Troubleshooting Warnings and Errors Encountered During Translation of Boundary Condition Packages

Drain Boundary Conditions

Warning Message: “Drain elevation are below the bottom of the cells. Your model may not converge”

This warning message is related to the drain boundary condition in Visual MODFLOW FLEX, during project translation. Since the message is just a “warning”, it can simply be by-passed and you may continue with your work. It is important however, to understand what the message tells you about your model.
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Cause

With open pit mining applications, it is necessary to estimate how much water must be dewatered for various phases of the pit geometry.  Often this is modeled with MODFLOW Drain boundary conditions. However, MODFLOW does not allow boundary condition cells to change layers at different stress periods (ie. over time).  Instead, this can be accommodated by using drain elevations that change over time, where the drain elevation is set at the estimated future pit face elevation.  For this reason, when you translate a model in VMOD Flex, some drain elevation values may be below the cell bottom (below the layer where the drain boundary condition was assigned). You may see the warning: “Drain elevation are below the bottom of the cells. Your model may not converge”. The particular cells can be inspected in using the column/row view.

This warning is common for the application mentioned above, for which lower drain elevation is intentional.

For example, the image above shows the warning for a drain during translation. The drain was assigned on the third layer with an elevation of 250.

In the row view, hovering the cursor over the third layer cell bottom (layer 3, row 53 and column 16) where the drain was assigned, we can see the elevation (Z) is 271.58. The assigned elevation for the drain (which in this case is 250) is less than the cell bottom at this cell. This is what the warning in the translation log indicates:

drain-2

Constant Head Boundary Conditions

Error Message: “Starting/Ending head must be greater than the cell bottom”. “Starting/Ending head must be greater than the model bottom”

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Cause

The head values defined in the Constant Head (CHD) package must be greater than the cell bottom in the layer to which they are assigned. This is a restriction imposed by MODFLOW, and applies to both the start and the end head. As such, this is displayed as an error that must be addressed before proceeding to the model run.  In VMOD Flex, when you create a constant head boundary condition in the numerical grid, or convert from a conceptual model object, the constant head will be assigned to the appropriate layer based on the head values.  However, there are few cases where this validation/check may fail, such as when you edit the head parameters (eg. through CSV), import from a MODFLOW or VMOD Classic project (where constant heads were not appropriately assigned) or where the conversion from the conceptual model was unable to determine the correct layer.

In the following example, you can see the particular cell (layer 2, row 26, column 69) for which the assigned starting and ending head (210) is less than the actual elevation of the cell bottom (221).

Similarly, when the starting/Ending head is below the model bottom (below the lowest layer of the model), the “Starting/Ending head must be greater than the model bottom” error message shows.

Solution

This can be addressed by finding those particular cells (the translation log will have the particular layer row and column) and editing them in the Define Boundary Condition > layer view > Constant Head: Edit > Single and changing the particular cells starting and ending head to have a value that is greater than the cells minimum Z (elevation).

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For more details…