Flux Boundary Condition With Fixed Head Constraint

I am a new user of FEFLOW - any help would be much appreciated.

I have a model with inflow on the model boundary that has the highest elevation.
The general flow of water through the model (and the prototype aquifer) is from this boundary to the opposite side of the model, which has the lowest elevation.
The model is steady state with two layers, both unconfined, with a free surface.
Slice 1 is free & movable, slice 2 is unspecified, and slice 3 is fixed.
The model also has surface recharge.

Using rounded numbers:
- the total inflow on the boundary is 1,000 m3 / day
- the inflow boundary is 5 km long and the model is 500 m thick at the boundary.
I have applied a flux boundary condition of 0.0004 m / day to both layers (i.e., to the total thickness of the model at that boundary.

Is that the correct way to apply the flux boundary condition?


I have also applied a fixed head constraint to the boundary, because the ground level along the boundary is not horizontal.
I applied a maximum hydraulic head equal to the minimum ground level along the boundary.
This hydraulic head is about 450 m above slice 3 (the bottom of the model), which is horizontal.

Is this the correct way to apply a maximum head constraint to a flux boundary condition?

If this is correct, does this mean that the flux boundary condition is never actually applied by FEFLOW to more than the bottom 450 m on the inflow boundary of my model?

Thanks,
Keith Adams.

Keith,

your setup means that at each single node of the boundary the hydraulic head is checked. If it is below the constraint you have applied, the flux boundary condition will be effective, if it is above, a head boundary condition with the value of the constraint will be applied. In steady-state, it might be difficult for FEFLOW to determine where to put the flux and where the head condition. If you encounter problems, try to run the model for a long time in transient mode to see whether this works and what the result is.

Peter

Hai,

I am a new user of FEFLOW and now develop 3D groundwater flow model in 9 layers and 10 slices. For first two slices (Layer 1) are unconfined aquifer and slices 3 to 6 are aquitards while rest are confined aquifers. I have a problem when to assign flux boundary in the aquifers layers.

I have a question: How to calculate the flux boundary in FEFLOW? Because the unit for flux boundary (L/T) in FEFLOW is different from flux boundary unit in the other method (finite difference in MODFLOW), they have L3/T.  Firstly, I will run the model in steady state condition and after calibrated I will run the model in transient condition.

Thank you for your attention.

Regards
Thomas

Kieth, First question: yes. second question: if the head at the constrained node exceeds 450 m constaint, then fixed head = 450 applied rather than flux.  The head at any of your nodes could fall below 450 with the constant flux depending on the conductivities, recharge, etc, you specified.  I agree a long-term transient may be needed.

Thomas, flux value = volumetric flow rate in L3/T divided by cross-sectional area (L2) through which flow occurs.

Pete

Thank you Pete for your reply,

but I still have a question regarding your answer, because when I calculated flux rate, I use equation Q=KiA, so I will get get flux rate in unit L3/T, then for assign flux boundary in Feflow, I just calculate hydraulic conductivity multiply hydraulic gradient (K.i), isn't it?

Thomas

Yes, however if the nodes that define the area across which the flow will occur are "inside" the model then the flux will be double the input value.  By "inside" I mean that flow could occur across the face of two adjacent elements.  This is easy to check in a simple model set up where you know what the answer is supposed to be.

Also, the flux may vary for each element for given K and gradient.  If you compute the average flux over several element faces and insert the average but have varying K, you may find the model computes heads that are too high or too low.  Also, if you are working with a situation in which the saturated thickness varies, then you may have to use the "integrated" version of the 2nd kind boundary condition. Please refer to the manuals and help file for more explanation for integrated 2nd kind.

By convention, inflow is negative.

Pete