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Hi,

I have a problem in my new flushing simulations.
If I change the friction law from Strickler to Nikuradse (incl. adaption of the values), I get unphysically high bed shear stress values (>1000 N/m2) in the shallower water areas with water levels smaller than 1m (e.g., inner side of the river band).

How can I solve this problem?

Many thanks,
Gabi

Hello,

I understand it is a coupling with Sisyphe, in which code do you see this problem? The only known problem so far is when you have BED ROUGHNESS PREDICTION in Sisyphe and different laws in Telemac-2D an Sisyphe. This is under investigation, but maybe you found something else.

With best regards,

Jean-Michel Hervouet

Hello Jean-Michel,

thank you for the quick answer. I am modelling a small flood wave and I just made a test with TELEMAC3d only and I have the same problem there. There are also very high bed shear stresses in some nodes, but in this case only in nodes with no or very low (a few centimeters) water depth.

In my first simulations the results are much worse, but I made the mistake that I used an previous sedimentation file computed with the Strickler law. So Telemac3d does not like that.

However, the problem is smaller but still there.

I attached my cas file, maybe you have some suggestions.

Many thanks,
Gabi

Hello Gabi,

I understand more the problem if it is Telemac-3D. When you choose the Nikuradse law the bottom shear stress becomes the result of a computation assuming that we have a logarithmic profile on the vertical and that the first plane above the bottom is in the logarithmic profile (with other friction laws it is only based on friction coefficient and depth-averaged velocity). It becomes tricky when there is only a few centimeters of water. A worse thing is that when the water depth tends to 0, we want that the friction, hence the shear stress tends to infinity, so that the velocity tends to 0. Sediment modellers have already complained about that and I think we should correct the shear stress on tidal flats when it is meant for sediment transport, so probably it would be another array just for sediment. The solution so far in the next version of Telemac-3D (beta version within one week, and already in the trunk) is that erosion and deposition terms are cancelled when the depth is less than a given threshold value that will be a keyword.

With best regards,

Jean-Michel Hervouet

Dear Jean-Michel,

thank you for your answer. I am looking forward to the next version.
Is there a possibility to test the beta-version?

Many thanks,
Gabi

Dear Jean-Michel,

Sorry for the short question, but will this keyword be also available for telemac2d? Is it included in Version 7p0r1 and if yes which keyword should be used?

Many thanks,
Christian

Hello,

Things are not like this in Telemac-2D, where the Nikuradse law is treated like a Chézy law with a specific coefficient, and... actually I do not really understand in your question which keyword you are refering to...

Regards,

Jean-Michel Hervouet

Dear Jean-Michel,

Thank you for your fast response and I am sorry about my unclear formulated question.

I was wondering whether there is an option to cancel erosion and deposition terms in telemac2d/sisyphe when a water depth is less than a given threshold?

I am running a t2d-sis internally coupled simulation of a channel with an adjacent tidal flat(mixed sediment transport). In general sediment transport patterns and sedimentation erosion behavior is as expected, except at low water levels adjacent to creeks and at the edge of the tidal flat I observe "spikes" of very high sedimentation.

Since the manual suggests to treat tidal flats via telemac2d(I am using FE).
I tested several advection schemes(e.g. 14 5 14, 1 5 14),
Option for Tidal flats=1
TREATMENT OF NEGATIVE DEPTHS: 2
MASS-LUMPING ON H: 1
SUPG OPTION:0;0

in sysiphe I use Type of advection:14 as suggested.

I made one test with using H Clipping, which reduced the sedimentation spikes, but caused different problems, and is also not my preferred option since not mass conservative.

Do you have any suggestion how to better improve the treatment of tidal flats in respect to sedimentation erosion at low water depths??

Many thanks,
Christian

Hello Christian,

Your options seem correct, and we have other similar cases that work, so I am not sure of what happens. The last case reported was in fact a problem at a boundary, since you need to have :

TREATMENT OF FLUXES AT THE BOUNDARIES : 2;2;...

to avoid large depositions of sediment at entrances, on small depths, otherwise the prescribed concentration will immediately deposit. We normally expect that inside the domain mass conservation will prevent any spurious deposit (but there is no artificial cancelling of deposition like in 3D, so maybe this is questionable). If you still have problems we would need to get your case, the man in charge of Sisyphe will be back next week.

With best regards,

Jean-Michel Hervouet

Dear Jean-Michel,

Thank you for your fast reply, and excuse me for the delay in my reply.

I was running tests on further parameter settings to see whether the problem persists, and unfortunately I keep having the same problem with spurious deposits.

Would it be possible to send you my case, so you could have a look?

Many thanks,
Christian