Critical grain size (diameter)


Sediment transport is the movement of solid particles (sediment), typically due to a combination of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained. Sediment transport occurs in natural systems where the particles are clastic rocks (sand, gravel, boulders, etc.), mud, or clay; the fluid is air, water, or ice; and the force of gravity acts to move the particles along the sloping surface on which they are resting. Sediment transport due to fluid motion occurs in rivers, oceans, lakes, seas, and other bodies of water due to currents and tides. For a fluid to begin transporting sediment that is currently at rest on a surface, the boundary (or bed) shear stress exerted by the fluid must exceed the critical shear stress for the initiation motion of grains at the bed. Critical grain size is related to the Reynolds number ( the ratio of the total momentum transfer to the molecular momentum transfer).

Related formulas


DCritical grain diameter (m)
νThe kinematic viscosity (m2/s)
Re Reynolds number (dimensionless)
ULocal free stream velocity (m/s)