Environmental Engineering Reference
In-Depth Information
greatly reduced. If enough such frames are put onto the river bed in the middle reaches of the Yangtze
River, the flow energy consumption and bed protection effect of the frames is the same as if a lot of bed
load is fed into the river. River bed incision will stop and flood stage will rise to the level before the dam.
Therefore, the water diversion from the river into Tongting Lake will soon be regained and the migration
of fish between the river and lake will be not affected. The effect of the human made “bed structure” on
the navigation channel must be studied before application of this measure.
11.2.3 Sediment Budget—Size Distribution Method
The sediment budget studies the distribution of sediment in different parts of a river basin. The method
deals only with the sediment amount without consideration of the transport mechanics. The International
Association of Hydrological Sciences (IAHS) organized a symposium on the theme “Sediment Budget”
in 1988. In selecting the theme, the organizers consciously chose a topic which draws on a wide range of
research and which represents an important area of current interest (Bordas and Walling, 1988). As yet
the study of the sediment budget is in its infancy and more research is required to develop the necessary
monitoring and modeling strategies and to improve understanding of the processes involved. In the past
many studies have focused on the erosion processes operating within a basin and sedimentation at its
outlet. Now there is an increasing awareness of the need to integrate the two and to establish sediment
budgets, which attempt to qualify the relations among the various components of the overall drainage
basin erosion-transportation-deposition system.
Wang et al. (1997) studied the sediment budget and sediment demand of rivers. There are various
demands for sediment, for instance, sediment mining for building material, land creation using sediment,
maintaining river regime equilibrium, and preventing the channel bed from eroding. Both sediment yield
and sediment demand need to be studied. Sediment budget studies balance between the yield and the
demand and consider the disturbance to the balance by human activities.
11.2.3.1 Sediment Budget of The Yangtze River
Figure 11.42 shows the 6300-km long Yangtze River, which is the largest and longest river in China,
with a drainage area of 1.80 million km 2 . The figure also shows the locations of hydrological stations,
meteorological stations, tributaries, riparian lakes, and debris flow areas. Below the map is the diagram
of the water shed topography on an east-west line from Batang-to Chongqing-to Poyang Lake, showing
the elevation of the basin from the origin in the west to the river mouth in the east. The Yangtze Basin has
elevation varying from 5,000 m to 0 m with latitude from N25 e to N35 e . The river flows through the
Qinghai-Tibet Plateau, Yunnan-Guizhou Plateau, Sichuan Basin, Three Gorges, Jiang-Han Plain, Lower
Yangtze Plain, and pours into the East China Sea at Shanghai. From the source to Yichang (Three Gorges
Dam site) is the upper reach, from Yichang to Hukou (Poyang Lake Mouth) is the middle reach, from
Hukou (Wuhan) to Datong is the lower reach, and below Datong is the estuary.
In China, the Yangtze River is called Changjang (long river), with special names for different stretches:
the lower reach and the estuary are called the Yangtze River; the middle reaches are called the Jingjiang
River; the reach from Yichang to Yibin is called the Chuanjiang River, from Yibin to Batang is called the
Jinsha River; and from Batang to the origin is called the Tongtian River (heaven river).
Investigation of the temporal and spatial variability of sediment transport within the upper Yangtze
River catchment leads to the following paradox. There is evidence that the extent and magnitude of soil
erosion across southern China has increased dramatically during the last 30-40 years (Smil, 1993; Wen,
1993; Lu and Higgitt, 1999; Edmonds, 1994). But there is no evidence of sediment load increasing at the
Yichang Hydrological Station delivered from the upper Yangtze River catchment. Moreover, many studies
have indicated that the total soil erosion from the upper reaches of the Yangtze River is around 2.2 billion
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