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mgozdor

Transient MODFLOW NWT and MODPATH Crash

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I am running a transient MODFLOW scenario using the NWT solver with UPW.  We are doing significant dewatering in this scenario and the water is drawn down through several layers (necessary due to geology).  The MODPATh model will run for a while and then crash due to "cell thickness is negative in" some cell.  I believe particles are entering cells where the CHD and NWT have the water table below the bottom of the cell which is causing MODPATH to crash.  Is there any way to fix this?  I already have MOODFLOW outputting head at the end of every timestep and the IPHDRY set to a positive number.  Thanks for any help you can provide.

modpath_error.JPG

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Interesting.  I don't really have time to look into it further, so I'll brainstorm with you.  I'm wondering if setting IPHDRY to a positive number could be causing the problem, depending on what you are setting HDRY at.  Try putting IPHDRY to 0 to see what happens.  The cell would then clearly not be available for particles.

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IPHDRY is a flag that indicates whether groundwater head will be set to HDRY when the groundwater head is less than 1×10-4 above the cell bottom (units defined by LENUNI).

If IPHDRY=0, then head will not be set to HDRY.
If IPHDRY>0, then head will be set to HDRY.

If the head solution from one simulation will be used as starting heads for a subsequent simulation, or if the Observation Process is used (Harbaugh and others, 2000), then HDRY should not be printed to the output file for dry cells (that is, the UPW Package input variable should be set as IPHDRY=0).

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