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
3.2.2 Particle Shape
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:
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.