Why didn't the Three Gorges Dam empty the water before the flood season?

2020-09-03 | Science on the planet Original |

As we all know, every flood season of the Yangtze River, many areas in the Yangtze River Basin suffer from different degrees of floods, bringing serious disasters to the local area.

Because there is the Three Gorges Dam in the upper reaches of the Yangtze River, so as long as the flood season comes every year, no matter whether the Yangtze River floods or not, there will always be a common topic on the Internet. Why has the Three Gorges Dam not solved the Yangtze River flood? Or why the Three Gorges DamDoes the dam not empty the water before the flood season? Do you have to save up to a large wave?

The scale of the Three Gorges Dam.

The Three Gorges Dam is the largest hydropower project in the world today. The dam is 2,335 meters long, 181 meters high, and has a normal water storage level of 175 meters. The flood control reservoir has a capacity of 22.15 billion cubic meters.

The amount of excavation and filling of the main building and diversion tank of the project is about 134 million cubic meters, the amount of concrete pouring is about 28 million cubic meters, and the use of steel bars is 463,000 tons, which is 11 times the amount of steel used in the Bird's Nest.

The metal structure other than steel bars is 256,500 tons, 26 sets of hydro-generator sets are installed, and the installed capacity is as high as 22.5 million kilowatts. The data are amazing. Therefore, the Three Gorges Dam is of great significance to China.

Flood control capacity of the Three Gorges Dam

The Three Gorges Dam is a concrete gravity dam, and the gravity dam itself can rely on its own weight to resist the pressure and impact of the upstream water. At the same time, the Three Gorges Dam is equipped with 77 holes and 67 holes on the spillway dam.

From top to bottom, it is divided into three layers, namely the overflow surface hole, the flood discharge deep hole and the diversion bottom hole. These spill holes are usually not used because the water used for the power generation and navigation locks of the Three Gorges is the same as that of the Yangtze River.The water flow is almost the same, but if there is a flood, some spare spill holes will be activated.

The annual flood flow rate of the Three Gorges Dam is 98,800 cubic meters per second, and the annual flood flow rate is 113,000 cubic meters per second. The design standard of the Three Gorges Dam is to prevent the once-in-millennium flood, but its checkThe standard is to prevent the once-in-10,000-year flood add 10% to the flow of the once-in-a-year flood.

That is, when the once-in-a-millennium flood comes, all functions of the dam can operate normally. When the once-in-10,000-year flood is encountered, some of the dam's functions may be in danger, but the largeThe main dam and main buildings are still safe.

Since July, my country has entered a flood season, and many places have suffered precipitation that has not been seen in a hundred years. Among them, more than 28 provinces have been affected by floods. The average precipitation in the Yangtze River Basin reached 346.9 mm, which exceeded the precipitation of the 1998 catastrophic flood by 331.9 mm .

Why not empty the Three Gorges Dam before the flood season?

Although the construction of the Three Gorges Dam can effectively prevent floods, the function of the Three Gorges Dam is not only flood storage, but also power generation, shipping, and water supply.

First of all, the Three Gorges Dam also contributes greatly to power generation. The total installed capacity of the Three Gorges Dam is as high as 22.5 million kilowatts. According to data, as of 2018, the cumulative power generation of the Three Gorges Dam is 1.2 trillion kilowatt-hours.

If calculated according to the on-grid electricity price of 0.25 yuan/kWh, the cumulative power generation income exceeds 300 billion yuan, then the Three Gorges Dam can generate 100 million yuan a day. However, the Three Gorges Dam can only use hydropower when the water storage capacity of the Three Gorges Dam is increased., If there is no water storage capacity, that is, if the water level is not high, the Three Gorges Dam cannot be fully charged to generate electricity.

Secondly, the Three Gorges Dam also plays a huge role in agricultural irrigation and water supply in other areas. If the Three Gorges Dam empties the water in the reservoir in advance before the flood season, then the water supply in some areas will become a problem.It may even cause severe drought problems.

Finally, in terms of shipping, the normal water storage of the Three Gorges Dam is 175 meters, and the total reservoir capacity can reach 39.3 billion cubic meters. The water level can be emptied to 55 meters, and there are several sand discharge holes, and the bottom elevation is 75 meters.

Generally speaking, this 75-meter elevation should be the dead water level. If the water is emptied and there is no water level, the large tonnage cannot be navigable.

But the water level of the Three Gorges Dam is too high. Therefore, the current dispatching interval of the Three Gorges Dam is in the annual flood season. The upstream water level is guaranteed to be at the flood control limit water level of 145 meters, and the storage capacity of 30 meters is extra to meet the flood peak, ensuring its availability.The storage capacity of the dispatching reservoir is 22.1 billion.

Why is the Three Gorges Dam releasing flood during the flood season?

Anyone who knows the Three Gorges Dam should know that the annual water storage of the Yangtze River reaches 960 billion cubic meters, of which 70 to 80% are concentrated in the rainy season in June and July.

And the total storage capacity of the Three Gorges is 39.2 billion cubic meters, and the dispatchable storage capacity is 22.1 billion. This storage capacity cannot stop the sudden and heavy rainfall. Therefore, when the upstream heavy rainfall arrives.

If the Three Gorges Dam continues to store water, it will be unable to withstand the upstream flood peaks, which will lead to some disasters. To avoid such a situation, the Three Gorges Dam must wait for the flood in the middle and lower reaches of the river to be controlled.

Open gates to release floods and reduce the reservoir capacity of the Three Gorges to welcome the next heavy rainfall. This is the main reason why the Three Gorges Dam needs to release floods during the flood season. In fact, it is not only the Three Gorges Dam that needs to release floods during the flood season. my country includes the Yellow River. The main reservoirs are also releasing floods during the flood season.

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