Branches and unstabilities in structures

Hi everyone,

I have problems with Mike11.
- It creates two branches (surround in attach file) which do not exist in my source files (overview network). I would like to delete them and unterstand why it puts these branches.
- After many unsuccessful trials, the result is unstable in structures despite of a constant flow and a low time step (10 sec).

Do you have any idea why this might be?

Thanks,
SCPsho

SCPsho

The two branches that you can see when loading the result file into MIKE VIEW is in fact the results of adding structures of the type 'Side Structure' in your model network.
The side structures will convey water from the main river through a side structure and out of the main water course at the particular location where the structure has been defined and that is done in MIKE 11 by creating a small (100 m) additional branch for each side structure.
The handling of side-structures are further described in the MIKE 11 documentation.
If you want to remove these branches then you must simply remove the structures - or alternatively change them into regular structure types, which means that they are located as structures within the main stream.
If you really need structures as side structures then there is no way to get rid of the additional 'side-structures branches'. These will always be made automatically by the MIKE 11 engine if structures are defined as side structures in the model network.

Stability can be an issue around structures and it is not possible to answer that question from the supplied information, as there can be many reasons for instabilities in structures defined in a model. One of the most typical reasons for structure instability though is if the structure geometry is defined to be larger than cross sections geometry up- and downstream of the structure, as this means that the structure is not a contraction with a head-loss in the river. It may be worth examining your structure geometry and compare it to cross sections geometry and eventually adjust somehow to make sure that you have a contraction/expansion loss around such a structure definition. That will surely make the structures in models more stable.

/Torben

Torben,

Thank for your quik response.
The problem of branches is solved thanks to your help. I didn't need side structure, so i removed them.

The problem of stability stay. I verified that structures make a contraction with a head-loss in the river thank to the plot.
In attach file you can find the differents files of model.
Do you hane an idea for the cause of instabilities ?

Thanks,
SCPsho

Hi,

I looked at your model, and I could see that you actually do have some issues with your structures' geometry: at least for one of them, the bottom level is lower than the thalweg of the upstream cross-section. So you should edit these structures in order to improve your model.

I've also seen that you have cross-sections really close to each others (grid step of approx. 1m): in such cases, you should consider reducing the time step. In some cases, a time step smaller than 1s may be required.

Please also remember that the Delta parameter (HD11 file \ Default values tab) can be slightly increased : it often significantly improves stability on structures.

Regards,
Mathieu