thickness Through Interpolation of Point Values

In the original model, there were three zones with differing thicknesses. We could reproduce that exactly by making the top of the layer 0 and the bottom of the layer either -10, -20, or -40. We could just outline the appropriate areas with closed contours on the Elev Bottom Unit1 layer to assign the appropriate bottom elevations. However, the original model was probably set up the way it was, not because those thicknesses were appropriate, but because it would have been much too difficult to calculate appropriate thicknesses for every cell with the tools available at the time. With Argus ONE, that limitation no longer applies.

We will enter point values for the top and bottom of the layer and allow Argus ONE to calculate the aquifer thicknesses for us. The only information we have for the top and bottom of the aquifer is a single cross section (Figure 19.2 Andersen, 1993). For this exercise, I have assumed that other parallel cross sections would be similar in shape. In real examples, you would probably use data from borings to determine the vertical positions of the aquifer boundaries. I estimated that the points shown in the illustrations below would provide an acceptable estimate of the aquifer configuration.

To enter these point values, switch to the "Elevation Top Unit1" layer, select the Point tool, and click where you want to assign a value. If the point tool is not visible, click on the open contour or closed contour tool. Hold the mouse button down until a popup menu appears, and select the point tool from it. When you are done, select all the points and copy them to the "Elevation Bottom Unit1" layer. (You can easily select all the points with the "Edit | Select All" menu item.) Now edit each of the points on the "Elevation Bottom Unit1" layer so that the bottom of the layer is 10, 20, or 40 feet below the top depending on each point's location.

If you don't feel like entering all these points yourself, you can just import the points I used. There are two ways to do this.

The points you import may not correspond to their correct locations on the map. Go to the map layer, select all the contours and move them so they lines up correctly with the points you have imported. You can move all the contours, once the are selected, by clicking on a contour and dragging. Don't click on a black square and attempt to drag. That will edit one of the contours rather than moving them.

To make Argus ONE interpolate between points, you must tell it to interpolate. You do this by opening the Layers dialog box, (View|Layers...) and changing the choice in the bottom of the dialog box where it says "When probed for values use:" to "Interpolation method". With the current version of the MODFLOW PIE (version 4.0.0.0). The "624 Interpolation" method is the default method for top and bottom elevations. It is possible that a different method will be the default in a future version of the MODFLOW PIE.


Transferring Data from the DXF Map to the Model

On the Maps layer, select the Arrow or Lasso tool and outline the area to be modeled to select all the lines that define the model boundary. Copy them to the clipboard (Edit | Copy). Switch to the MODFLOW Domain Outline layer. Paste the contours from the clipboard to the MODFLOW Domain Outline layer there (Edit | Paste). Some of the contours lie exactly on top of one another so you will get an error message telling you that some of the contours weren't transferred. If the model domain outline was transferred, you will need to get rid of all the other extra lines. You can get select lines in the interior by just clicking on them and deleting them (Edit | Delete). You can also use the lasso tool to outline contours and delete them. You select the lasso tool by clicking on the button next to the arrow tool.

If the line representing the domain outline doesn't get copied, make a new map layer, copy the contours from the DXF layer to the map layer and edit them there until you just have the domain outline you want. Copy that to the MODFLOW Domain Outline layer.

To create a new map layer, open the Layers Dialog box (View|Layers...). select the Maps layer and click on the "Duplicate" button. (You could also click on the "New" button and then click on "Information" under "Type" and select "Map" from the popup menu.)


The Layers Floater

You may wish to hide the Map layer. One good way to do this is to use the Layers floater. Select "View | Show Layers Window". The Layers floater allows you to easily switch between layers and to hide or show them.

 

Hiding Contour Values and Labels

It may be useful to hide values associated with contours so they don't completely cover some of the smaller contours. Do that by selecting "View | Show Contour Value". If contour values are shown, there will be a check mark next to that menu item. If not, the check mark will be absent. There are also buttons on the layers floater that you can use to show and hide labels and other properties of contours.

Once you get rid of all the contours in the interior, you should show the contour values again. You will see that there are still many contours. There is one long contour that surrounds the model domain and many shorter ones that lie exactly under or over it. If you select any path on the exterior, you will choose one of these lines. If the contour you select surrounds the model domain, select it and cut it to the clipboard (Edit | Cut). Delete the remaining lines, and paste the contour on the clipboard back onto the MODFLOW Domain Outline layer. If the contour you select makes up only part of the boundary, delete it. When you are left with only a single contour, assign it a "Density" of 125. We will use a uniform grid for this problem so there is no need to use the MODFLOW Grid Density layer.

There is a much simpler way of getting rid of all the extraneous contours. The contour that surrounds the model domain is a closed contour. On information layers, closed contours can be selected by clicking anywhere in their interior. Create a new information layer. On the new layer select "Special | Allow Intersection". Copy the contours from the DXF file to the new layer. Unselect all the contours and then click in the interior of the model domain to select a closed contour. If it is the one that surrounds the model, cut it to the clipboard, and paste it on the MODFLOW Domain Outline layer. Then delete the new layer.


Setting Other Model Features

Go to the MODFLOW FD Grid layer, select the Magic Wand tool and click it inside the model domain to make the grid. Uncheck the automatic angle calculation for this example if you wish to reproduce this example as closely as possible.

Next, go to the Initial Head layer and assign it an initial head of 30 feet. The easiest way to do this is with the Expression editor. Bring up the Layers Dialog box (View | Layers...). Click on Initial Head Unit1 in the upper half of the dialog box. Then click on the arrow under value to bring up a popup menu with a single choice; "Expression". Click on "Expression" to bring up the Expression editor. Enter a 30 in the top half of the expression editor.

You may have noticed that, in some cases, the bottom of the layers will be higher than 30 feet. What will happen to them when you assign cells a head that is lower than the bottom of the aquifer? For a fully confined aquifer, MODFLOW will not be given a value for the aquifer bottom. It will only get the aquifer transmissivity so the model will run as if that portion of the aquifer was always fully saturated. We are running the model this way to better match the original model on which this example is based. You could, however, change the aquifer type from "Confined" to "Convertible" on the geology tab of the Project Info Dialog box. (If you did this, you would also need to assign a specific yield to the layer.) Under these conditions, the part of the convertible layer in which the initial head was lower than the aquifer bottom would go dry immediately. To avoid that, you could make the model more realistic by assigning a varying initial head across the model domain.

You can assign default values for the horizontal hydraulic conductivity (0.00685) and specific storage (0.0015) Next, switch to the "Wells Unit1" layer. Use the point tool to put a well in the position of the pumping well. Be sure that the Map layer is visible so you know where to put it. The pumping well is the unlabeled circle. Its pumping rate should be -0.963 cfs. Its top and bottom elevation should be specified so that it is entirely within the aquifer. I used values of 10 and -10 respectively for the top and bottom of the well screen. We don't need to do anything about the observation wells because they do not affect the flow field.

Finally, we should add the river. First, switch to the "Maps" layer and select the river. It is the black line. Copy the river to the clipboard by selecting "Edit | Copy" or just press Ctrl and C simultaneously. Now switch to the Line Rivers Unit and paste it from the clipboard by selecting "Edit | Paste" or by pressing Ctrl and V simultaneously. Edit it so that the conductance per unit length is 0.0002, the river bottom elevation is 20 and the Stage Stress is 30.


Running the Model and Viewing Results

You have now completed data entry for the model. You can run it by going to the MODFLOW FD Grid layer and selecting "Pie | Run MODFLOW" as you have done before. (You will get warning messages about the initial heads being lower than the bottom of the cell.)

When you are done, you may wish to look at a contour map of the drawdown in the last time step. You can import the map as before ("Pies | MODFLOW Post Proc..."). The file you will need to open will have the extension ".fdn". When you have made the contour map, you will probably want to change the contour interval to a more appropriate value. Change to the "MODFLOW Post Processing Charts" layer and click on the contour map. Change the maximum to 2.5, the minimum to 0 and the contour interval to 0.25. You should get a map similar to the one shown below.

To determine whether the modeled aquifer properties are sufficiently reliable, you could compare the observed heads in each of the observation wells with the modeled heads.

Summary

This example illustrates several more useful methods for entering data for a MODFLOW model using Argus ONE and the MODFLOW PIE.

Example Problem 5, Landfill
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