Inconsistency in 1D/2D model by Lateral Structures

[Alquier]

I am currently working on a 1D/2D model with lateral connections everywhere on the left overbank and right overbank.
So i have lots of lateral structures (about 70 along 3km length) ones beneath the others above the banks.

0) My model is hardly stable.
1) I succeed to run floods but there are inconsistencies (see picture).
2) Mass balance isn’t respected. My model creates water (V_output= 2V_input almost), that’s weird.
3) Stage hydrographs don’t match with the profile plot display. Has anyone already noticed that ?

I don’t know what to change so as to correct this.
I guess obvious errors have obvious solutions but i can’t find out what is the problem.
Has somebody already built this kind of model ?
What should i check ?

Thanks for your help

[Toby]

I have had lots of issues with the 1d / 2d connections.

Check out this post I did and check for these errors first. Your LS’s look like they are snapped to the XS as mine where.

http://hec-ras-help.1091112.n5.nabble.com/1D-2D-Lateral-Structure-Volume-Accounting-Errors-solution-tp3974.html

I have also found areas with high velocities at the flow boundaries, can cause mass balance issues.

Good luck. It would be interesting to know if you find the same issues I had. I have been in contact with HEC and they think there might be a bug, so I sent them my model files.

[Anonymous]

Hi,

I’m trying to simulate 1D/2D and I’ve had many problems, bus I don’t know why. I’m using v. 5.0.3 and I have follow this document “http://www.hec.usace.army.mil/misc/files/ras/Combined_1D_and_2D_Modeling_with_HEC-RAS.pdf”.
The message error is:

**** ERROR: Solution Solver Failed ****

Unstable for Initial Iteration at 02SEP2008 0006

***** Warning! Extrapolated above Cross Section Table at: *****
(The extrapolation may have been caused by model instability)

Urubici01 teste from R.S. 7181.094 to 6662.051
Urubici01 teste from R.S. 6326.739 to 3667.368
Urubici01 teste from R.S. 3310.332 to 1236.624
Urubici01 teste from R.S. 1079.714 to 619.4326
Urubici01 teste from R.S. 331.0952 to 10.95156

**** The Model Has One Or More Error(s) ****

Unsteady Flow Simulation Terminated

Can you help me? Do you have any document, step by step. I’am using lateral strucutures too.

Thanks.

[Anonymous]

“I am currently working on a 1D/2D model with lateral connections everywhere on the left overbank and right overbank.
So i have lots of lateral structures (about 70 along 3km length) ones beneath the others above the banks. ”

If you haven’t already, go to the lateral structure editor and select the “normal 2D Equation Domain” instead of “use weir equation” and see if that helps any.

[Alquier]

Thank you very much Toby, your advice were usefull.

I have allmost no more balance mass issue in my model.
Splitt cell across lateral structures generated the bug.
You can easily spot these cells by looking carefully your Ras Mapper with the 2D mesh.
I had 8 cells like that along my banks so it was quite a huge bug.
Splitt cells don t bug everytime, and when they do, sometimes I had also to have at least one whole cell between my LSs to avoid the bug.

I’ve noticed that the normal 2D Overflow Computation Method was stabler than the weir equation

[Toby]

Glad the fix worked for you too. I have not heard anything back from HEC about the bug, but hopefully they are fixing it or, set the pre-processor to trigger an error so you know if you have a split cell in your mesh/1d boundary.

Cheers!

[Josevar]

I often use 1D/2D scheme to model flooding along dammed river and -in general- I have no problem for unsteady solution.
However some problem I noticed that if 2D area is overlapping 1D elements and then I check the model to remove any overlapping faces (next image).

Currently, I did not find a solution me too (but I hope to resolve soon) for stability of 1D/2D model in urban areas where a long lateral structure follow the border of 1D model. In fact, the 1D river is very strict (5meters) and then some points of the 2D area are within the 1D model.

I stay connected to future developments.

[Author]

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