GMS

Working with Inactive Cells in MODFLOW 6

By aquaveo on February 14, 2023

When working with a MODFLOW 6 model in the Groundwater Modeling System (GMS) it can be important to mark specific cells inside a project as inactive to keep it out of the computational domain. Using inactive cells can speed up the performance and accuracy of your model. Incorrectly designating cells as inactive or active can cause inaccuracies in your model or keep it from converging. Here is some guidance for working with inactive cells that will help create a better MODFLOW 6 model.

Modflow 6 model with inaccurate z values

When choosing which cells to mark as inactive in GMS, it is helpful to know how to find information about the cells in the first place, as well as what information MODFLOW 6 is using to calculate the X, Y, Z, and S values of that cell. The display above the graphics window that shows the values of the selected cell is using the cell center for its calculated values. If the inactive cells have a low elevation that is inaccurate for the model, it will drag the Z value for the active cells on the edge down to meet the inactive cell. This happens because the outside active cell has nodes in the Z value that still need something to attach to as there cannot be gaps between the nodes. This will distort the cells in unintended ways and create errors in the model.

MODFLOW 6 uses IBOUND to determine which cells to mark as active and which to mark as inactive. Marking a cell with an IBOUND value of zero will make it inactive.

There are a few ways to inactivate cells in a MODFLOW 6 model. The first is to create a polygon that will separate the active and inactive cells. Select the polygon that contains the cells that are to remain active. Go to the Feature Objects menu and select Activate Cells in Coverage(s). This will automatically make any cells that aren’t selective inactive.

The second way is to select a group of cells with the cell selection tool. Then go to the MODFLOW menu | Advanced | Cell Properties… and change the IBOUND dropdown menu to inactive. This will inactivate only the cells that have been selected in the graphics window.

The cells can also be set as inactive by editing the IBOUND Array directly.The IBOUND dialog can be found by clicking on Global Options under the MODFLOW menu. There is an IBOUND button in this dialog window where the IJK coordinates can be set to zero manually.

Making use of inactive cells can improve you MODFLOW 6 model. Try out MODFLOW 6 in GMS today!

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GMS

Converting Between 2D and 3D Unstructured Grids

By aquaveo on January 17, 2023

Many projects in the Groundwater Modeling System (GMS) start with an unstructured grid (UGrid). GMS supports both 2D and 3D UGrids. The type of UGrid you will need to use will depend on which numeric model you are using for your project. For this reason you may need to change a 2D UGrid to a 3D UGrid or vice versa. This blog post is going to review some of the tools for converting between 2D and 3D UGrids.

Typically, models like MODFLOW-USG or MODFLOW 6 will require a 3D UGrid. If you only have 2D UGrid data, this may require taking a 2D UGrid and converting it to be a 3D UGrid. Likewise, you may need to convert a 3D UGrid to be a 2D UGrid. This is more often done to correct uses with the 3D UGrid, but also to be able to use the data from the 3D UGrid with other applications.

GMS provides two tools in the Toolbox for changing between 2D and 3D UGrids: the Extrude to 3D UGrid tool and the UGrid 2D from UGrid 3D tool. The Toolbox is accessible through the Toolbox macro and these two tools are located with the Unstructured Grids tools.

Example of the Unstructured Grid tools in GMS

The Extrude to 3D UGrid tool converts a 2D UGrid to a 3D UGrid. The tool will allow you to select a 2D UGrid for extrusion, and then it lets you specify the number of layers and layer thickness for each of the new layers. This will extrude the 2D Ugrid to be a 3D UGrid with the specified number of layers that are at a uniform thickness. If you need layers to be at different thicknesses, you will need to edit the 3D UGrid using other tools in GMS.

The UGrid 2D from UGrid 3D tool will collapse a 3D UGrid to be a 2D UGrid. The tool allows you to select a 3D UGrid then select if the 2D UGrid should be created from the top or bottom of the 3D UGrid layers.

The unstructured grid tools in GMS allow you to have more options with how you work with UGrids and grid data for your groundwater projects. Try out the unstructured grid tools in GMS today!

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GMS

Using the MODFLOW HFB Package

By aquaveo on December 27, 2022

Sometimes in a MODFLOW simulation, you need to simulate very thin barriers to flow that aren't accurately represented by assigning values to entire cells. Fortunately, MODFLOW has the Horizontal Flow Barrier (HFB) package that facilitates accurately modeling thin flow barriers. Today, we explore how the HFB package can meet your needs, and how it functions.

The HFB package can meet your need for a more realistic approach to simulating horizontal barriers in your model. Whereas many packages in MODFLOW assign values to entire cells, that might poorly reflect reality for horizontal flow barriers with negligible width. These barriers might include slurry walls, sheet pile walls, or diaphragm walls around wells. Instead of assigning values to whole cells, the HFB package uses cell boundaries to simulate horizontal barriers. Doing so can more accurately reflect the actual situation.

To use cell boundaries to simulate horizontal flow barriers, the HFB package uses a hydraulic characteristic. You calculate the hydraulic characteristic by dividing the hydraulic conductivity of the barrier by the real-life width of the barrier. This value is assigned to cell boundaries. Then, MODFLOW uses that value to modify the regular flow between cells. Thus, you get modified flow at the cell boundaries that have a defined hydraulic characteristic.

The following is a suggested workflow for using the HFB package:

Make sure that the HFB package is turned on in the MODFLOW Packages / Processes dialog.
Set up a coverage that can include a barrier by checking Barrier in the Coverage Setup. Define the layers that the barrier affects using the Default layer range in the Coverage Setup.
Draw an arc representing the barrier. In the Attributes table for that arc, set its boundary condition to "barrier". Define its Hydraulic characteristic as you have calculated it.
Map from that coverage to MODFLOW.

The values for the HFB package can be edited using the HFB - Horizontal Flow Barrier command in the MODFLOW menu.

While using the HFB package, keep the following in mind:

There are certain assumptions that this package uses to function. It's assumed the barrier has no storage capacity. It's also assumed the barrier has negligible width. Therefore, the HFB package's sole function is to reduce conductance between adjacent horizontal cells.
This blog post primarily applies to standard MODFLOW versions. The HFB package is also available for MODFLOW 6.

Try out the HFB package in GMS today!

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GMS

Exporting Contour Shapefiles from a 3D UGrid

By aquaveo on December 7, 2022

Have you ever wanted to export the linear contours on a 3D UGrid as a shapefile? By exporting GMS contours as shapefiles, you can open contour shapes in other GIS applications or in other GMS projects. GMS provides functionality for exporting UGrid contours to shapefiles. This article gives some guidance for using this functionality.

In GMS, linear contours display on the top and the bottom of the 3D UGrid. This can result in contour shapefiles with undesired shapes that make contours unclear. Fortunately, GMS 10.7 can now display just one cell face (top or bottom) of single layers in a 3D UGrid. This enables exporting cleaner contour shapefiles from one face of a layer in a UGrid.

For example, GMS can display just the top of layer 1, or it can display just the bottom of layer 3. This can make for cleaner contour shapefiles because the files will have contours from only one face of the 3D UGrid. This is useful if you want the contours from a UGrid but only need the top- or bottommost contours.

Example of exporting UGrid contours to a shapefile

Do the following to export contour lines from one face of a UGrid layer as shapefiles:

Have a 3D UGrid visible in the Graphics Window. Make sure that it has a dataset that can be visualized using contours.
Activate the desired dataset.
Set the contours to linear in the dataset Display Options.
Turn on Single layer in the UGrid Single Layer toolbar and select the desired layer. By default, the UGrid Single Layer toolbar is to the right of the XYZS Bar at the top of the GMS window.
Adjust the drop-down in the UGrid Single Layer toolbar to show the cell faces desired: top, bottom, or all. GMS displays the top faces by default. This also adjusts which contours display. Now you have a certain set of contours singled out. These will be made into contour shapefiles.
Right-click on the active UGrid and select Export.
In the Export UGrid dialog that appears, select "Arc Shapefile of Linear Contours (*.shp)" from the Save as type drop-down and save the contour shapefile with the desired name.

If you move the shapefiles, make sure to move them with all of their accompanying files. That includes the files ending in "*.dbf", "*.prj", and "*.shx".

There may be breaks in the displayed faces of the UGrid when only showing the top or bottom face of a layer. This is because some adjacent cells are joined only by vertical cell faces; the edges of the top or bottom of the cells don't touch. Thus, when the vertical cell faces are hidden, there is only empty space. This often affects the display of the contours and the contour shapefiles.

Try exporting your UGrid contours as shapefiles in GMS 10.7 today!

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GMS