After visiting Dujiangyan and Qingcheng Mountain, Mr. Yu wrote the following sentence: "Ask Qingcheng Mountain after visiting Dujiangyan". There are many mountains and rivers in our country, which are different from each other. Chengdu's landscape culture can be seen. Dujiangyan, as an important water conservancy project in Chengdu Plain, has been running for 2000 years, benefiting many future generations.
Since Dujiangyan is so effective, why didn't China copy Dujiangyan as a water conservancy project in a large number throughout the country?
The fundamental function of Dujiangyan is flood control and irrigation.
Minjiang River is an important tributary of the upper reaches of the Yangtze River, and it was even once considered as the positive source of the Yangtze River in history.
Its total length is 1.279 km, and its basin includes Aba, Chengdu, Leshan, Yibin and other important cities in Sichuan, and it is a first-class river in Sichuan. On the other hand, Dujiangyan is located at the key point where Minjiang River enters Chengdu Plain from the western mountainous area of Sichuan Basin.
Because the Minjiang River here in Dujiangyan has just flowed out of a huge mountain area, the Minjiang River in Dujiangyan has a large difference and a high water volume. In ancient times, when the summer floods, the Chengdu Plain immediately became a land of thousands of miles, with no people or animals.
About 250 BC, that is, about 2300 years ago, Li Bing was appointed as the prefect of Shu County by order of Qin Dynasty. From then on, Li Bing and his son built Dujiangyan at the entrance of Minjiang River into Chengdu Plain, turning decay into magic and Minjiang River into profit, creating a "land of abundance" for more than 2,000 years.
In the flood season, due to the high water level at this time, more river water will be discharged from the wider outer river, estimated at about 60%.
As for the remaining 40% of the river water entering Neijiang, there is a treasure bottle mouth at the bottom of Neijiang. As shown in the following figure, Baokou Port diverts part of the river water to Chengdu Plain for irrigation.
Because part of the water in the bottle mouth flows, the Minjiang River will not be flooded during the flood season, and the diverted water can also be used to irrigate farmland, killing two birds with one stone.
From the two key facilities, namely, the water distribution fish mouth and the treasure bottle mouth, we can know that the fundamental function of Dujiangyan is flood control and irrigation.
Therefore, when the water level is high in the wet season, the outer river is divided into more rivers for flood discharge, and when the water level is low in the dry season, the Neijiang River is divided into more rivers because of its deep riverbed to ensure the irrigation water demand of farmland.
According to the modern observation and calculation of water quantity, the water quantity distributed in the outer river accounts for 60%, and the inland river accounts for 40%.
In the dry season, the proportion of water distribution is completely reversed, from 40% in the outer river to 60% in the inner river. At this time, more water is distributed to Neijiang, which ensures the irrigation water demand of bottle mouth in dry season to the greatest extent.
It is precisely because Dujiangyan has this magical effect of dynamically adjusting the water quantity of Minjiang River in different seasons that it has won the eternal reputation that "floods and droughts are caused by people".
What is the function of flying sand weir?
Sha Fei Weir is a low dam about 2 meters higher than the riverbed, and its main body is composed of cobblestone cages submerged in water.
In the final analysis, the function of flying sand weir is actually two functions-controlling water quantity and discharging sand from Neijiang.
Although the sand and gravel content of Minjiang River water is not as good as that of Yellow River, there are more sand and gravel in flood season.
If these sand stones are not treated for a long time, they will block the Neijiang River and raise its riverbed, which will lead to the failure of the "four-six water diversion" mechanism mentioned above.
In order to solve the problem of "sand and gravel accumulation in Neijiang", a facility, Sha Fei Weir, was built in Dujiangyan General Project.
The first important function of flying sand weir is to stop water.
In the dry season, because the overall water quantity of Minjiang River is not large, it is necessary to introduce 60% of the river water from Neijiang into Baokou as irrigation water as much as possible, and the flying sand weir plays the role of stopping water.
However, in the rainy season, the Minjiang River sometimes breaks out in great floods. This situation has nothing to do with human resources at all, but only with the rainfall that summer.
When this happens, although the water distributor is still working, the water flow in Neijiang will become very large, and if the water flow in Neijiang exceeds the flood discharge of Baokou, it will also cause floods.
At this time, the second function of the flying sand weir is reflected-flood discharge.
Because the flying sand weir is made of cobblestone cages that sink to the bottom of the water, it will be washed away automatically by the huge flood impact, which will make the Neijiang River completely unobstructed and play a role in flood discharge. When the flood passes and the dry season comes, people will find that the cobblestone cage rushes downstream in summer, and then sinks back at the position of the flying sand weir, so that it can continue to bear the role of retaining water in the dry season.
In other words, the ancient flying sand weir was actually a movable dam, which burst when the flood came and existed when the water was low, so it was simple to make and low in cost.
In addition, the flying sand weir has a third function-sand washing.
It uses the principle of curve cycle.
Dujiangyan The whole Minjiang River is in a bending state, especially Neijiang. Such a place, using the principle of "curve circulation", is naturally a wonderful place to wash sand.
The principle of circulation in a bend is an important theory in fluid mechanics: when water flows in a bend, it will produce centrifugal force, which will make the upper water flow point to the concave shore and the lower water flow point to the convex shore, forming a whole water flow that keeps rotating forward. The schematic diagram is as follows:
Neijiang in Dujiangyan is just a bend, so there will be many vortex undercurrents in the river. When these eddies travel to the flying sand weir, they will be stopped by the underwater weir. Under the action of vortex, the sand and gravel at the bottom of the river will be thrown into the flowing water above by the rotating water flow, thus leaking out over the flying sand weir.
In other words, in front of the weir facing Neijiang, there are a lot of eddies and undercurrents all the year round. Although these eddies appear and disappear constantly, they, like conscientious cleaners, constantly clean up the sand and gravel deposited in Neijiang and discharge them from the river passing above the Sha Fei weir.
This is also the origin of the word "flying sand" in the flying sand weir, which is extremely vivid.
This weir was built later. The original flying sand weir sank to the bottom, only 2 meters high, and the river could pass through it.
The mystery of Dujiangyan construction
The three main parts of Dujiangyan water conservancy project, namely fish mouth, bottle mouth and sand weir, constitute the main body of the whole Dujiangyan water conservancy project. It automatically divides water, discharges sand, irrigates and discharges flood. The whole operation is not only highly automated, but also has been used for more than 2000 years, and its reliability is staggering, and it still plays an irreplaceable role today.
With today's technology, no matter what machines are made, I'm afraid they can't run continuously for 2000 years, but Dujiangyan did it!
In fact, until today, it is extremely difficult for human beings to completely reproduce another Dujiangyan with the existing technology, because even if we travel around the world, we can't find another Minjiang River, another place with the same geographical characteristics and similar annual water volume as Dujiangyan.
Its success only exists in a short section of Minjiang River in guanxian. With the current infrastructure capacity of China, it may be possible to completely replicate in another river at the expense of manpower and material resources, but the cost is too high, which loses the original meaning of Dujiangyan's high cost performance.
Fortunately, today's technology has completely explored the scientific mystery of Dujiangyan. Although it cannot be completely copied, its various operating principles can be used for reference by other water conservancy projects and benefit people all over the world in another way.