Sediment Properties - Sediment Transport in Irrigation Canals: A New Approach - page 49

3.3 SIZE AND SIZE DISTRIBUTION

A first attempt to classify the individual, non-cohesive sediment particles,
is based on their size; more specifically their diameter. Usually, sediments
are referred to as gravel, sand, silt or clay, which refer to the size of the
individual particles; listed here in decreasing magnitude. Many attempts
have been made to describe the sediment by one distinct diameter and
a range of sizes has been suggested, but none of them has proven to be
entirely suitable. Some of the main definitions for the diameter (Bogardi,
1974) include:
• Sieve diameter ( d ): diameter of the square mesh sieve, which will just

let the particle pass. The sieve diameter is generally used for fractions
greater than 0.1 mm.

• Settling or sedimentation diameter ( d s ): diameter of a sphere with the

same density and same fall velocity as the given particle in the same
fluid and at the same temperature.

• Nominal diameter ( d n ): diameter of a sphere or cube with the same

volume as the particle.

• Tri-axial dimensions ( a , b , c ): the dimensions of a particle in the direc-

tion of three orthogonal axes. The largest dimension is called ' a ' and
the smallest dimension is called ' c '.
The sizes can be determined by direct measurements, by sieving, by
sedimentation or by microscope analysis. Table 3.2 shows a classification
of sediments as recommended by the American Geophysical Union.

Table 3.2. Classification of sediments according to their size.

10 − 3 m 10 − 6 m

Classification mm = m =

µ

Very coarse gravel 64-32

Coarse gravel 32-16

Medium gravel 16-8

Fine gravel 8-4

Very fine gravel 4-2

Very coarse sand 2-1 2000-1000

Coarse sand 1.00-0.50 1000-500

Medium sand 0.50-0.25 500-250

Fine sand 0.25-0.125 250-125

Very fine sand 0.125-0.062 125-62

Coarse silt 0.062-0.031 62-31

Medium silt 0.031-0.016 31-16

Fine silt 0.016-0.008 16-8

Very fine silt 0.008-0.004 8-4

Coarse clay 0.004-0.002 4-2

Medium clay 0.002-0.001 2-1

Fine clay 0.001-0.0005 1-0.5

Very fine clay 0.0005-0.00024 0.5-0.24