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 dimensionalization of a shear stress, and is typically denoted
) describe the conditions when the sediment starts moving.
Note that the shear stress is a property of the current, while the critical shear stress is a property of the sediment.
By multiplying the top and bottom of the Shields parameter by D2, you can see that it is proportional to the ratio of fluid force on the particle to the weight of the particle.