Surface Water Packages - Assigning Altitudes

[Bruce Campbell]

Hi -- Have a suggestion for assigning the altitudes for the various MODFLOW surface-water packages such as SFR2, STR, DRN, or RIV. In simple models this is usually not a problem but in more complex models with topography that varies across the model, it's a challenge. Using TIN's to assign the altitudes works well usually but I've had it lead to problems with hundreds to thousands of model cells when the process assigns the surface water feature to a altitude below the bottom of the cell. I suspect this is caused by the various interpolation steps required to get the cell altitudes into a TIN and then into a MODFLOW input file.

I've got a Python script that goes through the discretization file and pulls the top of each cell that has a surface water feature (streams or rivers) and uses the top of the cell altitude directly to build the MODFLOW input without any interpolation. Of course doing this means it's an extra chore to get these data back into GMS and use the functionality of the program.

Just an idea, but seems like a better way to assign the altitudes without multiple interpretation steps.

[Alan Lemon]

I think this is a great suggestion. What do you think about adding <Cell top> and <Cell bottom> to the drop down where you can currently select <constant>, <transient>, <TIN>, <Raster>? 

[Bruce Campbell]

Hi Alan -- That would be great!

[Alan Lemon]

After doing a "deep dive" into the Map->MODFLOW code we have decided that adding the <Cell top>, <Cell bottom> is not the best way to address this issue. Part of the problem is that we like to keep the data completely independent of the grid in the conceptual model. Also, we have an option to auto assign the layer based on elevation. I not sure how that could work with the <Cell top>, <Cell bottom> option.

If you really want to make sure that your boundary conditions get the same value that is assigned to the grid top or bottom you can convert the MODFLOW grid to scatter points using the Grid | MODFLOW Layers -> Scatter Points command. Then you can convert the scatter to a TIN and that TIN can be associated with your boundary condition. This should make it so you boundary conditions get the same value assigned to the grid cell and you won't have to do any extra processing outside of GMS.

[Martin1994]

Hi Alan.

Im having this problem with a complex model build in Modflow-USG.

Im trying to add a stream network in the SFR2 package, using the same tin to define ElEVUP and ELEVDN as i used to create the ugrid( topmost horizon)

I cannot sem to find the option to extract the top grid elevation in USG. My geology is really complex and the surface layer varries from being layer 1 to 10 in the model area.

Is there any way to fix this?

Martin