open TELEMAC-MASCARET The mathematically superior suite of solvers

Can someone explain to me, or point me to some references, two concepts:
VELOCITY DIFFUSIVITY
Chezy's Law of Friction (law of bottom friction = 3)

When I say explain, give a basic understanding in simple terms.
I've searched the documentation and I've searched the forum and can't find something that explains these concepts.

VELOCITY DIFFUSIVITY:
What is it? What exactly does it do in the calculation?

Chezy's Law of Friction:
What is it? How can I calculate the bottom friction using it? Are there any sources that list typical frictions for various bottoms (sand, rock, gravel, etc)?

Some background:
I'm calculating the velocities of a tidal flow (ebb and flood) of a fjard. I simulated it by increasing the level of water at the liquid boundary.
I just want to understand all the bits of the analysis, and I just haven't found a good source(s) that explain these two concepts.

Thanks,
ME

Hello,

Velocity diffusivity is the turbulent diffusion for the velocity, which is used instead of the laminar viscosity in Reynolds Averaged Navier-Stokes models.

The Chézy's law is like the Strickler law, but without the dependency on depth in the formula, so friction is only proportional to 1/C**2. Anyway the Strickler law is better, because the Strickler coefficient is supposed to depend only on the bed roughness, unlike the Chézy coefficient which in principle is valid only for a given depth, reason why it is seldom used, except perhaps in maritime applications where the depth does not vary much.

With best regards,

Jean-Michel Hervouet