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Fall velocity

The fall velocity is by default calculated using the Rubey (1933) formula1:

\[ w_{s} = \sqrt{(s - 1)gd}\ \left( \sqrt{\frac{2}{3} + \frac{36\upsilon^{2}}{(s - 1)gd^{3}}} - \sqrt{\frac{36\upsilon^{2}}{(s - 1)gd^{3}}} \right) \]

Equation: Rubey formula for the fall velocity of a sediment grain.

This formula expresses the fall velocity as a function of the grain size (\(d\)), relative density (\(s\)) and laminar viscosity (\(v\)).

It is also possible to set a user-defined fall velocity, overriding the value calculated by the Rubey formula. Some non-cohesive sediment transport formulas will not allow zero fall velocity due to their mathematical formulations, a nonzero value is the only constraint on the user-defined fall velocity.

  1. Rubey, W. W. (1933). Settling velocity of gravel, sand, and silt particles. American Journal of Science, (148), 325-338. ↩