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Settling velocity for a particle of sediment can be derived using Stokes Law assuming quiescent (or still) fluid in steady state. The resulting formulation for settling velocity is,

$$ w_s = \frac{g~(\frac{\rho_s - \rho}{\rho})~d_{sed}^2}{18 \nu} $$,

where $$ g $$ is the gravitational constant, 9.81 $$ m/s^2 $$; $$ \rho_s $$ is the density of the sediment; $$ \rho $$ is the density of water; $$ d_{sed} $$ is the sediment particle diameter (commonly assumed to be the median particle diameter, often referred to as $$ d_{50} $$ in field studies); and $$ \nu $$ is the molecular viscosity of water. The Stokes settling velocity can be thought of the terminal velocity resulting from balancing a particles' buoyant (proportional to the cross-sectional area) and gravitational forces (proportional to the mass). Small particles will have a slower settling velocity than heavier particles, as seen in the figure. This has implications for many aspects of sediment transport, for example, how far downstream a particle might be advected in a river.