Building and using fully unstructured (disu) grids

[RyanConway91]

I would like simulate underground mine workings in detail in a regional (aka large model). Seems like a full on disu would be good for this. So far I have stuck to disv.

What are other experience with disu?
What software can be used to generate the grid (I have GWV, which can do octree, but I dont think this will get me quite what I want).
Once you have the grid, intersecting (volumes, point, surfaces) seems non-trivial. How have others done this?

[hjia1005]

I have been using mf6-disu for some complex models. I am happy to share some of my experiences. To create the mesh, you can use:

1. Algomesh (commercial)
2. Gridgen (free and you can create the mesh via Flopy)

For Algomesh, you need to prepare the elevation rasters for each layer and you can set a thickness threshold. Then the extent for each layer can be determined and connections can be set up by Algomesh. You will have a disu file, gsf file, gnode to rnode indexing files, which can be read and processed in Flopy and set up your mf6-disu model in Flopy.

For Gridgen, unfortunately, I have to spend more time on the elevation raster and work out the extent of each layer as polygons beforehand. Then you can use

for layer in nlay:
         g.add_active_domain(f'your_path_to_layer_extent/layer_extent_{layer}', [layer])

By doing this, you get a different number of cells for each layer - that's why we use disu.

After this, you can apply the elevation rasters to your model layers and you need to make your model structure CORRECT before you add any packages - this could involve a lot of iterations of work. The checks may include:

• positive thickness for each cell
• for a specific rnode, top of layer n equals to bottom of layer n-1
• and other checks, until every rnode is correct.

After the above is done you can set up the final mf6-disu file and create a gsf file (this is normally used for pilot point processing because plproc doesn't support mf6-disu yet). You can also create your full mesh as a shapefile and set up your final gnode-rnode file with gnode, rnode, layer, coordinates, top, bottom, thickness for each single cell.

Once you have the mesh in shapefile and the gnode-rnode file (this can be converted to points), you can do all geospatial intersections using geopandas to create other packages (GHB, DRN, RIV, LAK...).

The same way helps to build up a mfusg model, but I personally found that mf6 has a better performance in convergency, especially for a complex model. Also keep in mind that CLN is only available to mfusg, and not for mf6, if you have to use it.

I hope the above gives you some idea to start with.

[emorway-usgs]

@BrockOliver, per @RyanConway91's suggestion, you can quickly tell the format of the DIS file exported from a 3rd party software by inspecting the GWF name file. The three different grid types are identified by DIS6, DISU6, and DISV6; one of those acronyms will appear in the name file generated by GWV (or GUI of choice) in order for MF6 to run.

[christianlangevin]

@BrockOliver, if your gsf file isn't too big and you can share it, feel free to drop it into this discussion. I'm curious to take a look and see what it would take to get have flopy provide all of the information needed to construct a MODFLOW 6 flopy.mf6.ModflowGwfdisu object. The arrays you mention (ihc, cl12, and hwva) are different from what MODFLOW-USG reads, but they can be easily calculated.

[RyanConway91]

It would be nice if flopy could make model grids directly from .gsfs. My current GWV approach works but is slow and often crashes with large files.

[christianlangevin]

On second thought, I'm not so sure that the content of the grid specification file is enough to recreate a numerical model grid. There is enough information to plot the grid, but I'm not sure the full connectivity, cell tops and bottoms, and other information can be determined only from the gsf.

[BrockOliver]

Hi All,

Thank you for the responses. GW Vistas indeed wrote "disu6" to the name file, so all good on that front. I agree with @RyanConway91, if possible it would be nice for flopy to have the ability to go from a .gsf file to a disu object. @christianlangevin, if you're still interested in taking a look, what's the best way to send my .gsf file your way? It's ~15 mb.