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Sam Marginson

SGS - implications for choice of grid size

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Hi TUFLOW people 🙂 suddenly I am unsure of something as basic as what grid size to select!  Looking at the first chart in section 3.2.1 of the release notes, I'm left wondering whether the seeming lack of impact of grid size on results is limited to Quadtree users or whether the results would be similar for standard HPC users.  Normally I'd try to select a grid size so that channels & bridges are represented by at least 5 grid cells, but if I'm dealing with a relatively wide floodplain that's a bit demanding.  No doubt the usual rule of "you should satisfy yourself that the results are reasonable" applies, but do you have any experiences you can share in that regard?

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Hi Sam,

If you were using just HPC with a fixed grid then i would agree with your above reasons - basically you want to select a grid size to accurately represent the size of the channels you are concerned about and hydraulically significant structures - i.e. those in your areas of interest and all those that route significant flow to and out of said areas. 

The graph in question in the release notes, you can see that with QPC being used in higher resolution you get a significant increase in routed flow compared to a lower res. In which case when considering grid size of course the higher level of detail the less artificial loss in channels (caused by saw-tooth effects) and less chance of course cells 'trapping water'. Of course when thinking about grid size another limiting factor is the results size and needed memory to run the simulation. The use of QPC allows the user to specify that differing levels of detail in specific areas while having courser elsewhere. 

The graph really highlights the power of using both QPC and SGS to take advantage of the sub grid elevations to assist in routing flow.

Cheers,

Joe

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Hi Sam,

Happily the result on that plot is not limited to quad-tree users, and works for 'plain' HPC too. (Though as Joe says, when you combine quad-tree and SGS then things get really good, as you can still get good expansion losses from structures with smaller cells and have the improved flow conveyance from lack of saw-teeth and trapping water which SGS brings)

So you ask the very sensible question "what grid size to select?". I think though, that you've probably already given the best answer with "you should satisfy yourself that the results are reasonable"! On the plus side, all your trial runs with coarser than 'normal' meshes will at least be nice an speedy as you look for some result convergence and satisfaction that results are fine.

For my part, and without much evidence to back it up, I'll say the following... For those less interesting channels one finds around the place, I'm much happier to have them at 1-2 cell kind of resolution (with more careful z-shape definition required) compared to when one was relying on a gully line to do the job. But any channel whose capacity you really care about should still probably be at least 4 and preferably 6 or more cells across. But now you can do that and switch on SGS! Referring back to the plot, you'll see that the high-res result is still a ways off the SGS result (and we'd like to hope the SGS results presented are the 'best' answers in this instance), so while you can maybe make your cells bigger to some extent, I think the important thing is to turn on the SGS.

Others' views may differ of course. 😉

Happy modelling!

Peter.

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