1D dam breach analysis – error

[Yuri2020]

Hi all,
I am trying to reproduce the unsteady flow analysis of a level pool routing model in https://therassolution.kleinschmidtgroup.com/ras-post/how-to-use-a-storage-area-to-define-a-reservoir/

However, the analysis encountered an error saying that “A change (or known value) in WSEL or EG at a node at river station in reach caused a WSEL below the bottom of the cross-section.” The river station indicated in this error is the first dummy cross-section upstream to the inline structure (dam). I tried to increase the initial flow rate of my hydrograph, increase the Manning’s n value of the cross-section, and even apply pilot channels. However, the error is still there.

Could you please help me out? Thank you very much.

[cameron]

Did you set your initial wse for the dummy cross-section?

Is the storage area elevation invert lower than the dummy cross-section invert?

Is the breach elevation lower than the dummy cross-section invert?

When does the error occur?

What is your minimum flow from your hydrograph?

[Yuri2020]

Hi Cameron,

Thanks for your reply. I checked all the settings according to your questions. And now I have another error: “Minimum error exceeds allowable tolerance at 22JUL2020 24:00:00”. That is the beginning of the analysis (analysis time zero).

The two dummy cross-sections are the same and have a flat geometry data with an invert elevation higher than the final breach elevation. The storage invert elevation is lower than the breach elevation. The initial flows and elevations of the two dummy cross-sections are equal to the initial flow and elevation of the storage. The minimum flow of my hydrograph is 1 cms.

When I remove the inline structure, the analysis runs smoothly. This may prove that the cross-sections are ok. However, as I add the inline structure, the error occurred.

I also would like to run a sediment analysis by using this level pool routing method. However, I cannot add any boundary condition (hydrograph) for the storage area in the quasi unsteady flow editor. Should I add the hydrograph to one of the dummy cross-sections?

[cameron]

Did you set a pilot flow for the inline structure? Cross-sections need to stay wet and the dam will block all flow unless you have outlet works, but that flow may not be enough to be stable.

Is there a reason why the two dummy cross-sections have different invert elevations than the storage area?

Sediment transport and dam breach in the same model will be difficult to keep stable and get good results due to the high velocities and other issues. I would use the Unsteady sediment transport option rather Quasi-Unsteady.

It also may be easier to do the breach model only first and take the breach hydrograph to use in the sediment transport model.

[Author]

Posts