Presets
Playback
Snow flow
Friction, turbulent drag and the flow depth of the moving snow.
Slope profile
Steepness and length of the starting zone, then the transition to the flat.
Layers
Model checks
The science
Voellmy-Salm block model
The moving snow is treated as one block sliding along the slope with arc-length coordinate s. Its speed obeys du/dt = g (sin(theta) - mu cos(theta)) - g u^2 / (xi h): gravity along the slope, dry Coulomb friction (coefficient mu) and velocity-squared turbulent drag (coefficient xi in m/s2, flow depth h). On a long steady slope the speed saturates at the terminal value u_t = sqrt(xi h (sin(theta) - mu cos(theta))).
Terrain and runout
The profile has a steep starting zone of angle theta_s, a transition of linearly decreasing angle and a flat runout. The avalanche stops where u reaches zero. Practitioners summarise the geometry by the alpha angle (from the top of the release area to the stopping point, tan(alpha) = drop / horizontal distance) and the beta angle (to the point where the slope falls to 10 degrees). The empirical Lied-Bakkehoi relation, alpha = 0.96 beta - 1.4 deg, is drawn for comparison.
Simplifications
A single block with constant flow depth: no snow entrainment, no splitting into dense flow and powder cloud, no lateral spreading or channelling, mu and xi are constants (real values depend on snow, terrain and speed). The model is a teaching tool for the competition of gravity, friction and drag, not for hazard mapping.
What to try
- Raise mu from 0.2 to 0.5: the avalanche stalls on the slope instead of reaching the flat.
- Raise xi (smoother, less turbulent drag): top speed rises toward the frictional limit.
- Lengthen the transition zone and see how the stopping point and alpha angle change.
- Watch the alpha-beta graph: the simulated stop lies near the empirical line for typical parameters.