Basic Concepts and Management Issues of Rivers - River Dynamics and Integrated River Management - page 10

classification of sediment size must follow a geometric scale; i.e., an appropriate ratio between neighboring
groups must be taken. For the Atterberg and the Chinese classification, the ratio is 10, and for the
American Geophysical Union classification the ratio is 2. In connection with this aspect, sieve openings
for granulometry also follow a certain ratio. For example, the sizes of the openings in the Tailor sieves
follow the ratio of 2 0.25 . The grain sizes such as 0.005, 0.05, and 2 mm are used as demarcations of sediment
groups because sediment particles larger and finer than these tend to show quite different characteristics,
as shown in Table 1.1 (Ruhin, 1957).
Table 1.1 Relation between particle diameter and sediment properties (after Chien et al., 1998)
Note: D is diameter of particles and Z is the fall velocity of a single particle in water
Sediment is the product of weathering of rocks, and thus, is closely related to lithology. Figure 1.9
shows different sediments from the areas with different lithologic compositions. In the upper reaches of
the East River, which is a tributary of the Pearl River in China, most of the drainage area is composed of
granite. Feldspar in the granite changed into kaolinite in the process of weathering, which results in quartz
and mica particles and fine kaolinite particles (Fig. 1.9(a)). There are few gravel, cobbles and boulders in
this area. In the Xiaojiang watershed, the rocks consist of metamorphic rocks, including phylite, layer
stone, mud stone, and sand stone. Weathering of these rocks generates solid particles with various sizes
from clay to boulders. Figure 1.9(b) shows the sediment deposits of debris flow in the area. In the
limestone area in Dali in Yunnan Province in South China the rock is weathered mainly by dissolution by
rainfall containing CO 2 . In the process no granular material is produced. Therefore, there is a very thin
layer of soil on the hills and very poor vegetation may develop in this area (Fig. 1.9(c)). Generally speaking,
granite produces boulders, coarse sand and small amount of clay; limestone produces boulders, cobbles
and gravel; shale produces clay and silt; sandstone produces boulders, cobbles, gravel and sand; and
metamorphic rocks produces various sizes of sediment.
Size distribution —Sediment refers usually not to individual particles but rather to a mixture with
innumerable particles of different sizes, shapes, and mineral compositions. Although sediment particles are
not cemented together to form an entity, many properties pertinent to sediment manifest themselves mainly
through the existence of an ensemble of sediment particles. Size distribution of sediment particles,
including the degree of uniformity, reflects directly the properties of parent rocks and the intensity of the
sorting process by river flow; it is also closely related to the amount of sediment transported. The most
useful method to describe the size distribution of sediment is the cumulative size- frequency curve, in
which sediment size (its logarithm) is taken as the ordinate, and the percentage by weight of sediment