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Problems when draining the groundwater level to second, less permeable layer


clarabg
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Dear GMS community,

I have two layer model where the upper layer is quite permeable (K=10^-4 m/s) and the lower one consists of clay (K=10^-9 m/s). My stationary model works fine and is nicely calibrated with only a small error.

However, when I try to lower the groundwater level with the DRT-package to a level in Layer 2, I get huge errors and my model can't converge. If I change the conductance of the drain so that the groundwater level is still in layer 1 everything works, although I get the dubious response of the groundwater level barely being lowered in layer 1 and the hydraulic potential being at the desired level +37 m in layer 2 (please see the attached image).

I’m using MODFLOW NWT and UPW flow package, as well as the HBF-package to simulate sheet pile walls in layer 1 within which the groundwater level will be lowered.

Can anyone please help me and give me some ideas as to why the model cannot reach a solution simulating this scenario? And also if you have an idea of how the model behavior in the attached picture is possible, I would greatly appreciate it.

Best regards,

Clara

Model_side_view_210202.jpg

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  • 3 weeks later...

My first thought is that you are running this as a steady-state model.  That would set up a situation where your starting heads and where the solution wants to be is too far apart.  The solution immediately oscillates significantly and can't converge.  I suggest that you make it transient.  That should help.

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Thank you greatly for your reply! I will try to make it transient.

If I want to look at the effects of the drawdown on nearby buildings, do you recommend me to look at the level depicted in the upper layer only? Or does the lower layer also contain vital information since the levels are vastly different from the upper layer? I have a hard time interpreting this "behaviour" of the model and which results to use in further analysis...

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Based on the information you have provided, it sounds like you may have a confining layer where your lower aquifer has a higher head than the upper aquifer (upward gradient).  If you were measuring this with wells out in the field, you would see the shallow overburden wells with a lower water elevation than the deeper overburden (or bedrock).  In this scenario, the actual water table (which you would look at for nearby effects) is typically the water elevation of the shallow overburden, unless something allows water to get past the confining layer (like a well that is screened across all intervals.....or digging through the confining layer).  

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