Flood Defense and Water/Sediment Management—With Particular Reference to The Yellow River - River Dynamics and Integrated River Management - page 385

Retrogressive erosion will occur because the stage reduction at Lijin increases the energy slope in
upstream reaches. A 200-400 km long section upstream of Lijin can be affected by the retrogressive
erosion and the accumulative siltation of the river channel can then be stopped. The high capacity of the
river channel can be preserved by the strategy. Nevertheless, the injection of seawater in the Yellow
River may cause salinization of the farmland and cause ecological problems to the areas downstream of
the injection point.
To minimize the negative effect of the strategy, the second scheme is recommended. In the scheme, the
injection point is Xihekou, 47 km downstream of Lijin. From Xihekou to the river mouth there is little
agricultural land-use and the floodplain is now mainly used to plant shrubs and trees, and raise shrimp.
The canal is only 24 km long to transport water from the sea to Xihekou (Fig. 6.21). The seawater will be
directly pumped into the Yellow River at a rate of flow of 1,000 m 3 /s. Figure 6.54 shows the result of
numerical model studies for scheme 2—injection at Xihekou (Lin et al., 1998). After 10 years of seawater
scour and retrogressive erosion, the thalweg at the injection point may be scoured down by 7-8 m but the
flood stage (3000 m 3 /s ) may be lowered by 4.4 m.
Fig. 6.54 The thalweg profiles of the river before and after 10 years of scouring with seawater and retrogressive
erosion for scheme 2—injection at Xihekou (after Lin et al., 1998)
Difficulties are still likely in the implementation of the new strategy. For instance, the power needed
for pumping water has a high cost input and the ecological impacts of pumping sea water into the river
will be very complex. The following studies have to be conducted before the strategy is adopted: (1) the
effects of the seawater scour should be further studied with physical scale model(s); (2) Impacts of
seawater injection on the ecological system in the delta should be carefully studied. The maritime section
of the Yellow River is only 20 km long because the tidal difference at the river mouth is only about 1 m.
An artificial sea water flood in the river may cause alkalization of the riparian land. The riparian habitat,
land use, fauna and animal population could be changed. (3) Comprehensive study is required on the
riverbed scour with seawater, transportation of sediment by sea currents land creation at the river mouth,
and the delta evolution and development. (4) Conduct a comparative study on seawater scour with
dredging and other strategies.
6.6.3 Artificial Hyperconcentrated Floods
The main problem in the lower Yellow River is high sediment load and insufficient water flow. The
average sediment concentration in the lower Yellow River is about 25-40 kg/m 3 . Hyperconcentrated
floods may transport sediment at concentration up to several hundred kg/m 3 . Generation of artificial
hyperconcentrated floods is suggested to transport sediment at high efficiency. Hyperconcentrated flow
scours the main channel and causes sediment deposition on the floodplain. Therefore, hyperconcentrated flow
can create and maintain a deep and narrow channel (Qi, 1993). Recorded data show that hyperconcentrated