Hi,

Lots to ask....

Scratching my head here. I think my problems are with the modelling rather than being conceptual, but if you can help me out either way I'll be happy.

The problem.. 3D, steady state, saturated, unconfined with phreatic surface with the default constraints for the phreatic surface set for 'falling dry' 'touching top surface'.

Model boundaries, pretty much fixed head all the way around. I know I have too much water coming in to my system, but I'm limited to working from a previous regional model thats been okayed.

I have also lots of streams, sub surface and surface drains which I'm required to model explicitly, for water balances etc. I figured 1D discrete elements might help me out otherwise it would all get a bit messy.

So, Ive set my transfer boundary conditions as you would, stage for streams, elevation for surface drains, and below elevation for the others..

Transfer boundary constraints for minimum heads are set appropriately for streams/drains.

My feature element are set along my transfer boundary conditions with conductance, transfer in/out etc set, I hope correctly (Im assuming that flow is Darcy along my (porous) rivers with a conductance == 1m/s).

As anticipated from the boundary conditions I have to work with my model heads are way to high ~10m above ground surface. This is pre sensitivity analysis, which is likely to be futile, but I have some questions regarding my base case.

In summary, my questions are.

1) Do the default constraints (in my case unconstrained touching top surface) during the iterative phase and at final solution ignore, say, any maximum boundary constraints you may have set on your transfer conditions? I ask this as the option to constrain your heads to ground level by applying the dirichlet conditions over the model domain seem to supercede any internal boundary condition. ie you don't get any flow in/out your tranfer boundaries even if the head distruibution suggest you should.

2) Do transfer conditions, in the instance of using the 1D feature elements, only need to be applied themselves in 1D along the defined discrete element conditions? They would seem to work.

3) I'm a little uncertain as to the best way to represent a good approximation for a stream/drain using the linear elements. Is a high K value of 1m/s high enough, or would the maximum you can get away with in the model be more appropriate?

4) Convergence in my model seems highly sensitive to the 1D elements. Specifically with regard to transfer in/out values (the lower the better), conductance and cross sectional area and not discretisation. This appears to be in the case where I have high head gradients. I thought increasing conductance and increasing cross sectional area would be the way forward, but it seems the converse is true. I confess to not understanding the reason why, and that the attachment of 1D elements to a finite element grid is new to me. Im assuming that the transfer conditions apply directly to the 1D element nodes, and also that the modelled heads do too. Its the simulation of flow along the 1D element, in addition to whatever is going on in the element below that I do not fully understand. I've tried the white papers, but Im non the wiser.

5) Is the simulation of a cutoff wall using a low K vertical quadrilateral fracture element a suitable approximation? For a single layer do I need to apply this condition to an upper and lower slice?

cheers

Simon