Christopherus Braun

Gienmarco,

In my opinion there is no "easy way" to discribe those boundary condition in FEFLOW.
But there do exist several possibilities to do it.

1.) You could define a first type mass boundary condition on the top slice of your model. This would represent the concentration in your soil. All water passing through that slice (so also your recharge) will get that concentration.

2.) You could define a second type mass boundary condition on the top slice of your model. This would a mass flux (concentration in soil x water flux = mass flux).

3.) You could define the source in the material property menu. This is probably the method you were talking about. In this case you have to take the volume of the layer into account. For your problem I would not use it (this is ment to some problems where mass concentration are "produced" by the soil and not brought in from outside).

Zebra

Amelia,

I do agree with you, you should not increase the convergence criterion. Actually it seems allready a little bit to rough for your model.

What about the problem class you use? Is it a confined or unconfined model? Allthough most models are unconfined in nature, it maybe helpful first to calculate a confined model. Maybe the difference could be neclectable for a regional scale.

The solving of the problem should be more easy in a confined model. So you could try first to solve it for the confined case. Analysing that solution you maybe detect some problems in your model (e.g. some inconsistend BCs ...). Also this could give you better starting heads for the unconfined model.

In using an unconfined approach, I made some good experiences with the "phreatic" option. Here you can force more stability in the solution by increasing the parameter for the "residual water depth" in the "specific options" settings. This water depth should be somehow related to your vertical discretization.

Unfortunately FEFLOW gives no control over spatial distribution of occuring errors an convergance problems. But if you check your results carefully you will find places with strange hydraulics which indicates normally also numerical problems.

The most frequent error source in models as yours are the boundary conditions. You could try to eliminate them step by step and run the model to find out which one makes the most problems.


Zebra

According to your boundary conditions:
keep in mind, that FEFLOW will treat each flow boundary as a freshwater boundary (c=0) if you do specify any concentration at the inflow nodes or massfluxes.

Another thing you could check is whether or not your steady state transport solution has converged. To do that I normally run the modell as a transient one (or maybe steady flow / transient transport). I had made some bad experience with the steady state transport simulation. Espacially if you want to simulate a density driven flow (as you will have in saltwater intrusion) the solution will be much more stable and meaningfull in a transient / transinet model run.

Hi,

What kind of boundary condition did you apply in your confined aquifer under the sea? Maybe there is a freshwater BC for the transport part?

To my knowledge the only way to import discrete elements is a join (a-join with polygons, l-join with lines). So you have to create first some line or polygon features from your points. Since the discrete elements are supposed to be "discrete" there is no interpolation routine given for them and maybe that dould not make sence?

I do agree with you about C++
But the awk scripts under the cygwin enviroment look a little bit "old fashioned" to me...

As far as I know this is only possible for the hydrograph-window where the results at reference points are shown.

But I agree with you, that it maybe helpful to be able to switch to a different veriable.

You can join boundary condition constraints (same procedure as for the Boundary conditions).
If you want to set both, min and max constraints you have to do the joining twice.

Does that not work?

Hi Chris,

Actually I do not know whether it is yet possible or not. I think it will not work for IFM, but it may work for filter progrmming. At the moment I amthinking of writing some import/export filter. Here Python could be a "state of the art" alternative to awk.

We still use C++ (under Visual Studio) for our IFMs.

Maybe your numerical problems came from the parametric model you use for unsaturated flow. if you define some very sharp curves FEFFLOW may not produce meaningful results.

You have to keep in mind, that for the stability of the numerical simulation not the total value of capillar pressure or relative  conductivity are important, but there deviation over saturation. So avoid any steep functions.

I made some good experience with the modified Van Genuchten approach. Here you have two possibilities to smooth your simulation:
1. Reduce the parameter A: this doews introduce some capillara difussivity into the simulation and helps a lot.
2. Use a value for the parameter delta that does not make the relative permeability curye to steep (e.g. 1.1 - 1.3)

Of course the parameters you use should have some physical meanings, but there maybe a large range in data.


The other thing you could try is a mesh refinement. Not so much in hirizontal direction, but in vertical direction. This shoud help much more the unsaturated processes take place in the vertical direction.

The vertical element length you should use has also something to do with the parametric model you use. You have to assure that an unsaturated profile could be computed an not just " wet or dry ". There is a rule of the thumb: if you use a (modified) Van Genuchten model the vertical spacing of your slices should be in the range of 1/A

Zebra