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Hi

for a Telemac2D Simulation of a river section I have too high water levels especially in the region of the upstream boundary. I varied the Strickler coefficient to a value of 40, but still the water level is approx 0,5 m too high compared with measured values. For a Strickler Value=52 I get correct water levels. But a normal (physically right) Strickler values for the river bed should be lower than 40 in my opinion. Do you have an idea?

Thank you

PS: I attached the previous computation file condInit.ser, geometry file, cas file

Hi steffienew,

Telemac uses FICTION COEFFICIENT, not FICTION VALUE. For example, Mannings Number of 40 means: Fiction Coefficient is 1/40= 0.025. I am not familiar with Strickler Law of Fiction, but just want to make sure you understand the difference.

Also, you can try to apply other fiction laws to see the difference. if you model sediment transport, Chezy is usually recommended (what I have been taught).

Best Regards

Huy

Hi,

do you know that your water levels are correct (the ones for the validation)?

Roughness and the parameters of the turbulence model are usually the calibration parameters, therefore, which turbulence model do you use?

Best regards,
Gabi

Thank you Huy and Gabi for your quick answers.

@Gabi: I use K-EPSILON Model. I didnt change anything in the turbulence model parameters, so use the default parameters. What can I try for calibration?

@Huy: I will check if other Friction Law will give differences.

Best regards

Hi,

normaly, the k-epsilon Model gives lower water levels than constant eddy or the Elder Model in T2D. Therefore I would modify the roughness.
The roughness in a river consists of the bed roughness and the grain roughness, if your mesh is very fine and represents the bed roughness, there is thus just the grain roughness left, which is lower of course.

To the friction laws: As far as I know, all friction coefficients are converted to Chezy in T2D, except the Nikuradse one.
Therefore it shouldn't make a difference if you use Chezy or Strickler if the values are equivalent.
But you can check the conversion in the subroutines.

Best regards,
Gabi

p.s Are you sure about your reference water level or why do you know that Strickler 52 is to smooth?

Hey Gabi,

Can you explain what you mean with grain roughness? Up to this I only used the Friction coefficient for the river bed to calibrate the model.

I try to check the conversion in the subroutines but I am a beginner with fortran so I am not sure to get this. Anyway, thank you very much for the hints.

Yes I am sure about the reference water level. I read in the literature that normal Strickler friction coefficents for a river like the Rhine in the lower part which I consider are between 30 and 40. So I think a coefficient of 52 is not physically correct.

Thank you and Best regards

Hi,

tau_tot = tau_grain + tau_bed + tau_susp

tau_tot is the total shear stress, tau_grain is the shear stress due to the grain roughness (skin friction) and tau_bed is the shear stress due to the bed roughness and tau_susp is the shear stress due to suspended sediments according to Zanke (1982) (also Van Rijn, Engelund ..)

I do not think Strickler 30-40 is a correct value for the lower Rhine (maybe this is taken form 1D-calculations and correspond to the total roughness).
A Strickler value of 30 corresponds to grain sizes of about 5-8cm!!!

Try to calculate your Strickler value from the grain sizes in the river or use the Nikuradse value ks.

Best regards,
Gabi

Thanks Gabi a lot.

I have also picked something new from your explanation.

Best Regards

Huy

Hey Gabi,

Thank you very much for your explanation!

A first literature research gave me a Strickler value of 47.4 now. I found grain sizes of 18.8 mm for my river-case and calculated with:

n(Manning) = 0.041 * d(50)**(1/6) (Found inthe Book Hydraulics of Open Channel Flow)

and then

k(strickler) = 1/n(Manning)

Kind Regards
Stefan

Hello,

If you are not happy with your Strickler coefficient to get a correct free surface, the first thing you should investigate is the correctness of the river channel representation in your mesh.

Have you compared for a cross profile the difference between your representation and the actual cross profile ?



Regards
Alexis