Minimum Layer Thickness (with fixed top elevation) Commands

This post describes how to use the minimum layer thickness and minimum layer thickness fixed top elevation commands to enforce nodal elevation rules when building your model mesh or grid. These commands help prevent pinchouts, which occur when nodes in upper layers have lower elevations than those in layers beneath them—a common issue when working with large surface data files in complex geological systems.

We find these commands particularly useful for maintaining geological consistency in HydroGeoSphere models. A sample problem demonstrating their application, based on the Abdul verification problem, is available for download. In this example, pinchouts are identified and corrected using both commands, illustrating their effects on model grid adjustments.

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Figure 1: Command description (Minimum layer thickness)

This week we’re highlighting a pair of commands that can be helpful when building your model mesh/grid. The two commands (minimum layer thickness and minimum layer thickness with fixed top elevation) can be used to enforce nodal elevation rules. In other words, nodes in upper layer should always maintain an elevation greater than those in lower layers. When this basic rule is violated it’s known as a pinchout. Pinchouts can be a common problem when using large surface data files (i.e. raster, ASCII) to define layer elevations in complex geological systems.

Minimum Layer Thickness

Figure 2: Layer pinchout

The reference manual entry for the minimum layer thickness command is shown in Figure 1, and we can see that this command requires a single instruction (min_thick) to specify the desired minimum thickness.

A simple 2-D cross-section is shown in Figure 2 to demonstrate a pinchout. Applying the minimum layer thickness command to the cross-section in Figure 2 results in a correction to the ‘Surface’ nodes within the pinchout zone. The new ‘Surface’ node elevations are set to the elevation of the associated node in the layers base (Layer 1 top) elevation plus the specified ‘min_thick’ value (Figure 3).

Figure 3: Pinchout resolved by increasing elevation of upper node-sheet

Minimum Layer Thickness with Fixed Top Elevation

In situations where it is preferable to maintain the original elevation of the layer top you may consider using the minimum layer thickness with fixed top elevation command. This command is pretty similar to the minimum layer thickness command except for two important differences:

Figure 4: Command description (Minimum layer thickness with fixed top elevation)

1. The base elevation of each impacted layer will be altered, as opposed to altering the layer top elevation.

2. This command impacts all layers beneath the pinchout if the top elevation is less than the base elevation.
   a. In other words, it impacts all layers affected by the pinchout, and leaves the remaining layers alone.

The reference manual entry for the minimum layer thickness command is shown in Figure 4, and we can see that this command requires a single instruction (min_thick) to specify the desired minimum thickness.

Another simple 2-D cross-section is shown in Figure 5 to demonstrate the minimum layer thickness with fixed top elevation command. In this case we can see that surface layer pinches out the top of layer 1 and layer 2.

Figure 5: Layer pinchout

Applying the minimum layer thickness with fixed top elevation command to the cross-section above results in a correction to the ‘Layer 1 top’ and ‘Layer 2 top’ nodes within the pinchout zone. The new node elevations for ‘Layer 2 top’ are set to the elevation of the surface node elevation minus the specified ‘min_thick’ value. The new node elevations for ‘Layer 1 top’ are treated the same way, except using ‘Layer 2 top’ as the reference point (Figure 6).

Figure 6: Pinchout resolved by decreasing elevation of lower node-sheets

Note: since the minimum layer thickness with fixed top elevation command applies to all layers it must be applied outside the new layer command block, but inside the generate layers interactive…end command block. If you suspect that your layer elevations may result in a pinchout the best practice would be to include the minimum layer thickness with fixed top elevation command before assigning any individual layer elevations.

!minimum layer thickness with fixed top elevation
    !0.1

    new layer
        layer name
        Middle layer

        uniform sublayering
        5

        !minimum layer thickness
        !0.1
        
        elevation from raster file
        ./mesh/abdul_dem_constant_3_5.asc

        offset top
        -0.5
    End

Figure 7: Grok.exe displays error due to layer pinchout

Sample Problem: ‘Abdul_AlgoMesh – minimum layer thickness’

To help demonstrate these commands, a sample problem based on the familiar ‘Abdul’ verification can be downloaded here: Abdul_AlgoMesh - minimum layer thickness.zip

Figure 8: Resulting layer distribution with “minimum layer thickness” command

In this example the Abdul domain has been recreated using AlgoMesh and the elevation of the middle layer top has been set to a constant elevation of 3.5m. As you can see from the code snippet above, the minimum layer thickness and minimum layer thickness with fixed top elevation commands have both been included in the code, but are commented out (also note that minimum layer thickness with fixed top elevation appears outside the new layer code block).

If a pinchout is encountered when you execute your grok script you should see a warning like the one in Figure 7 (generated by running the unaltered grok script included in the sample problem):

If you activate the minimum layer thickness command then grok will execute successfully, and a portion of the resulting grid will look something like Figure 8 (note how the top layer has been pushed up above 3.5m elevation in some areas):

Figure 9: Resulting layer distribution with “minimum layer thickness with fixed top elevation” command

If instead the minminimum layer thickness with fixed top elevation command is activated the same area of the grid will look like Figure 9 (note how the maximum grid elevation is 3.5m throughout, and the layers beneath have been lowered to accommodate).

Note: Tecplot layout/data files have been included in the example problem so you can easily recreate these cross-sections yourself.