**We have chosen
to study wet-snow avalanches in order to be able to run simulations with
Telemac2d.**
**Globally, you can describe
the forces which are acting on a pack of snow as follows:**

**Then you obtain the following
equation:**

**where U is the speed
of the solid of weight m (here U is the average speed of the avalanche)**

**and F is the sum of the
external forces which are acting on this solid.**

**Fp represents the driving
forces (drawn in blue)**
**Fn represents the drag forces
(drawn in red)**

**The only driving force is
of course the driving part of the gravity, which can be written as:**
**The drag forces, linked
to different wasting processes, are difficult to modelize, depending on
the rheological model chosen. Otherwise, many authors agree to express
this force as follows:**

**The first term represents
the solid rubbing:**
**The second term is due to
the viscosity forces, linked to speed gradient in the flow. Those forces
are predominant in very slow flows and can be in consequence negligible
in many models.**
**The third term represents
the flow turbulence. The coefficient of turbulence rubbing, c, can be defined
as follows:**

**Finaly, you obtain this
basis equation:**