open TELEMAC-MASCARET The mathematically superior suite of solvers

Hi there,

I have setup a very basic, non-cohesive sediment model chained to a previous hydrodynamic model to experiment with movable bed sediment transport. The Sisyphe model is setup with a single layer, and sediment class, with all bed roughness prediction and slope effects neglected.
The drag coefficient is being specified through the use of a constant Chezy coefficient in the steering file and the Meyer-Peter bedload formula is being employed.

I am able to successfully validate the bed load transport and total bed shear from the model output against hand calculations for this basic case when the SKIN FRICTION CORRECTION = 0 (I am taking scalar velocity from the sisyphe output). Initially I was having some difficulty and realised that by default this parameter actually specifies a skin roughness length, ks' proportional to the sediment diameter (ICR = 1) which is different to what the manual states.

My understanding is that in terms of bed load the skin friction correction should only affect the skin friction shear and hence Shields parameter for the Inception of bed load movement in the Meyer-Peter formulation. It seems however that the TOB (Total Bed Shear) output to the SELAFIN results file is being reduced when the skin friction correction is activated (almost as if it is no longer the total bed shear by the estimated skin shear stress). Should this not solely be a function of the Chezy and depth averaged velocity?

If anyone could assist if helping me to try and understand what is happening here it would be greatly appreciated. Please let me know if you need more clarification.

Best Regards,
Toby

Hello,

yes, as per default the skin fiction correction is on.
Hm, the 'TOB' output should give you the total bed shear stress, hence without skin friction correction. So you say, when you activate the skin friction correction, you get another TOB in the output? This would be strange, unless in a newer Telemac version something has changed.

You can give in the Sisyphe steering file also the output of the variable 'MU' which ist the skin correction friction factor. The grain shear stress is then TOB * MU.
If you have a coupled simulation with Telemac, then you can output the friction velocity 'US' in the Telemac steering file. Then you could compare the Telemac total shear stress with the Sisyphe total shear stress TOB.

Hope this helps,
Clemens

Thanks for the reply,

I just wanted to follow up on the issue to let you know that there was no difference in the total bed shear between the two simulations (ICR=0 and ICR=1) as should be expected.

It was a simple oversight of my own in checking a node where I had considerable difference in the evolution of the seabed between the two simulations. Because Sisyphe attempts to adjust the velocities due to conservation of mass in uncoupled mode my total bed shear was changing as a result between the simulations.

This issue was not apparent at nodes where no seabed evolution was noticed.

Best Regards,
Toby