User Forum of Software BASEMENT

nicolas

User

Registered: 2024-01-10

Posts: 4

ERROR -> Newton-iteration to calculate water surface elevation (z)

Hello,

I'm trying to run a BASEchain model but I get the following error message :

ERROR -> Newton-iteration to calculate water surface elevation (z) as a function
   of the wetted area (A) failed for cross section 'km870.0'. Maximal number of
   iteration (1000) was reached.
   Requested A = 1.58873e+32
   Current z   = 0
   A-Anew      = 1000
   [z0,z1]     = [565.746, 579.115]
   [A0,A1]     = [0, 753.951]
   Please try to (1) start simulation with wetted initial condition, (2) check
   tags in HYDRAULICS -> PARAMETERS -> SECTION_COMPUTATION.

I'm modelling a section of about 1 km long of a small torrent (bottom ~2m, banks ~1.5m high) on an alluvial cone.
The distance between the CS is quite large (10-15m) as there would otherwise be to many CS to treat manually.
The CS were generated from a scan with a width of 50m, but there are nodes that are separated by a few cm.
However, there is no abrupt change in the CS.

I first tried to run the model without defining the main channel, and the error message came for the 3rd section.
I thought it came from the fact that I'm modeling an alluvial cone, so I defined all the main channel and bottom ranges.
Now the error message comes from a CS further downstream (~200m of the 1km long section).

I tried to model smaller sections of the stream and sometimes it works.

I also tried what was proposed in https://forum.basement.ethz.ch/viewtopic.php?id=555 without success.

I also tried to delete the problematic CS, but the error just moves downstream.

Does the problem comes from the fact that CS are to far from each other or because there are too many nodes in my CS?

Thank you for your help.

Nicolas

Lukas Vonwiller

Developer

Registered: 2014-09-04

Posts: 127

Re: ERROR -> Newton-iteration to calculate water surface elevation (z)

Hi Nicolas

Try to have a look at the _restart.bmg in the 1-D Grid Editor (Tools -> 1-D Grid Editor). My guess is that you have some large deposition at these cross sections with too many nodes (especially at the embankments). A possible solution would be to reduce the number of unnecessary nodes. In case of large depositions this could help!

Hope this helps!

Best, Lukas

nicolas

User

Registered: 2024-01-10

Posts: 4

Re: ERROR -> Newton-iteration to calculate water surface elevation (z)

Hi Lukas,

Thank you for your answer.
I haven't added the transport block yet, this is a hydraulic-only simulation for now. Thus, there cannot be deposition.
I was able to remove some nodes, and now a longer reach works without error.

However I still get this error for CS that are on the alluvial cone (meaning the outside of the levee (flooded area) is lower than the river bed).
Defining water flow and active ranges between the levees (= main channel range) didn't change anything.
Is it a problem you have already encountered or it should work and the problem comes from somwhere else?

Best, Nicolas

Lukas Vonwiller

Developer

Registered: 2014-09-04

Posts: 127

Re: ERROR -> Newton-iteration to calculate water surface elevation (z)

Maybe its your downstream boundary condition that causes the problem? What do you apply there?

nicolas

User

Registered: 2024-01-10

Posts: 4

Re: ERROR -> Newton-iteration to calculate water surface elevation (z)

Hi Lukas,

Yes, you are right! I set the slope of the downstream boundary condition to high.
Now the hydraulic model works well.

Thank you for your help, I wouldn't have looked there otherwise.

Best, Nicolas