Integrated River Management - River Dynamics and Integrated River Management - page 734

Figure 11.16 shows the velocity-discharge relations for the Yellow River at the Huayuankou and Gaocun
hydrological stations (Liu, 2007). The Yellow River carries an extremely high sediment load, therefore, it
is very dynamic. The cross sections vary every year and the bank-full wet areas consequently vary. In
general the main channel has been shrinking in recent years due to sedimentation and less high flood
events scouring the channel. The velocity increases with discharge faster in a small channel than in a large
channel. Therefore, the points greatly scatter. Nevertheless, the velocity does not surpass a limit velocity
between 2.5-3 m/s for channels with different bankfull areas.

Fig. 11.16 Velocity-discharge relations for the Yellow River at the Huayuankou and Gaocun hydrological stations

In mountain rivers the development of the cross section is constrained by bed rock and bank rock. At a
gorge section the velocity may be higher than 2.5 m/s during extremely high flood events. Nevertheless
the high velocity sections are rather short. Figure. 11.17 shows the average velocity along the course from
Yichang to Chongqing and Yibin on the Yangtze River at a discharge of 30,000 m 3 /s, which equals the
normal flood discharge. The velocity was calculated with the discharge and cross sectional areas at
narrow sections and wide sections. Although the velocity at a few locations is higher than 2.5 m/s, the
average velocity for all reaches is well below 2.5 m/s.
The limit velocity law has both morphological and ecological implications. As listed in Table 11.1, all
fish species cannot live in high flow velocities. River flow remains mostly within the velocity range of
0.3-2.5 m/s, which is the velocity most suitable for many fish species to live. On the other hand, natural
rivers develop complex channels to reach the highest stability. In other words, the natural complex channel