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Water ponding at 2D/2D boundary

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I have a problem with my multiple domain model (5m inside 20m grid) running with a direct rainfall approach. It seems there is a problem at the 2d/2d boundary as the flow from the larger grid (20m) struggles to flow through to the smaller grid (5m). So far I have tried the following (but still no improvement in the results) :


- Adjust the d attribute of the 2d_bc_2d2d file.

- Set the d attribute to 0 and add the vertices on the 2d_bc_2d/2d line manually. (I'm not sure what the recommended number of vertices to add though.)

- Applied 2d_zshp to smooth the boundary

- Adjusted Cell Wet/Dry Depth == 0.001 and Cell Side Wet/Dry Depth == 0.0005

- Set Map Cutoff Depth == 0.005 (Note that when the Map Cutoff Depth is off, the result seems to get a lot worse.)


The screenshot shows that the water just stops at the boundary though some manage to go through but majority of them is ponding at the boundary of the larger grid.


Has anyone come across this problem before and would anyone be able to give me some advice, please? Thanks.










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It's difficult to tell with any certainty from the image, but it looks as if no water is getting onto where the boundary cells should be in the coarse domain; there are two reasons I can think of for this:

1) Might the geometry in that domain not be quite what you're expecting it to be? Just double check that your DTM and zshapes are all doing what you think they should be! Check your zpts check file, and if things are not right, check your zln and/or zsh check  files to see what's going on.

2) Have you definitely referenced the 2D-2D link boundary in the coarse grid .TBC? It does need to be present in both, but I don't think there's any warning if it's only referenced in one domain! That would definitely stop the transfer of flow (and is my hot favorite for what's happened in this case). In which case, the limited flooding in the finer grid would have to all be from the pluvial flow, as you wouldn't get any transfer at all.


Might either of those be the case here?


Otherwise, you may just need to consider revising your model setup to not have a 2D-2D boundary there. This could be done either by moving your interface between domains so it coincides with a 1D watercourse, if you have one, or by removing it altogether and using 5m everywhere; it'd be slower but more accurate! The general view in our office these days is that 2D-2D linking is more trouble than it's worth and should be avoided where possible. :)

Other views welcome!



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Thanks for your suggestions, PHA, and you are absolutely right that 2D-2D linking is more trouble and time consuming than it's worth especially with the direct rainfall method. We gave up and ran two separate models instead.

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