Pseudo-Unsaturated Approximation in 2-D?

I am trying to do some quick 2D simulations (vertical cross-sectinon) for a simple "hillslope" scenario with a phreatic surface.  The geology is layered with different K.  I have applied recharge flux boundary at the ground surface boundary and a seepage face condition (flux boundary constraint) along a hillslope at the right boundary  (no flow conditions are imposed at left and bottom of model). These models are not calibrated models, but simply preliminary models to look at sensitivity to some parameters.
I am trying to to do these quickly, therefore am specifying saturated flow conditions (even though it is not actually saturated above the water table), but the results still provide an approximation of the water table (pressure = 0 isoline) and i can look at resulting flow pathlines.

My question is....is the approximation for the unsaturated zone in 2D the same as the pseudo-unsaturated approach for 3D that is indicated in the While Papers?  It is not specifically indicated in the White Papers what the 2D approximation is.

Laurie

Hi Laurie,

There's no pseudo-unsaturated approach for 2D vertical models. This means that unless you choose Richards' equation, the model will be fully saturated and calculated as confined. To get a quick solution, you might choose the unsaturated mode with simplified parametric relationships (e.g., the linear model).

Peter

When i model as saturated, I get a zero pressure isoline forming within the model (below the top boundary which is a ridge crest). Is that zero P isoline then an approximation of where the water table might be under actual phreatic aquifer conditions?  I will try with unsaturated conditions for comparison.
Thanks

This zero pressure isoline does not relate to an actual water table as the model is calculated as fully confined. I highly recommend to do an unsaturated simulation if the phreatic surface needs to be considered.

OK, thanks. Its running now, but slow.

An alternative method, which I've used in the past to verify 2D models is to create a thin 3D model with a number of layers.  This way you can use the unconfined methods (phreatic or free and movable).  It will also run much faster than your 2D unsaturated model and will be a lot more stable.

Great, thanks for the tip!