Posted Mon, 20 Jan 2020 18:20:01 GMT by Tana Yun
Hi All,
Any advice on the following questions would be deeply appreciated.

I am planning to assign a horizontal fracture for a whole layer in FEFLOW, for ex. with Transmissivity  1e-6m/s and aperture of 1e-4m. I know this can be done by creating a thin layer like 0.1m thickness, then assigning K value of 1e-5m/s and a porosity of 0.001 to be equivalent.

While for Discrete Feature: Darcy's law, I am just wondering if this will mean the same thing like a thin porous medium layer as I described above, which means selecting the 'slice faces' of an entire layer, then specify thickness, K value, and porosity as described above?

Can I do this by selecting slice edges? as I notice, in this way, cross section area instead of thickness is needed to be defined, so I am not sure if i can still find a fixed thickness value for this way.

The last question is that, as described above, a horizontal fracture can be achieved through assigning a thin layer with equivalent parameters or a discrete fracture, but a 0.1m thin layer would mean more refinement to the model layers which in turn will slow down the model run, I am supposing a discrete feature may not need the same degree refinement of the layer? or will it introduce more instability to the model?

Thanks in advance for any help.
Posted Thu, 23 Jan 2020 15:07:22 GMT by Peter Schätzl Grundwassermodellierer
The actual process of applying parameters to discrete features depends on the FEFLOW version you are using (6.x vs. 7.x). The same applies for the question of what to select in order to generate 2D discrete features: in version 6.x you'd use 'slice faces' (faces, not edges, and on a slice, not in a layer!), in 7.x you should use 'faces' rather than the legacy 'slice faces'.
It will not be exactly the same as using a separate layer, for two reasons:
1. The layer will separate the underlying and overlaying layers from each other, the discrete feature will not, as the nodes are shared.
2. The spatial discretization in vertical direction will be different, with more nodes vertically with the separate layers.
It highly depends on the parameterization of the different hydrogeological features whether the differences are significant or not. The more the flow is in horzonatal direction, and the higher the conductivity of the fracture compared to the layers above and below, the closer the results will be. Also stability will highly depend on the parameters - no general answer to that one.
Posted Tue, 28 Jan 2020 15:06:49 GMT by Tana Yun
Hi Peter,

Thanks so much for the reply. I am using a version of 7.x. It does offer both 'faces' and 'slice faces' as wells 'join faces', so you mean I should use 'select faces" if I want to assign a horizontal fracture to an entire slice?

I also feel confused about the parameters that need to be specified in Darcy Discrete feature: Thickness, porosity, K value.
Does the thickness means aperture? or the meaning would be different depending on the choice of 'faces' or 'slice faces' selection?
It seems the porosity should be 1 for all the time, as this is a fracture,
Then  K=T/aperture?
Not sure if I am right. I'd be appreciated if you could shed me some light.
Posted Wed, 29 Jan 2020 12:22:37 GMT by Peter Schätzl Grundwassermodellierer
'slice faces' are kind of a legacy mode, according to what was used in version 6.x (and earlier), thus I'd recommend to use 'faces'. For the modelling result, however, it should not make any difference. 'Thickness' is the fracture aperture, used to calculate the storage properties of the fracture. There's a detailed explanantion of the different parameters (in different cases) in the FEFLOW book.
Posted Wed, 29 Jan 2020 18:33:02 GMT by Tana Yun
HI Peter,
thanks so much for your explain, very helpful ;)

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