Author Topic: Hyraulic head in the first slice  (Read 461 times)

elena.cerino

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Hyraulic head in the first slice
« on: April 08, 2016, 04:03:06 pm »
Dear everyone,
I have two problems, maybe trivial: the first is how can I visualize the real velocity instead of Darcy velocity (elemental and nodal). The second is about hydraulic level. I have simulated a phreatic aquifer using phreatic status in the first slice and dependent for the others slices. My layers are 5 m thick and the water table is from 15 to 20 m below the ground level. My model has also a recharge on the top (inflow on top).Is that way correct? Because as result I obtain a phreatic level which is correct in value but which is represented also in the first three slides...and varies from first to second or third slice...in addition if I view the hydraulic head in a cross-section I see  isolines going from the top to the bottom of the model, but the top should consist of "dry cells", so the isolines should start after the "dry cells" (i.e. forth or fifth layer).Is that correct?Is there any way to visualise it in a different way?

Best Regards

Peter Schätzl

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Re: Hyraulic head in the first slice
« Reply #1 on: April 11, 2016, 01:56:57 pm »
1. You can set up an elemental or nodal expression, dividing the resulting Darcy velocity by the porosity.
2. FEFLOW does not use any dry cells. In the dry layers in phreatic mode, the hydraulic conductivity is reduced linearly with saturation of the element, but the element stays part of the simulation. Thus recharge is still also added on top of the model (into slice 1). This will lead to a vertical gradient in hydraulic head, as recharge has to be routed from the model top to the water level. The phreatic approch is a generally very stable solution for regional model with a predominantly horizontal groundwater flow. In more local situations, or situations with vertical flow dominating, I'd recommend to rather use a variably saturated/unsaturated approach applying Richards' equation.
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elena.cerino

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Re: Hyraulic head in the first slice
« Reply #2 on: April 12, 2016, 04:54:52 pm »
Thanks a lot for your reply and helpful suggestions. About my second question, due to the fact that in Feflow there are no dry cells even if I do not insert a recharge on top as result I see isolines going from the top to the bottom of the model even if my water table is 15-20 m below the ground and I can visualize the hydraulic head in the first layers which are actually not characterised by the presence of groundwaters.I wonder if there is a way to visualize the hydraulic head only in layer that are actually characterised by the presence of groundwater.
Thanks in advance
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Björn Kaiser

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Re: Hyraulic head in the first slice
« Reply #3 on: April 13, 2016, 09:24:18 am »
The hydraulic head is indeed available above the water table, because you solve the head at each node. In my point of view it is worth to show the heads also above the water table.

However, if you only want to show the heads below the water table I suggest to hide it via a map. You could export the distribution of the pressure as a polygon shapefile. All polygons with a negative pressure represent the areas above the water table. In contrast, all polygons with a positive pressure represent the domains below the water table. You could use GIS to delete all polygons which are attributed with positive pressures. Finally, load the map containing polygons with negative pressure only in FEFLOW to hide areas above the water level.

Björn Kaiser

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Re: Hyraulic head in the first slice
« Reply #4 on: April 13, 2016, 09:27:51 am »
Changing the range of the color-scale in FEFLOW is probably not an option, because a specific head value at one location could be above the water table, while the same head value at another location could be below the water table.

Peter Schätzl

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Re: Hyraulic head in the first slice
« Reply #5 on: April 14, 2016, 03:59:50 pm »
Another option would be using a stored selection as the plotting target: You can select all elements (in a layer or in 3D) where h<elevation (this is the case above the water table) via 'Select by Expression', then store the selection and plot the isolines onto this selection by clicking it in the'Selections' panel before double-clicking to show hydraulic head.
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elena.cerino

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Re: Hyraulic head in the first slice
« Reply #6 on: April 15, 2016, 10:00:33 am »
Many thanks for your replies!

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Elena

razipv

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Re: Hyraulic head in the first slice
« Reply #7 on: October 11, 2019, 06:49:19 pm »
How can I get the continuous simulation of actual water table to compare it with the observed groundwater level? because in first slice head is high and in lower levels it is decreasing