Problem FEFLOW: Hydraulic head

Good evening, I am a new member of the forum.

I'm getting close to little use of FEFLOW software. In particular, I ran a simulation of a [b]unsatured or variably satured media (Richards' equation)[/b] by constructing a mesh composed of two layers.

After entering boundary conditions (no flow limits on three sides of the structure) and recharge (annual applied to the first slice) I tried a simulation.

From the results I get a value of the hydraulic head significantly elevated even higher than the topography of reference!

I wanted to know if anyone had any idea of what could be a possible cause of the problem, in particular, may be linked to a wrong value of the reference system?

thanks

Karanka

If you are new to feflow, I would suggest starting with a simple model formulation (such as a confined, saturated flow problem) and move from there.  There are a number of additional considerations in modelling unsaturated media which require a greater level of experience and understanding. First question to ask would be: are you aware of the rationale for selecting an unsaturated modelling approach and whether this is the best approach for your problem?

Hello first of all thank you for your reply.

Yes, I realize that this approach is quite complicated but in my case I have a partially saturated aquifer or otherwise variable.

I tried to run a simulation with the first option (standard groundwater-flow equation) of Feflow considering:

- unconfined aquifer;
- phreatic surface
- constrained for head

In this case, the result shows a complete saturation of the structure that is not the reality.

For this reason I was thinking of switching to a simulation with variable satured media.

Thanks

Karanka

It seems that, both in the saturated and unsaturated simulations, you get an unreasonable high output head.
Maybe the recharge is too high?
And did you set in the only no-flow side a bc able to accept water?

Hello thank you for the suggestions, both in the saturated and unsaturated simulations I have not not acceptable solution, with values of hydraulic head in the unsaturated simultaion considerably high.

I do not think that the recharge is too high: I tried to remove it but the values are still too high or meaningless.

Thanks

Karanka

Hi Karanka

In reading your description again, it seems that you have recharge for introducing water to the model, but no mechanism for it to escape such as a boundary condition? This would explain excess saturation of the model domain.

You said that you had "entered no flow boundary conditions" on three sides? What about the fourth side? (Assuming it's rectangular).

Did you know that if you do nothing, then all external boundaries of the model are no-flow. Hence, you can't really "assign" a no flow boundary. It just is one until you assign a flow boundary. Perhaps you are assuming that water can leave through the fourth side when in fact it can't until you assign a type 1, 2 or 3 BC there...

Hello Blair, thank you for your interest and for your help.

You got that right, to isolate my system I added three no-flow bc on 3/4 of the structure.

The fourth side corresponds to a small river (so it may be a condition of the third kind) of which we have a few annual values; also from the analysis the river is disconnected from the underlying aquifer fed only by precipitation.

For this reason it was decided not to include a condition of the third kind on this side but leave the program to free simulate a water table.

I wanted to ask if this reasoning was totally wrong (considering that the river does not drain the aquifer and is not drained by the same)?

thanks

Karanka

I assume you have assigned type 2 boundaries with zero flux to represent no flow boundaries? This is a common mistake I've seen - simply delete these boundary conditions and they will remain as no-flow boundaries.

If recharge is occurring, there must be a mechanism for that water to leave your model domain. I suggest you spend some more time on your hydrogeological conceptualisation and consider inflows and outflows of your system, and where its boundaries should be defined in order to represent the system behaviour correctly.

Perhaps you have overly constrained the model boundaries thereby restricting lateral or vertical flow to/from adjacent areas? If the groundwater is not exiting via the river, where is it going?