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.