(a) A Set of Simple Solids with Grid Layer Assignments (b) The MODFLOW Grid Resulting From the Layer Assignments
The Solids -> MODFLOW command represents a powerful tool for modeling complex stratigraphy in a completely grid-independent fashion. As part of the overall conceptual model building process, the stratigraphy at a site is modeled as a set of solids. The solids are built using tools in the Borehole, TIN, and Solids modules. These solids can represent a wide variety of complex stratigraphic relationships. The user then assigns hydraulic conductivity (Kh and Kv) and storage coefficients to the solids as material properties and a mult-layer grid is constructed where the boundary of the grid occupies the same region of the solids in plan view. The Solids -> MODFLOW command can then be used to automatically define the elevation arrays in MODFLOW. If the grid is refined or edited in any way, this command can be selected again to rebuild the arrays in seconds with no further user intervention. Together with the feature objects in the Map module, a set of solids can be used to build a completely grid-independent conceptual model regardless of the complexity of the site.
Before executing the Solids -> MODFLOW command, a set of solids should be contructed that match the site stratigraphy. These solids are typically constructed using the horizons approach. When building these solids, it is best to build the primary TIN with a larger outer polygon boundary than the boundary that is used to define the MODFLOW conceptual model. This ensures that the stratigraphy will encompass all of the grid.
Next a layer range must be assigned to each solid. The layer range represents the consecutive sequence of layer numbers in the MODFLOW grid that are to coincide with the solid model. A sample set of layer range assignments is shown in the figure below (a). The example in the figure below is a case where each solid is continuous through the model domain and there are no pinchouts. Each of the solids is given a layer range defined by a beginning and ending grid layer number. The resulting MODFLOW grid is shown in the figure below (b).
(a) A Set of Simple Solids with Grid Layer Assignments (b) The MODFLOW
Grid Resulting From the Layer Assignments
A more complex case with pinchouts is illustrated in the next figure (a). Solid A is given the layer range 1-4, and the enclosed pinchout (solid B) is given the layer range 2-2. The set of grid layers within the defined range that are actually overlapped by the model may change from location to location. The layer range represents the set of grid layers potentially overlapped by the solid anywhere in the model domain. For example, on the left side of the problem shown in the figure below (a), solid A covers grid layers 1, 2, 3 and 4. On the right side of the model, solid A is associated with grid layers 1, 3 and 4 since the enclosed solid (solid B) is associated with layer 2. Likewise, Solid C is associated with grid layers 5 and 6 on the left side of the model but only with layer 6 on the right side of the model where solid D is associated with layer 5. The resulting MODFLOW grid is shown in the figure below (b).
(a) Grid Layer Assignments for a Set of Solids with Pinchouts (b) The MODFLOW
Grid Resulting From the Layer Assignments.
When assigning layer ranges to solids, care must be taken to define associations that are topologically sound. For example, since solid B in the figure above (a) is enclosed by solid A, solid B could not be assigned a layer range that is outside the layer range of solid A.
Layer ranges are assigned using the Solids Properties dialog.
The next step is to create a set of material properties for the solids using the Material Properties command in the Solids menu.
Once the solids are created and the layer assignments are made, the next step is to create a grid. The grid boundary in the xy plane (plan view) should either match the boundary of the solids or encompass the solids. The grid can be refined around wells if desired. The number of layers in the grid should be compatible with the layer assignments made to the solids. When the grid is first created, the z elevations can be ignored since they will be inherited from the solids. A sample grid is shown below.
After the grid is created, the cells outside the model domain should be inactivated using the Activate Cells in Coverage command in Feature Objects menu in the Map module.
Now you are ready to execute the Solids -> MODFLOW command. This command brings up a dialog listing the three basic options associated with the Solids -> MODFLOW command. Each option utilizes a different approach for converting the solid stratigraphy to the MODFLOW BCF input arrays. The three options are:
Boundary Matching
Grid Overlay
Grid Overlay with Keq
Related Links:
Solid Module
Solids to Layered Mesh