Integrated River Management - River Dynamics and Integrated River Management - page 741

The morphology of Yalutsangpu River watershed resulted from both geologic and morphologic
processes. In the past millions of years the Indian continental plate was underthrusting the Himalayan
crustal blocks in a relatively coherent and simple geometry (Ni and Barazangi, 1984). The Indian plate
thrusts against the Eurasnia plate resulting in deformation of Yalutsangpu River watershed and uplift of
the Himalaya Mountains. Figure 11.25(b) shows a conjectural drainage area of Yalutsangpu River millions
years ago. The drainage area was not so narrow and slender but rather two dimensional, similar to a
normal dentritic watershed. At present, the Yigongtsangpu and Yalutsangpu River flow in opposite
directions and collide with each other at the confluence (Fig. 11.25(a)). Nevertheless, such a phenomenon
of two rivers flowing in nearly the same valley but in opposite directions is by no means a result of
morphological processes. Figure 11.25(b) shows the conjectural stream network, in which the two rivers
(Yigongtsangpu and Yalutsangpu) flowed in different valleys. The thrust of the Indian Plate into the
Eurasian Plate changed the flow directions and resulted in the present “opposite rivers.”

11.2.2 Equivalency of Bed Structures and Bed Load Motion
Bed structures, such as step-pool systems, and bed load motion in rivers generate resistance to the water
flow and consume energy. Bed structure and bed load motion are mutually replaceable in energy
consumption and effect on fluvial processes. This is the law of equivalency of bed structures and bed
load motion.

11.2.2.1 Energy Consumption by Bedload Motion and Bed Structures
Sediment particles carried by river flow have larger specific weight than water, and tend to settle down
and stop motion. To maintain the movement of sediment particles, a force is needed to balance the
submerged weight of the particles and prevent them from depositing. Bed load particles move in different
ways depending on flow conditions and the size of the particles. One mode of bed load motion is by
rolling and sliding on the bed, in which the submerged weight of particles is supported by the contact
force. A second mode of bed load movement is by hopping or bouncing along the bed. Solid particles
moving in this way is supported by dispersive force and is known as saltation load. Saltation load
composes the major portion of bed load in intensive bed load motion.
The mechanism of the dispersive force is illustrated, in general, by the following example (Wang and
Qian, 1985). As shown in Fig. 11.26, particle P located at point 1 at instant t 1 moves at a velocity,

o
relative to particle P 1 , and it reaches point 2 at instant t 2 after collision with P 1 and its velocity changes
to

,

V o c Such an abrupt change in velocity, both in magnitude and direction, because of collision, causes
acceleration. The average acceleration during the time interval t 2 - t 1 is

.

oo o

( VV ) ' V

o

a (11.9)

( t t ) ' t

2 1

According to Newton's second law, particle P must be subjected to action of a force. An average value of
the force is given by

G oo o oo

oo ' '

M (( Vii ) ( V j j ) )

FMa (11.10)

' t

o o

where M is the mass of the particle P , i and j are the basic vectors in the longitudinal and vertical

o
' and its two components are shown in Fig. 11.26. They represent the force directions, respectively. V

o

and two force components in the longitudinal and the vertical directions. The vertical component of
the force is the dispersive force, which supports the particles and prevents them from depositing. The
longitudinal component generates the resistance to the flow. The work done by the resistance is the
energy consumption for the motion of one bed load particle:

F