I can think in two options, but you must check the impacts of each one in
the results of your model:

1) Import the terrain topography (1st layer top) and 1st layer bottom
(elevations for both will be the same for the cells where the first layer do
not exist), applying a minimum thickness for the 1st layer. This can be a
good solution or not, depending on your boundary conditions and parameters.

2) Apply inactive cells in the first layer for the cells where sand is not
present. I have never tried this option so I am not able to give you
any clues about the effects of doing this.... only remember that imposing an
inactive cell is equivalent to apply no flow boundary.

Good luck,

Stela Cota
Nuclear Tecnology Development Center - CDTN/CNEN
Belo Horizonte - Brazil
+55-31-3069-4610



2008/5/21 Dieter Plass <plass@...>:

> Hello,
>
> I have built a 3-layered model: quaternary sands on top of siltstone on
> top of sandstone. Now there are areas where layer1 (sand) does not
> exist. I do not know how to remove layer 1 for single cells in Visual
> Modflow. Please advise.