Settling Velocity and Suspension Velocity

The velocity of flowing water determines which particles will be suspended in the water.  Fast flowing water can hold very heavy objects while slow water only holds very light particles.  

Particles settle selectively by velocity.

Every material has its own suspension and settling velocity.  The suspension velocity is the speed of water above which the water will pick up the material and hold it in suspension.  The settling velocity is the speed below which the material will be dropped out of suspension and will settle out of the water.  

Large, heavy objects have higher suspension velocities and lower settling velocities - they require faster water to hold them in suspension.  Particles commonly suspended in water, listed from highest to lowest suspension velocity, include rocks, sand, silt, and organics.  

In addition to size and weight of the object, the suspension and settling velocities are influenced by shape and surface roughness of the object, and by temperature and viscosity of the water or fluid through which the material is moving.  

Suspension velocity is influenced by a material's size, shape, and density.  The particles with a ratio of a greater density and lesser surface area are able to resist the effect of water's increased velocity while the particles with a lesser density per size ratio are overcome and are picked up by the water.

The relative sizes of gravel, sand, silt, and clay particles are shown below:

Size of gravel, sand, clay, and silt particles.

Sand and gravel are both large and dense.  In addition, they have a small surface area per unit volume since they are roughly spherical.  So these types of particles have a high suspension velocity.

Soil particles.

Loam soil, which is magnified above, is made up of a combination of sand, silt, and clay.  Since silt and clay particles are so small, it is unsurprising that soil has such a low suspension velocity. 

Leaf litter.

The picture above shows a patch of leaf mold slightly reduced in size.  The particles can be seen with the naked eye and are much larger and more irregular in shape than the particles of the other three test specimens.  Despite their larger size, leaf mold particles have a very large surface area and a low density.  As a result, leaf mold is lifted into the water at a low velocity.