The Shields parameter, also called the Shields criterion or Shields number, is a

nondimensional number A dimensionless quantity (also known as a bare quantity, pure quantity, or scalar quantity as well as quantity of dimension one) is a quantity to which no Dimension (physics), physical dimension is assigned, with a corresponding International Sys ...

used to calculate the initiation of motion of

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...

in a fluid flow. It is a nondimensionalization of a

shear stress Shear stress, often denoted by (Greek: tau), is the component of stress coplanar with a material cross section. It arises from the shear force, the component of force vector parallel to the material cross section. ''Normal stress'', on the ...

, and is typically denoted

\tau_\ast

\theta

. This parameter has been developed by Albert F. Shields, and is called later Shields parameter. The Shields parameter is the main parameter of the Shields formula. It is given by: :

\tau_\ast = \theta = \frac,

where: *

\tau

is a dimensional shear stress; *

\rho_s

is the density of the sediment; *

\rho

is the density of the fluid; *

g

is acceleration due to gravity; *

D

is a characteristic particle diameter of the sediment.

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.

References

* *

External links

* {{Geology-stub Sedimentology Dimensionless numbers Fluid dynamics