Flood Defense and Water/Sediment Management—With Particular Reference to The Yellow River - River Dynamics and Integrated River Management - page 363

along the course; so that these rivers exhibit concave riverbed profiles. Equation (6.2) indicates the
direction of morphological processes and equilibrium state of longitudinal river profile. Sediment load
plays an important role in the speed of morphological process but does not change the direction and the
final equilibrium of the profile. The higher is the sediment load the faster is the morphological process.
For a low sediment load river the riverbed profile often does not meet Eq. (6.2) because it takes a very
long time to reach the minimum stream power profile.
The Yellow River carries heavy sediment load and the morphological processes is fast. The large
quantity of water diverted along the course of the Yellow River makes the term s d Q/ d x negative. For
instance, since 1986 the average discharge has decreased along the Yellow River course in the reach
downstream of Huayuankou, i.e. d Q/ d x<0 . According to Eq. (6.2) the term Q d s/ d x must be positive. In
this case, the riverbed profiles will develop toward a convex shape, which is different from the normal
concave curve. Figure 6.33 shows the bed profiles of the lower Yellow River for 1977 and 1997. The mean
bed elevation is the average bed elevation of the channel with a cross-section of wet area about 500 m 2 .
The figure shows that the lower section of the river is developing toward a convex profile. Because the
profile of the upper section is concave, the river shows an “ S- shape” longitudinal bed profile. The trend
will continue and the turning point in the profile will move upstream because the water diversion is
continuing and more water will be diverted in the foreseen future (Wang and Hu, 2004).

Fig. 6.33 Longitudinal bed profiles of the lower Yellow River in 1977 and 1997 (Wang and Hu, 2004)

6.4.6 Delta Channel Stability and Land Creation
The modern Yellow River flows into the Bohai Sea via Lijin since the river levee at Tongwaxiang (about
600 km from the present river mouth as shown in Fig. 6.1) was broken and flood water captured the
Daqing River channel, in 1855. Thence the reaches upstream of Lijin have been densely populated and
the levees in the reaches were enhanced and reinforced many times, and no avulsion and channel shift
occurred in these reaches since 1855. The population density down stream from Lijin was low and the
levees downstream of Ninghai were weak to resist the assault of flood. Therefore, nodal avulsions occurred
around Ninghai (60-100 km from the mouth as shown in Fig. 6.34) and the river changed its delta
channel 11 times between 1855 and 1976. The river channel swept over a fan-shape area of radius about
50 km. The present delta was created by rapid sediment deposition in the past 147 years, accompanied by
frequent shifts of the channel in the area. The 12 old river channels are shown in Fig. 6.34 (Wang and Liang,
2000), that each has its own name. The present channel-Qing-Shui-Gou Channel- has been in use since
1976. A recent minor shift of the mouth channel (about 20 km from the mouth) to Chahe occurred in
1996, which is also shown in the figure. More detailed data on the channel shift and location of avulsions
are presented in Table 6.7 (Yin and Chen, 1993, Zhang et al., 1997).