open TELEMAC-MASCARET The mathematically superior suite of solvers

Dear all,
I am new in TELEMAC.I read telemac3d_theory and sisyphe usermanual trying to learn how telemac3d couple with sisyphe.As far as i know:

1.If i choose Nikuradse law as LAW OF BOTTOM FRICTION then FRICTION COEFFICIENT FOR THE BOTTTOM :ks = 2.5*D50

2.Shear velocity = 0.41*Uplanel/ln(33*Z/ks)

3.Cf = 2g/(7.83*ln(12*waterdepth/ks))^2



4.Shear stress = -0.5*1000*Cf*(shear velocity)^2

5.shear stress then take into function of the Shields number and calculate bedload transport formula
Is there any wrong??

Many thanks!

Dear Jason,

The bottom friction is set in the hydrodynamics (in telemac3d e.g. to a unique value). This is given to sisyphe. And has at this stage nothing to do with the mean diameter.

The shear stress is calculated in tob_sisyphe.f from the shear velocity which is calculated in telemac3d according to the friction law (tfond.f). If it is Nikuradse it is
UETCAR=(KARMAN/LOG(30*Dist/KS))**2 * (U3D**2+V3D**2)
U3D, V3D: velocities at layer 2 (counting starts at the bottom)
DIST= Distance between the two bottom-most layers
Shear stress = UETCAR * density_water
KARMAN = 0.4 by default but can be set to another value in the steering file.

For the bedload only the skin friction should be taken into account (if skin friction correction=1, which is the default). In this case the skin friction is computed. For Nikuradse it is RATIO BETWEEN SKIN FRICTION AND MEAN DIAMETER=3 (default) * d50 (which is calculated in case of multi grain. A skin friction correction factor MU is calculated as ration between shear stress with skin friction and shear stress with friction from telemac.

In case you choose BED ROUGHNESS PREDICTION = YES the roughness will be calcualated by sisyphe and given back to telemac3d. This is possible but dangerous as the prediction of the full bed roughness (not only the skin friction) is very empirically. So for most cases I would not recommand to use this option.

Depending of the chosen bed load formula the dimensionless Shields parameter or the shear stress is using to calculate the solid discharge.

I hope this helps you a bit.
Best regards,
Rebekka