# Water table variation in FLAC

**URL:** <https://forum.itascainternational.com/t/water-table-variation-in-flac/1974>\
**Category:** FLAC2D\
**Created:** [March 7, 2025, 9:30am UTC](https://forum.itascainternational.com/t/water-table-variation-in-flac/1974 "2025-03-07T09:30:09Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![agapanto](https://avatars.discourse-cdn.com/v4/letter/a/f4b2a3/32.png) [@agapanto](https://forum.itascainternational.com/u/agapanto)\
**Post date:** [March 7, 2025, 9:30am UTC](https://forum.itascainternational.com/t/water-table-variation-in-flac/1974/1 "2025-03-07T09:30:09Z")

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Hi all,

I’m using a water table in my FLAC v.8 model with the command `water table`.  
I need to change the water table level, by lowering it from a level L1 to a level L2. The Command Reference section of the FLAC manual reports as follows for the `water` command:

_If the water level is instantaneously changed (e.g., lowered), the densities above and below the new water table may need to be changed, and the existing total stresses for zones beneath the changed water table need to be changed. These changes are necessary to account for elastic deformations resulting from the changed water table. For example, if the water table inside a shaft is instantaneously reduced by 1 meter via the WATER table command, then the total stress for material inside the shaft needs to be reduced by an amount equal to the product of the water density, gravity and water-level height reduction. In SI units, if the water level is reduced 1 meter,_  
_then the total stress needs to be reduced by 104 Pa. A convenient way to make the change in FLAC is with the INITIAL command:_

```auto
ini sxx add 1e4 range
ini syy add 1e4 range
ini szz add 1e4 range

```

_The range would be all zones below the region where the water table is reduced. The values are added to reduce the total stresses, which are negative in compression._

I have two questions:

1. The `range` includes: i) all the zones below the original L1 level, ii) all the zones between the L1 and L2 levels, or all the zones below the L2 level?
2. Let’s now assume that I wanna **raise** the water level from L2 to L3. Should I correct the total stress too, or this modification is already performed when the new water table level is higher than the previous setting?

Thanks a lot for your help.

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**Author:** ![jwang](https://avatars.discourse-cdn.com/v4/letter/j/b782af/32.png) [@jwang](https://forum.itascainternational.com/u/jwang)\
**Post date:** [March 7, 2025, 3:44pm UTC](https://forum.itascainternational.com/t/water-table-variation-in-flac/1974/2 "2025-03-07T15:44:13Z")

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Have you tried ‘ATS’ mode in the configuration? the ‘ATS’ mode, will automatically adjust the total stresses.

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<div class="post-metadata">

**Author:** ![agapanto](https://avatars.discourse-cdn.com/v4/letter/a/f4b2a3/32.png) [@agapanto](https://forum.itascainternational.com/u/agapanto)\
**Post date:** [March 7, 2025, 4:27pm UTC](https://forum.itascainternational.com/t/water-table-variation-in-flac/1974/3 "2025-03-07T16:27:06Z")

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Thank you jwang! I will try with this configuration.  
If I want to manually correct the total stresses anyway, how should I consider the aforementioned `range`?

Thank you,

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<div class="post-metadata">

**Author:** ![jwang](https://avatars.discourse-cdn.com/v4/letter/j/b782af/32.png) [@jwang](https://forum.itascainternational.com/u/jwang)\
**Post date:** [March 7, 2025, 4:40pm UTC](https://forum.itascainternational.com/t/water-table-variation-in-flac/1974/4 "2025-03-07T16:40:01Z")

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1. You have to specify the range where the initial command applies either via keyword ‘region’, ‘group’, or i j numbers. 2, you have to adjust the total stress too.