Defining River as polygon

[Iva 0]

Hi everyone,

I am working on my multi layer model in MODFLOW-USG. 

In the middle of the model I have a big river. For that river I have defined river bed and head stage as TINs.

I am trying to Map it in MODFLOW-USG as polygon but there are too many errors (although conceptual model looks fine).

Most common errors are "Stage elevation is below river bed" and "River stage is below cell bottom elevation".

A few time I succeed  to start MODFLOW but the heads were too high and low (-2e-17 to 2e+12).

Also there are many dry cells close to the river.

 

What is the best way to deal with this kind of problem?

Is River Package even good enough for problems like this or should I use Stream (STR or SFR2)?

On the photo there are my TINs for river bed and head stage.

 

Thank you for your answers

[Rob Virtue 0]

If you have a river stage below bed level, you will get a warning but the model may still run. Unfortunately, the result will be garbage, sometimes spectacularly so. This is because the package includes the calculation stage elevation - bed elevation to calculate gradient, so then the value becomes negative. It could be solved by inserting a line of code that sets flow to zero if stage is at or below bed level, but for some reason that has never been implemented by USGS (perhaps I'm missing some consequence of doing so). I once got someone to modify an earlier version of the MODFLOW source code to include this and it seemed to worked well.  Assuming you can't do that, then all you can do is make sure your river bed TIN matches your model cell grid (turn 2D grid into TIN) and either truncate your stage elevations so they are no lower than the bed elevation or set the bed elevation to below the lowest stage elevation. Then make sure the river cells are assigned to layer corresponding to the lowest level. Note that for transient TINs the layer assignment is based on the first time step, not the lowest (or at least it used to be) so make sure you have the lowest values in the first time step, even if you have to "turn of' the river with a very low conductance in the first (short) time stem.  Sorry there is no easy solution.

[Sean Czarniecki 8]

If your river stage is below the river bottom, then you definitely want to switch to one of the stream packages.  The stream packages allow the river/stream to go up and down based on the surrounding groundwater elevation and flows in the stream.