One of the three basic options associated with the Solids -> MODFLOW command is the Boundary Matching option. The goal of the boundary matching algorithm is to compute a set of elevation arrays that honor the boundaries between the stratigraphic units as closely as possible.
Once the grid is created, the final step is to select the Solids -> MODFLOW command in the Solids menu. The layer elevations and material properties for the 3D grid will then be automatically assigned from the solids as shown below.
The following images represent cross-sections at selected locations of the grid shown above. Notice that the grid elevations precisely match the stratigraphic boundaries defined by the solids while maintaining the continuous layers required by MODFLOW.
When the Solids -> MODFLOW command is executed with the Boundary Matching option, it is common to have seams that occupy only a portion of a layer as shown in the above cross sections. The top and bottom elevations for cells adjacent to these seams must be adjusted by GMS using a "smoothing" process to ensure that there are not drastic cell size differences in the horizontal direction from one cell to the next. The smoothing is accomplished by iteratively changing the elevation of selected cells until the cell elevations change less than the Smoothing Tolerance specified in the Solids -> MODFLOW Options dialog.
This option enables users to avoid extremely thin layers at edges of pinchouts and represents the minimum thickness of grid cells created from the solids. Solids with thickness less than this amount are ignored and the surrounding material is used instead. This property is assigned in the solids properties dialog. The figure below demonstrates the application of this property.
Small minimum thickness
Larger minimum thickness
The top cell bias is the percentage of the thickness which is assigned to the top layer of the MODFLOW grid create from the solids. The thickness of the top layer increases as the top cell bias increases. A large top cell bias can be used to prevent top-layer cells from going dry. This property is assigned in the solids properties dialog.
Related Links:
Solids to MODFLOW Command
Grid Overlay
Grid Overlay with Keq