Shields parameter

The Shields parameter, also called the Shields criterion or Shields number, is a nondimensional number used to calculate the initiation of motion of sediment in a fluid flow. It is a nondimensionalization of a shear stress, and is typically denoted formula_1 or formula_2. It is given by:
where:
Physical meaning.
By multiplying the top and bottom of the Shields parameter by "D"2, you can see that it is proportional to the ratio of fluid force on the particle to the weight of the particle.