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Define Child Grid (for Local Grid Refinement (LGR)

A higher-resolution block-shaped child grid can be created within any numerical grid (parent grid) and used for running Local Grid Refinement (LGR) simulations with the MODFLOW-2005 LGR package. Refined child grids are often used to improve simulation accuracy around areas of interest within your simulation domain. For example, refined grids may be needed in:


Regions where hydraulic gradients change substantially over short distances, as would be common near pumping or injecting wells, rivers, drains, and focused recharge.

Regions of site-scale contamination within a regional aquifer where simulations of plume movement are of interest.

Regions requiring detailed representation of heterogeneity, as may be required to simulate faults, lithologic displacements caused by faulting, fractures, thin lenses, pinch outs of geologic units, and so on.


Visual MODFLOW Flex allows you to create up to nine child grids within a single parent grid. However, you cannot create a child grid within a child grid, and child grids cannot not overlap and must have at least one parent grid cell in between them.


Adding Child Grids

Child Grids can only be added in the conceptual modeling workflow.  Visual MODFLOW Flex currently does not support adding Child Grids onto models that have been imported. If you wish to run LGR on an existing project, you can re-build this using the Conceptual Modeling workflow.



For more information on the local grid refinement package, please refer to the online guide for MODFLOW-LGR and to the published documentation by Mehl and Hill (2005, 2013).


There are two ways in which you can define a child grid within a numerical model grid:


When defining the parent grid; check the Create Child Grid box in the Vertical Grid dialog, and click the [Next] button.

After the parent grid has been defined; right-click on the numerical grid from the Model Explorer tree, and select Create Child Grid.


Horizontal Grid Refinement

Horizontal child grid refinement involves specifying the location of the child grid within the parent grid, and defining the row and column refinement ratio.






You can add data objects (e.g., boundaries, wells, site maps) from the data explorer to the 2D Viewer preview to assist you in determining the placement of the child grid within the parent grid. Select the desired data objects from the data explorer, and the data will be displayed in the 2D Viewer preview.


Enter a unique name for the child grid in the Name field.

Next, select the refinement ratio from the Ratio combo box. A ratio of 3:1, for example, will refine the parent cell by a factor of three, resulting in nine horizontal child cells within one parent cell.

Finally, specify the Row Refinement interval and the Column Refinement interval, by selecting the starting row/column and ending row/column, for where the grid refinement should be applied within the parent grid. The child grid can be placed anywhere within the parent grid as long as it does not overlap another child grid.

Please Note: The child grid cannot be rotated; it must be in the same orientation as the parent grid.

Click the [Preview] button to preview the child grid in the adjacent 2D Viewer.

Click the [Next] button to proceed to the next step.



Vertical Grid Refinement

Vertical grid refinement involves selecting which model layers to refine and specifying the refinement ratio for the selected layers.



The top of the child grid must always coincide with the parent grid and therefore the Start layer will always be 1. However, the End layer can be any model layer below the top model layer in the simulation domain.

There are two options for defining the refinement ratio. Select Globally for all layers to assign a single refinement ratio to all layers. Alternatively, select Specify each layer to assign a refinement ratio layer by layer.

Please Note: Although the top layer must be the start layer, vertical refinement does not have to start at the top. Assign a refinement ratio of 1:1 to the top layer and it will not be refined.

Click the [Finish] button to create the child grid.




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