Model half-confined and half-unconfined?

Hi,

I need to build a model where half of the domain is confined, but the aquitard pinches out and I need to model the other half as unconfined. What is the best way to go about doing this?

Also, how does FEFLOW deal with dry cells in unconfined layer? Is it ok to define layer elevations such that the water table may be below the bottom of the upper layer in places?

Thanks,

SB

The way FEFLOW handles dry cells depents on the model type: In unsaturated mode, the capillary relationships are taken into account to model the capillary updraft in this area. In saturated/unconfined/free+movable mode, the top slice is shifted after each time-step to match the water table. In saturated/unconfined/phreatic mode, the conductivity of a dry cell is set to a very small value (a semi-saturated cells conductivity is scaled linearly with the saturation).

Which model to choose depends on the actual model details; however from your description I would guess that the saturated/phreatic mode would be appropriate. It is important to set the right settings here (3D model): in the free+surfaces editor, open "constraints" and set the "touching the top surface" to unconstrained". In the "specific options" dialog, choose "confined". In this way, FEFLOW will handle the top surface as an impermeable boundary (this is for the confined part of the model). In the unconfined part, just a sufficiently high elevation for the top slice that avoids that the water table "touches" the top boundary.

A 2D model works similar, here set the "constraints" to unconstrained and apply a sufficiently high "Aquifer top elevation" in flow material properties for the unconfined part of the model.

Thanks, that sounds like the right kind of thing. The only problem is that the model has a few layers due to the need to model a significant downthrow across a fault. The confined aquifer is the bottom layer...hence there will be a couple of layers above representing aquitards (in the unconfined section the parameters are changed so that the top layer represents the aquifer).

Does the confining layer need to be the top layer of the model? If not will there be a problem with layers going dry?

Cheers.