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Im working with a 5m grid and would like to have an area of it reduced to a finer grid, at the same orientation and within the same extent. Is there a simple way of doing this?

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Not an amazingly simple way, no, not yet. Sometime soon I hope! But the old multi-domain with the 2d-2d boundaries aren't too difficult to set-up, and work fine in most cases.

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Slightly off the original topic, but does anyone know whether there are any limitations on grid size choices for 2D2D linking? E.g. do they have to be multiples of each other (so 2m and 5m)? Is it better if one is half of the other as a minimum ratio (e.g. 5m and 10m)?

I've got a FloMo-Tuflow model where only a small section of it is an area of interest and i'm considering running a 4m grid in domain of interest, with a 20m grid in domain of less interest. The area of less interest is very big and we need to include it due to hydrology calculations (on a budget) but need to minimise run times.

Other combinations could be:

- 4m:24m

- 8m:24m

Any comments?


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In large part that's going to depend upon your topography/water surface at the location you'd like to join these different resolutions... Here's why:

  • The link happens by using hidden 1D nodes that are connected to each 2D domain using HX boundaries
  • This means that the water surface can only 'bend' along the boundaries every time there is a hidden node
    • It's worth noting at this point that the user has control over the spacing of the hidden nodes!
  • As with other 1D elements connected to a 2D domain via HX then, it is important to have both
    1. Enough nodes to capture and curve in the water surface
    2. Not too many nodes so they're always at least 3 or four cells apart.

So, if your water surface is well enough behaved, or your boundary can be carefully digitised along a water surface contour, then you should be ok with whatever cell sizes you fancy! You're limited by your larger cell size, as you don't want your 1D nodes less than three cells apart; in your main example, that's a spacing of 60m for the hidden nodes. You then need to consider whether that will be sufficient for the finer resolution you wish to use. If your water level is nice and flat, you'll be fine! Or if your land is sloping nice and uniformly parallel to the direction of flow, you'll probably also be all right. Either way, do check your results look sensible! If your water surface is NOT flat or constantly sloping, you might get away with it if your cell sizes are not too different, but by far my strongest recommendation would be to re-site your boundary somewhere where you do have a tame water surface. The ultimate preference would be not to have your different domains touch and link 2d-2d at all, by transitioning somewhere dry (two disconnected bits off flood plain with a river that flows from one to the other), or transitioning either side of a 1D channel (i.e. left bank high res, right bank low res), but that's often not possible.

I think for my own purposes, I've not wished to reduce by more than a factor of 5 when refining, but the nature of the link means there's neither requirement nor benefit in having multiples or factors of 2 between your 2D cell sizes. With a good location for sitting your boundary, any of the combinations you suggest would be fine.


I hope that makes sense and helps! Do feel free to ask further clarifying questions if needs be.



PS. To the devs, any progress on the original question in this thread? The whole nested 2d with quad-tree (or nine ways) refinement of cells thing that was discussed so many moons ago?

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