Flood Defense and Water/Sediment Management—With Particular Reference to The Yellow River - River Dynamics and Integrated River Management - page 346

artificial flood created with the reservoir. The concentration of the released flow is planned at 10-20 kg/m 3 .
In order to control the concentration released sediment, the water is released from the bottom, middle,
and top outlets. The released concentration from the bottom outlet is high dying the water yellow and red,
and the water from the top outlet is clean and white. The flood scoured the river channel bed and the
average concentration of suspended load increased from less than 20 kg/m 3 to 30 kg/m 3 . From 2002 to
2004 three experiments were conducted to scour the lower Yellow River with artificial flood before the
flood season. Each year 40-60 million tons of bed sediment were scoured by the artificial flood. Now the
artificial flood has become a routine operation of the Xiaolngdi Reservoir before the flood season.
6.2.8 Dredging
The main flooding risk is due to the quick siltation and capacity reduction of the channel. Therefore,
sedimentation control and increase of the water-conveying capacity of the channel is main aim of river
training and the key criteria of the new strategies of river management. Besides the traditional strategies
and regulation of sediment and water by using the Xiaolangdi Reservoir, Yellow River channel dredging
has become an important auxiliary measure. Training of the river by human actions become much more
common than before, and, therefore, people have begun to use dredging for controlling sedimentation.
They dreamed that through dredging the river channel shrinkage at selected locations can be alleviated or
stopped and a high capacity for carrying sediment, ice, and water can be preserved.
The functions of dredging are to: (a) widen and deepen the shrinking river channel at selected sections;
(b) remove coarse sediment of diameter larger than 0.025-0.05 mm from main channel to the floodplain
so to reduce the accumulative siltation of the main channel; (c) raise the elevation of the surrounding
ground, reinforce dikes, improve soil quality, and create new wetlands at the river mouth with the dredged
sediment.
Historically, the Yellow River channel was dredged many times, but the results were not satisfactory.
On the one hand, annual sediment load was quite high and the dredged channel was soon resilted. On the
other hand, dredging was not supported by advanced technology and suffered from lack of experiences.
Today, the conditions and requirement for dredging are different. Many river training projects in China
and other countries provide rich experience on dredging and the development of technology and dredgers
has greatly improved the efficiency of dredging. Furthermore, there are less and less extreme events and
the channel bed is rarely scoured. In the meantime economic development has enabled advanced dredgers
to be used for river training. The Yellow River can be dredged by mechanical excavation and transportation,
agitating with jets and explosion. Various dredgers have been used: dredge boat; agitating dredger; dipper
dredger; hauling scraper; excavator and bulldozer; and trailer dredger (Zhang et al., 1997). Figure 6.18(a)
and (b) show two types of agitating jet dredgers scouring sediment during flood season.
At the river mouth the width becomes larger, the depth also increases, the slope becomes gentler, and,
therefore, the velocity here greatly reduces. The sediment carried by the flow is unloaded and a mouth
bar is formed. The river mouth bar causes a resistance and results in higher flood stages in the upstream
reaches. Sediment was, hence, deposited in the upstream channels before the 1980s, which was one
reason for the frequent avulsion and channel shift. In the 1980s, dredging projects were implemented to
remove the mouth bar. Different dredgers were used and the river mouth has been since then maintained
free from a mouth bar. Because of the dredging of the mouth bar, the Qing-shui-gou channel has been
used for much longer than the average life span of previous channels.
The success of dredging the mouth bar encouraged people to dredge the river channel as a solution of
the channel sedimentation problem. A dredging test was conducted in a 11 km long section near the river
mouth (35-46 km downstream of the Lijin Hydrological Station) in January to May 1998. An 11 km long,
200 m wide, 2.5 m deep ditch was dug in the almost flatted river bed and 5.48 million m 3 of sediment was