Encyclopedia of Environmental Science and Engineering, Volume I and II

SEDIMENT TRANSPORT AND EROSION 1065

significant quantities in alluvial beds; often wash

load is arbitrarily taken to be sediments finer than

0.062 mm, i.e. silts and clays.

2) Suspended bed material load which is the portion

of the suspended load derived primarily from the

channel bed; generally the bed material is assumed

to be the sediment coarser than 0.062 mm, i.e.

sands and gravels.

Table 1 indicates the terminology used by the American

Geophysical Union in describing various sizes of sediment.

Properties of Sediments

An excellent review of the properties of sediments is presented

by Brown.^1 He discussed the determination and significance

of the following:

a) properties of the individual particle,

b) particle size distribution and

c) bulk properties of sediments.

Properties of the Particle Neglecting interaction effects, the

behavior of an individual particle in a stream depends on its

size, specific weight, shape, and the hydraulics of the stream.

Two commonly used methods of determining particle

size are: (1) mechanical sieve analysis and (2) the fall velocity

method. The sieve analysis method differentiates particle size

on the basis of whether or not the particle will pass through

a certain standard square opening in a sieve or mesh. This

method is applicable for sands or coarser particles. Except in

the case of spheres, “sieve size” will only be an approxima-

tion to the true equivalent diameter of the particle since the

results depend to some extent on the particle shape.

The fall velocity method of determining the effective sedi-

ment size is gaining popularity in sediment transport research.

On the basis of the particle’s terminal velocity, in a specified

fluid (water) at a specified temperature, the particle is assigned

a fall or sedimentation diameter equal to the diameter of the

quartz sphere which has the same terminal velocity in the

same fluid at the same temperature.^1 This particle size inte-

grates the effects of grain size, specific weight and shape into

a single meaningful parameter for sediment transport studies.

Researchers at Colorado State University have developed a

Visual Accumulation Tube to aid in the determination of the

fall diameter distributions for sediment samples.

Particle Size Distribution On the basis of a sieve analysis

of fall diameter analysis, of a sediment sample, a cumulative

frequency curve for the particle size can be drawn. Figure 1

shows typical particle size frequency curves for a sample taken

from a sandy stream bed and for a sample of suspended load

over the same bed.^6 The frequency curves are usually plotted

on logarithmic-probability paper.

TABLE 1

Sediment grade scale

Group Particle size range, mm

Boulders 4096–256

Cobbles 256–64

Gravel 64–2

Sand 2–0.062

Silt 0.62–0.004

Clay 0.004–0.00024

FIGURE 1 Typical particle size frequencies curves for stream sediments (after Bishop et al.).

GRAIN SIZE (mm.)

IN TRANSPORT (by dunes)

PERCENT FINER (by weight)

ON THE

BED

1.0

.01 .05 .1 .2 .5 1 2 5 10 20 30 40 50 60 70 80 90 95 96 99 99.5 99.9 99.99

.10

.08

.20

.30

.40

.50.

.60

.80

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