Particle size Ds may be defined by its volume, diameter, weight, fall velocity, or sieve mesh

size. Except for volume, these definitions also depend on the shape and density of the

particle. The following definitions are commonly used to describe the particle size (U.S.

Interagency Subcommittee 1943):

1. Nominal diameter - The diameter of a sphere having the same volume as the particle.

2. Sieve diameter - The diameter of a sphere equal to the length of the side of a square

sieve opening through which measured quantities (by weight) of the sample will pass. As

an approximation, the sieve diameter is equal to the nominal diameter.

3. Sedimentation diameter - The diameter of a sphere with the same fall velocity and

specific gravity as the particle in the same fluid under the same conditions.

4. Standard fall diameter - The diameter of a sphere that has a specific gravity of 2.65 and

also has the same terminal settling velocity as the particle when each is allowed to settle

alone in quiescent, distilled water of infinite extent and at a temperature of 24C.

In general, sediments have been classified into boulders, cobbles, gravels, sands, silts, and

clays on the basis of their nominal or sieve diameters. The size range in each general class

is given in Table 3.1. The non-cohesive material generally consists of silt (0.004-0.062 mm),

sand (0.062 - 2.0 mm), gravel (2.0 - 64 mm), or cobbles (64-250 mm).

The boulder class (250 - 4000 mm) is generally of little interest in sediment problems. The

cobble and gravel class plays a considerable role in the problems of local scour and

resistance to flow and to a lesser extent in bed load transport. The sand class is one of the

most important in alluvial channel flow. The silt and clay class is of considerable importance

in the evaluation of stream sediment loads, bank stability and problems of seepage and

Generally speaking, shape refers to the overall geometrical form of a particle. Sphericity is

defined as the ratio of the surface area of a sphere of the same volume as the particle to the

actual surface area of the particle. Roundness is defined as the ratio of the average radius

of curvature of the corners and edges of a particle to the radius of a circle inscribed in the

maximum projected area of the particle. However, because of simplicity and effectiveness of

correlation with the behavior of particles in flow, the most commonly used parameter to

describe particle shape is the Corey shape factor, Sp, (Albertson 1953) defined as:

lc

Sp =

(3.1)

l al b

where: 4a, 4b, and 4c are the dimensions of the three mutually perpendicular axes of a particle

4a the longest; 4b the intermediate; and 4c the shortest axis.

3.2

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