How does the Three Gorges Dam work in Yangtze River flood control?
The flood capacity of Three Gorges Reservoir is 22.15 billion cubic meters (29 billion cubic yards). Every year before the flood season comes, the water in Three Gorges Reservoir is discharged to the allowed lowest level, leaving the space as much as possible to store possible coming floods.
If a regular flood comes, it can be totally stored and intercepted by Three Gorges Dam. In the event of a big flood, flood diversion and flood storage measures will be taken to reduce the flood. For instance, the first flood in 2016 flowed into Three Gorges Reservoir with a speed of 50,000 m3 per second, but it was discharged at 31,100 m3/second. In 2018, the first flood flowed in at a speed of 53,000 m3/s, and out at 40,000 m3/s. After floods, the stored water would be evenly discharged bit by bit.
Besides, when a flood occurred in the downstream, Three Gorges Dam would intercept the part of upper stream water beyond safety discharge of downstream, prevent the flood growing bigger.
Scientific Arrangement of Flood Storage and Discharge
The Yangtze River runoff is almost one trillion cubic meters every year, of which 70-80% is concentrated on the rainy season. But the flood control capacity of Three Gorges Dam is only 22.15 billion cubic meters. Therefore, it is necessary to control flood scientifically. If just simply store water in rainy season for later use, the reservoir would be fully filled soon; in the event of subsequent floods, Three Gorges Dam will lose the effect of flood control. However, Three Gorges Dam also plays roles in electricity generation, navigation and irrigation, which all need water, so the time and the amount to discharge water should be scheduled reasonably, taking all the situations of main stream and tributaries, upstream and downstream, and both banks into consideration.
According to the current dispatching regulations, the water level of Three Gorges Reservoir should be reduced to 155 meters by May 25th each year, and to 145 meters by June 10th, the allowed lowest level. In order to avoid downstream flood because of the water discharge, the water level goes down cannot exceed 0.6 meter per day. Throughout rainy season, Three Gorges Reservoir is always prepared to block floods. When the situation of downstream improves, the flow out of the reservoir would be increased, thus to set room aside for the next flood.
Difficulties for Three Gorges Dam to Control Flood of Yangtze River
1. The flood storage capacity of Three Gorges Dam is limited. Together with all the reservoirs in Yangtze River Basin, the total flood control capacity is less than 20% of the annual runoff of the Yangtze River.
2. There are several large tributaries in the upper reaches of the dam. It is difficult to total their runoff into the Yangtze when all their flood peaks meet. Therefore, the best discharge time and discharge volume cannot be figured out accurately. It will be even more difficult taking into account the largest flood capacity of the downstream at the same time.
3. Many other reservoirs have been constructed on upstream of the Dam. It is hard to coordinate all the large and small reservoirs to work for Yangtze River flood control together. Not a few reservoir operators keep their flood control capacity and discharge plan as business secrets, which also influences the runoff into Three Gorges Dam, too.
4. Three Gorges Dam is the main project for Yangtze River flood control in need of embankments and other supporting projects, which are not complete. The provinces in Yangtze River Basin has strengthen the dikes after the flood disaster in 1998, but those along the tributaries have not. Thus the high standard flood control system has not yet been formed.
5. Soil erosion in Yangtze River Basin has not been totally controlled, which requires higher flood control capacity year by year.
Further Reading: Major Causes of Yangtze River Flood
In the long history of China, the Yangtze has always had huge water runoff from upper reach and tributaries in middle reach, which leads to flood easily almost every year either big or small, especially in rainy season. In addition, there is no big lake in middle and upper reach to store the floods, and the river course there has been too winding to let water flow away. In modern time, soil loss becomes a major culprit of Yangtze River flood. Since 1950s, about 20,000 square kilometers (7,722 sq mi) lost, which is equivalent to 50 billion flood control capacity, 2.2 times of the flood control capacity of Three Gorges Dam.
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