Voids in vadose zone

How would you go about modelling a void in the vadose zone?  The discrete feature elements don't seem appropriate because they are 1-D or 2-D.  I am modelling karst-like voids, which are 3-D. 

I have experimented with setting the unsaturated porosity to 1 at all nodes within the void and with setting a hydraulic head BC of 0 (atmospheric) at all nodes adjacent to the void, but I am not happy with the result. 

Any thoughts?

Thanks!

Jeff

You can use the discrete features, which have some channel flow capability (these should work OK if the model is very large compared to the karst channels), or you can use 3d elements with large K values (ok for large or small models).  Or you can program up a fancy ifm module.

Pete

Hi Pete:

Thanks for both replies.  The discrete features won't work because the scale I am working with is small - an experimental cube of soil 5m on a side with a void that is about 1 m in diameter, so the 1D and 2D discrete features will not accurately reproduce what is happening.  I tried using a large K-value for the void, but the model showed the void as a preferential flow pathway with a higher water content than the surrounding soil.  In reality, the void was just that -empty, with no water in it.

I have thought about ganging up a series of 2D discrete features along adjacent nodes to "build" a 3D feature, but have not tried it yet. 

I think I am wrong about using discrete features in the vadose zone. I think FEFLOW automatically disables them in the vadose zone due to the assumption of confined conditions (but perhaps this is only for the features that rely on Darcy's law).  If you cannot use DF, you'll have to define channels of hi-k elements.  Probably will also need very fine mesh and time stepping.

If anyone else is facing the same sort of problem, have a look in the FEFLOW reference manual, chapter 22.  It is not exactly the same issue, but it helped me in understanding what I needed to do to create a modelled void.