Drainage from field influenced by tidally varying boundary

[su-12345]

Hi all,

I’m building a simple conceptual model in ModelMuse using MODFLOW 6. The setup is:

• A flat agricultural field draining into a ditch/drain, which then drains into an adjacent estuary
• The estuary has tidal water levels (i.e. changing over time)
• The goal is to simulate conditional drainage:
  - If tide is below the drain level → drainage can occur
  - If tide is above the drain level → drainage is blocked

Ultimately, I want to explore how changes in this drainage window (due to tides) affect groundwater heads in the field.
I’m stuck on how to best implement this "floodgate-style" behaviour in ModelMuse.

Has anyone successfully set up tide-controlled drainage logic in ModelMuse or know a good workaround?
Would love any advice on:

1. Which package(s) would work best for this behaviour?
2. How to simulate the ditch/drain?
3. How to implement a simple "gate" that blocks or allows flow based on estuary head?
4. Whether this is feasible in ModelMuse, or if it is necessary to do in FloPy?

Happy to hear any suggestions or ideas, going in circles with my attempts currently!

[emorway-usgs]

Hello @su-12345, Perhaps you can try adding a drain type boundary condition (DRN package) to the cell(s) representing ag fields. Within the DRN package, you must specify the elevation and conductance of each DRN boundary. For stress periods corresponding to when the tide is above the pre-determined threshold, you could set the conductance equal to zero which will effectively shut-off the DRN boundary. Once tidal levels drop below your pre-determined threshold, you could specify a non-zero conductance for the DRN boundaries for that stress period. In the MF6 I/O guide, the input looks like:

BEGIN PERIOD <iper>
  <cellid> <elev> <cond> [<aux(naux)>] [<boundname>]
  <cellid> <elev> <cond> [<aux(naux)>] [<boundname>]
  ...
END PERIOD

Assuming DRN package applies to only a couple different cells and the tide level rises above your shut-off threshold in stress period 2, you could do something like,

BEGIN PERIOD 1
  10 5.0 1000.0
  11 5.5 1000.0
END PERIOD 1

BEGIN PERIOD 2
  10 5.0 0.0
  11 5.5 0.0
END PERIOD 2

BEGIN PERIOD 3
  10 5.0 1000.0
  11 5.5 1000.0
END PERIOD 3

Regarding the ditch that receives the DRN water, you can use MVR to facilitate the transfer of water from the DRN boundary to the ditch, assuming you represent the ditch with the streamflow routing (SFR) package. However, SFR does not currently offer functionality representing hydraulic structures, such as a gate.