Why is the water conservancy project from 2,300 years ago still in use today? The principle of Dujiangyan is amazing

If you were asked to quickly name two famous water conservancy projects, besides the Three Gorges Dam, the first one that comes to mind would probably be the Dujiangyan Irrigation System.

Some people may even think that Dujiangyan, like the Three Gorges Dam, was built in modern or contemporary times, but in fact, Dujiangyan was built in the Eastern Zhou Dynasty more than 2,300 years ago. Such a water conservancy project built more than 2,300 years ago is still in use today. We can even say that Sichuan, the "Land of Abundance", was built by Dujiangyan. So why did the ancients build Dujiangyan? In the Eastern Zhou Dynasty, Shu was not as prosperous as it was later. At that time, droughts often broke out in the Chengdu Plain, and floods often occurred on both sides of the Minjiang River in the west. It can be seen that Shu at that time was a disaster-ridden place ruled by droughts and floods. In 316 BC, the Qin State destroyed the Shu State and later established Shu County, which marked the beginning of Shu's transformation from a disaster-ridden place to a prosperous place.

Since Shang Yang's reforms, the Qin State has become increasingly powerful and has long wanted to annex the six states. After conquering Shu, it intended to turn Shu into a granary in the rear of the Qin State.

But Shu was plagued by disasters, so how could it become a granary? In order to completely change the situation in Shu, the Qin State finally decided to divert the Minjiang River into the Chengdu Plain. This would not only change the flood problem of the Minjiang River during its flood season by diverting the water, but also eliminate the drought near Chengdu, killing two birds with one stone. Although the idea was good, it was difficult to implement it. First of all, there was a Yulei Mountain between the Minjiang River and Chengdu. In order to divert the Minjiang River into Chengdu, an opening had to be made on the Yulei Mountain. With the construction capacity at the time, this was a huge project that would take 20 to 30 years to complete.

Furthermore, even if an opening is successfully made on Yulei Mountain, how will the water be distributed after the canal is dug to divert water? How will the water volume be regulated during the dry and flood seasons? How will the beach be dredged and sand removed?

These are all very thorny problems, and even in modern times, they are still quite difficult to design. So how did Dujiangyan solve this problem? It must be said that the principles contained in Dujiangyan are enough to amaze today's physicists. The people responsible for the construction of Dujiangyan were Li Bing, the governor of Shu County, and his son. It is hard to imagine that a person living 2,300 years ago could think of opening mountains by using the method of thermal expansion and contraction. Li Bing and his son first ordered people to burn the rocks with fire, and then pour cold water on them. The heat and cold made the rocks very brittle and easy to excavate, so a project that would have taken 20 to 30 years was completed in eight years.

Compared with opening mountains and digging canals, how to divide water, control water and drain sand are the real challenges.

It can be seen from the structural diagram of Dujiangyan that the location chosen by Li Bing and his son to build Dujiangyan is exactly at a bend in the Minjiang River. They built an island-shaped fortification for diversion at the bend of the river. The front end of this diversion island is called "Fish Mouth". The left side of Fish Mouth is called the Outer River and the right side is called the Inner River. The water in the Inner River will eventually flow to Chengdu through the Baopingkou below. There is a trick here. When Li Bing was designing it, he designed the Inner River to be deeper than the Outer River, and the Outer River was wider than the Inner River in terms of water surface. This design ensures that the Chengdu area will not be short of water during the dry season and will not be flooded during the flood season. Why?

During the dry season, the water level of the Minjiang River is lower, but because the inner river itself is deeper than the outer river, the inner river gets more water than the outer river, which ensures water supply for the Chengdu area.

When the flood season comes, the water level rises. Due to the water surface, the outer river is obviously wider than the inner river, so the outer river gets more water than the inner river. This controls the water flowing to Chengdu and prevents floods. What if the water volume is particularly high in a certain year? It doesn't matter. Do you see the Feishayan below? It is a very low weir. As long as the water level exceeds this Feishayan, the excess water will flow back to the outer river from here, so Chengdu will never be flooded. This ingenious design solves both water diversion and water control. The next step is to drain the sand. When the river water flows, it will carry a lot of silt, which will inevitably block the river channel over time. How to solve it?

Although Li Bing did not understand what physics was, physics was applied everywhere in his designs.

The sediment is not evenly distributed in the river, especially when the river turns. The sediment itself is heavy, so it will go to the inside of the bend when turning, while the clear water without sediment will be thrown to the outside of the bend. This is why Li Bing chose to build the Dujiangyan at the bend of the river. The river water carrying sediment will go to the inside of the bend and most of it will enter the outer river, while the clear water will go to the outside of the bend, so it will enter the inner river and flow to Chengdu. A small amount of sediment that enters the inner river will also go to the inside of the outer channel of the inner river, so it will eventually return to the outer river through the Feishayan. Since the sediment basically goes to the outer river in the end, the annual beach digging work has become very easy, and the Dujiangyan can continue to operate. Such a successful damless water diversion project was built 2,300 years ago, which can be called a miracle.