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katebradbrook

Multiple 2D domains MB calculations

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I have a multi-domain ISIS-TUFLOW model with 2 smaller cell-size domains for which I set a timestep of 1s and 2 larger cell-size domains for which I initially set a timestep of 5s (ISIS Timestep = 5s).

Model runs fine, results look ok, but large apparent MB errors (>7%).

MB error for each 2D domain on its own is ok (from MB2D.csv), but if I compare the volume coming in from ISIS (linked by SX boundaries), it is reported in MB.csv files as much less than it should be (according to ISIS) for the 2 smaller cell size domains. However, the total volume of water in all domains combined seems about right.

Additionally if I run all domains at 1s then the errors dissapear, reported total volume in from ISIS is correct in these domains, and total amount of water remains similar to run with MB errors. In conclusion, therefore, I don't think there really are MB errors in the mixed timestep version (correct amount of water is actually going in and being stored), TUFLOW just thinks there is because it is not reporting the inflow volumes correctly for the smaller timestep domains. I assume this is a result of the Screen/Log Display Interval == 100 being in units of timesteps rather than seconds and therefore 100 timesteps in smaller cell domains does not represent same amount of time as in larger cell domains? Does my interpretation sound correct and if so, is it only related to using ISIS and could the MB reporting for mixed timestep multi-domain models be corrected in future versions?

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Hi Kate,

The first thing to say is that any domains that are linked to ISIS must all share the same timestep. This is the root cause of your MB with the differing timesteps, and also is why it all works fine when you tried it with all 2D domains at 1sec.

It's not just a reporting issue, but is down to how ISIS and TUFLOW exchange information; they don't exchange at set times, but rather at every other timestep, so it all gets a bit confusing to describe what it's managing to do when the timesteps in the 2D don't match what ISIS expects (as specified in the .IEF). Indeed what amazes me the most is that linked models like this are able to run and don't just go splat!

However... Set all ISIS-linked 2D domains to have the same timestep, and make sure that timestep is also specified in the .IEF. If you have any 2D domains that are not linked to ISIS, then these may happily be whatever timestep you wish.

Hope that clears things up for you!

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Thanks for clarifying this. Unfortunately having to run all areas at the smaller timestep does remove some of the advantages of multiple domains. However, the difference in total volume was only 0.15% so whilst there are obviously small errors with how much is actually put in, I think there is still a reporting issue leading to the large mass-balance. It was only the large flows in my 1000-year that alerted me to the issue - all my smaller return periods seemed quite healthy! So maybe it wouldn't be too hard to sort out so it can cope? In the meantime, I am re-running all of this model at the same timestep (increasing run times from 15h to 60h) and in future may still make the 'mistake' of using different timesteps for model development but use a uniform one for final runs.......

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