Christopherus Braun

Hello,

The first question is what you are trying to model. If your are only using the unsaturated approach to get a better approximation of the water table and you do not vary the parameters in space then the parameters you choose should not affect this result very much because at this point all parameters should give the same solution.
But keep in mind that this is valid only for steady state computations. In transient computations the unsaturated properties are controlling the watering/dewatering processes in the unsaturated zone and will effect your modelling results.

About the parameters you use:
residual asturation Sr has to be <1 (typical values for porous media are in a range of 0.001 to 0.05
The parameter A in the Van Genuchten relationship can be intepretated as 1/hc where hc is the capillary rise. Typical values for hc are for example about 0.3 m for sand and about 2.0 m for loam. Thinking of a fractured media you can try to describe A with the diameter d of the bigger fracture in your rock. If think of the fractures as tubes the capillarry rise could be estimated as:
hc [m] = 3e-5/d[m]
So your value of A seems to be quiet large but could be realistic.

You are right that values of A less then your vertical mesh discretization could cause some numreical problems, because then a node is (more or less) dry or wet.

If you have problems with the model stability you could try to use some othe values for m and n: try to smooth the capillary pressure curve. Maybe some bigger values will help.
If you use the modified Van Genuchten function you can seperately define the relative conductivity function. To my experience small values of the parameter delta give more stable solutions (but delta < 1 is not physically correct!).

Maybe also a mesh refinement (in the region of the water table) could help with the convregence, but I would first try the parameter variation above.

Good luck
Zebra

The classical method would be to put a well BC with transient pumping rates and compare the measured and calculated drawdowns during the model calibration. Sometimes it is more easy to look only at the recovery period becasue you got then well defined "pumping" conditions.

I sometimes had problems with the constraint condition of BCs (for seepage face nodes FEFLOW does set a 1. kind BC with the head equals the z-coorinate of the node and applies a constraint that will no infiltration will be allowed). If your head is below the z-coordinate, the 1. kind BC will be converted to a 4. kind BC with flux=0, if the heads gets above and the constraint is chacked again the BC is reconverted to a 1. kind BC. In steady state models it could happen, that there is a convertion of the whole problem before all of the BCs have there finally state.

What you can do is, that you rerun the model sometimes and check whether something is changing. Or you can run the model in a quasi-steady-state (transient run with steady BCs) here the constraints are at least checked every timestep.

The storativity will only affect the transient behavour of your system. So if you look at a steady state or a quasi steady state problem (e.g. water levels after a "long" simulation time) the model results will show no difference. So should look at the hydrographs (observation points located in alice 1 or 2) in a transient run. Here you should see differences.

When I do run transient FEFLOW models I normally make a copy of the initial head before I start the run. An easy method to do this is to copy it to a nodal reference distribution (from the initial head menu --> copy). So then you got all in the femfile. If you want to rerun the model yuo can go to reference distribution and copy this back to initial head.

Zebra

You also can join the data using your super element mesh.

Hi Gemma,

to do the change in the GUI is a little bit tricky:
1. be sure you are in the "assign" mode
2. select the model type
3. click on the "model button" beside the model type button
4. now a number should appear in the "Keyboard request" field. You have to confirm this number (left click in the field and return)

Sometimes you could forget the last topic.

Zebra

Hi,

I had the same problem sometimes. I did not have it lately. Do you use an older version of FEFLOW?
When I had this problem I found that an old trick does work:
Edit the FEM-File (assign the model you want), save the the FEM-File and exit FEFLOW.

Zebra

First you should check your boundaries together with your slice elevation. Maybe you find here some reasons for falling dry.
You can supress this effect by setting the constraints and the residual water depth (you will find those settings in the "options-menu").

Zebra

I think FEFLOW could not directly handle contout lines. I do that mostly in the following way:
- create in ArcView or ArcGis a Surface TIN from the contourlines
- export the FEFLOW nodes as points (shapefile) and load them in the GIS
-  extract the elevation from the surface TIN at the points and add them to the shapefile
- import this to FEFLOW
This is a little bit amfward, but it works.