Basic Concepts and Management Issues of Rivers - River Dynamics and Integrated River Management - page 9

Mechanism of merging of streams —According to the minimum stream theory, the river network
develops to reach the minimum stream power per distance. Two parallel u -th order streams transporting
water at discharges Q 1 and Q 2 merge to form a ( u+ 1)-th order stream. The ( u+ 1)-th order stream is more
meandering and transports water with discharge equal to the sum of Q 1 and Q 2 . The difference of stream
power between the ( u+ 1) - th order stream and the two u- th order streams is:

' (1.16)
in which s is the slope of the larger stream, s 1 and s 2 are the slopes of the smaller streams. Replace the
slopes s 1 and s 2 with the average slope of the u- th order stream , s u , and replace the slope s with the average
slope of the ( u+ 1) - th order stream, Eq. (1.16) becomes

P s J J ( Qs Qs )

11 2 2

' (1.17)
From Eq. (1.3) s u+ 1 is smaller than s u , thus ' must be negative. Therefore, the stream power becomes
smaller as the two streams merge into a larger stream. In general, small streams always merge into larger
ones to minimize the stream power.

PQQs J ( )( s )

1 2 u 1 u

1.1.3 Sediment
Three primary geomorphic processes affect rivers, i.e., ķ erosion, the detachment of soil particles;
ĸ sediment transport, the movement of eroded soil particles in flowing water; and Ĺ sediment deposition,
settling of eroded soil particles to the bottom of a water body. In these processes sediment plays
undoubtedly the main role. The following section introduces the main concepts of sediment and sediment
transportation.
Sediment is defined as the solid particles found in a deposit after transportation by flowing water, wind,
wave, glacier, and gravitational action. Sediment discharge is defined as the mass or volume of sediment
passing a stream cross section in a unit of time. The typical units for sediment discharge is tons per second
or per day. To differentiate various types of particles, sediment is subdivided into groups. For a long time,
various kinds of terminology have been used for the different sizes of sediment particles.
Sediment classification —Attenberg made his classification at the beginning of the nineteenth century.
It was approved by the International Association for Soil Sciences in 1927 as the standard in soil analysis,
and it has been widely adopted in European countries. Most American geologists use Wentworth's
classification (Wentworth, 1922). In 1947 the American Geophysical Union drew up a new standard for
sediment classification (Subcommittee on Sediment Terminology, 1947). This standard is based on the
same groups that Wentworth used. The only difference is that each group was subdivided, so that the
denomination of classes is more complete. Classification of sediment in hydraulic engineering in China
followed that applied in the former Soviet Union. Some divergence remains between this classification
and those in European countries and the United States. This topic uses the Chinese classification. Two
classifications of sediment are given in the following:
Chinese classification:
Boulder (> 200 mm) > Cobble (20 - 200 mm) > Gravel (2 - 20 mm) >

Sand (0.05 - 2 mm) > Silt ( 0.005 - 0.05 mm) > Clay (<0.005 mm)

Attenberg's classification
Boulder (> 200 mm)

> Cobble (20-200 mm) > Gravel (2 - 20mm) >

Coarse Sand (0.2 - 2 mm) > Fine sand ( 0.02 - 0.2 mm) > Silt (0.002 - 0.02mm) >

Clay (<0.002 mm)
Despite the several classifications of sediment used in various countries, they have some points in
common. Most of the intervals for the sediment groups are unequal, because the sizes cover such a wide
range. Stone blocks and fine clay particles differ by a factor of more than a million. Evidently, the