open TELEMAC-MASCARET The mathematically superior suite of solvers

Hi!

I would like to have some details on how is computed the shear stress (skin friction) for pure cohesive sediments.
According to suspension_computation.f, TAUP = TOB * MU, TAUP being the skin friction used in Partheniades formula in suspension_erosion_coh.f.

This would mean that the skin friction for cohesive sediments is computed using the grain size assuming a log profile and turbulent flow in the boundary layer, which might not be true for very fine sediments (clays with d50 =< 0,06 mm). Is there a White-Coolebrooke formula programmed in Sisyphe that woud allow to account for transitional flow in the boundary layer? Do you recommend a specific method to estimate the skin friction for clays?

Thanks in advance for your help!

Regards
PL

Hi Pilou,

You are right, for the friction we assume in Sisyphe turbulent rough boundary layer(which is generally the case, even for a 1mm bed roughness).

For a smooth turbulent flow, you would have to program the White Coolebroke law based on what is done in Telemac 2D(see friction_calc.f).


When dealing with cohesive sediment myself, I generally assume a skin bed roughness of about 1 mm even if the grain is very small, there are always some irregularitiees in nature (presence of shells or con-cohesive). There are two ways to do this:
1. turn off the skin friction correction and assume a Nikuradse friction law (ks=ksp=1mm)
2. Keep the skin friction correction and increase either the diameter or the KSPRATIO.The cohesive grain diameter is only used to determine the skin friction (KSPRATIO= 100 for D=.01 mm)

Good luck, Cath

Hi Pilou,

You are right, for the friction we assume in Sisyphe turbulent rough boundary layer(which is generally the case, even for a 1mm bed roughness).

For a smooth turbulent flow, you would have to program the White Coolebroke law based on what is done in Telemac 2D(see friction_calc.f).


When dealing with cohesive sediment myself, I generally assume a skin bed roughness of about 1 mm even if the grain is very small, there are always some irregularitiees in nature (presence of shells or con-cohesive). There are two ways to do this:
1. turn off the skin friction correction and assume a Nikuradse friction law (ks=ksp=1mm)
2. Keep the skin friction correction and increase either the diameter or the KSPRATIO.The cohesive grain diameter is only used to determine the skin friction (KSPRATIO= 100 for D=.01 mm)

Good luck, Cath

Hi,

First of all, thank you for the information about estimating skin friction for cohesive sediments.

I am working with mixed sediments(cohesive/non-cohesive) and was wondering if I could apply a different KSPRATIO for my different sediment fractions??

Thank you for your help,

Regards,
Christian