Dujiangyan, a World Heritage Site: How did the water miracle 2300 years ago create a land of abundance?

China contemporary writer and scholar wrote after visiting Dujiangyan (a world cultural heritage and a national 5A tourist attraction): "I think the most exciting project in the history of China is Dujiangyan, not the Great Wall. On the surface, its scale is far less magnificent than the Great Wall, but it is destined to benefit for thousands of years. "

Dujiangyan belonged to the territory of Shu during the Spring and Autumn Period and the Warring States Period. As early as the reign of King Huiwen of Qin, the State of Qin greatly enhanced its national strength through Shang Yang's political reform, and started the process of conquering the six countries (Chu, Qi, Yan, Zhao, Wei and Han) and unifying the world. However, due to various conditions at that time, there were some differences in the order of conquering the six countries.

Represented by General Sima Cuo, he advocated attacking Shu first, because the geographical position of Shu is very important. Once Shu is laid, it can attack Chu downstream through the Yangtze River and its tributaries. There has always been a dispute between Shu and neighboring Pakistan. This is very beneficial for Qin to unite against Shu. At that time, Sima Cuo said to King Hui of Qin: "There is rebellion in Shu, and its country is rich, with enough cloth, silk, gold and silver for military use. Water is connected with Chu and has the power of Pakistan. You can get it by drifting a big ship eastward to Chu and Chu. If you get Shu, you will get Chu, and if Chu dies, the world will be merged. "

However, Sima Cuo's military proposition that "Chu is obtained when Shu is captured, and world cooperate is obtained when Chu is captured" accords with the actual situation that Qin unified the world, so the idea of attacking Shu first was quickly adopted by King Hui of Qin.

As we all know, today's Shu State is located in the Sichuan Basin, which is surrounded by plateaus and mountains with an altitude of 1000 m to 3000 m, such as the Qinghai-Tibet Plateau, Daba Mountain, Huaying Mountain and Yungui Plateau. The terrain is complex, easy to defend but difficult to attack. According to the poet Li Bai, it is "it is difficult to get into the sky through the Shu Road".

The reason why Qin took the lead in attacking Shu was related to a war in 3 16 BC. At that time, Shu sent troops to attack Pakistan, and Pakistan had to ask Qin for help, so King Qin Huiwen sent Sima Cuo to help Pakistan attack Shu. After Shu was destroyed, Qin conveniently destroyed Pakistan, so Bashu was under the jurisdiction of Qin.

After Shu and Ba were destroyed one after another, Qin took the opportunity to head for Chu, but the strength of Chu was second only to that of Qin in the Seven Heroes of the Warring States Period, which meant that the contest between Qin Chu could not be fought in three or two years (in fact, Chu was destroyed in 223 BC), and after Qin destroyed Shu, Shu rose up one after another, so Shu was revoked and Shu County was changed into the granary and garrison of Qin.

In ancient times, the soldiers and horses did not move food and grass first. Shu county (now Chengdu area) is located in the plain, with relatively low terrain, and the Minjiang River in the west flows from north to south. Although it is only twenty or thirty kilometers away from the center of the plain, it is not easy for the carriage to be the most advanced means of transportation in ancient times. More importantly, the overall terrain of China is high in the west and low in the east, so the Minjiang River is a veritable "hanging river" on the whole Chengdu Plain. In addition, the western Sichuan Basin where Minjiang River flows is a rainy area. When the Minjiang River flooded, there was a piece of Wang Yang in the eastern plain, but it was a thousand miles of sand after the flood receded. In addition, Leiyushan on the east side of Minjiang River blocked the eastward flow of Minjiang River, resulting in perennial drought and flood disasters in Chengdu Plain.

In 256 BC, under the military proposition that "Chu is the key to Shu and Chu is the key to the world", Li Bing, the magistrate of Shu County who is familiar with astronomy and geography, and his son (Erlang, the prototype of Erlang God) were ordered by the king to build Dujiangyan from 256 BC to 25 BC1year, which turned the Chengdu Plain, which suffered from severe floods and droughts, into a unified military granary of Qin State and also became the Chengdu Plain.

Minjiang River is an important tributary of the "mother river" in the upper reaches of the Yangtze River. It originated in the southern foot of Minshan Mountain at the junction of Sichuan and Gansu. Its east and west sources flow southward through Songpan County, Dujiangyan City and Leshan City, and merge into the mainstream of the Yangtze River in the northeast of Yibin City, with a total length of 7 1 1km (65438+ if Dadu River is the main source).

After taking office, Li Bing inspected the Minjiang River on the spot. He pointed out that in order to cure the flood and drought problem in Chengdu Plain, drainage and water diversion must be carried out. He found that when the Minjiang River flows through the west side of Lei Yu, Dujiangyan, because of the hard mountain, it blocked the way of water flow and made the river channel approximate an arc. Digging a hole in Leiyu Mountain to bring the Minjiang River water to Chengdu Plain can solve many problems such as irrigation, traffic, flood control and drought resistance.

Because the rocks are hard, it will take nearly 30 years to chisel with a pickaxe without gunpowder at that time. Li Bing thought of the method of expanding with heat and contracting with cold. First, heat the rock with fire, and then water it. After the rock is cracked and loosened, it is easier to dig. In this way, Li Bing worked for nearly 8 years, and Lei Yushan was finally cut into a hole 20 meters wide, 40 meters high and 80 meters long. This hole is named "Treasure Bottle Mouth".

Although the water diversion project of "Baokoukou" has been completed, the east bank of Minjiang River is blocked by mountains, and it is generally difficult for the river to flow from "Baokoukou" to Chengdu Plain as expected. At this time, Li Bing and his son led people to build a diversion weir not far from the upstream of the bottle mouth. They filled bamboo cages with stones and piled them in the middle of the river, forming a long and narrow artificial island similar to a fish mouth, which is what we see today.

After the completion of "Yu Zui", Minjiang River was directly divided into Waijiang River and Neijiang River. Outer rivers are mainly used for flood discharge, while a large part of the water in Neijiang flows from Baokou Port to Chengdu Plain. At first glance, the fishmouth dike is simple, but it actually contains profound scientific truth.

First of all, the width and depth of Neijiang and Waijiang are different, and the water distribution in different periods is also different. The inner river is narrow and deep, while the outer river is wide and shallow, so the amount of water shared by one deep, one shallow, one wide and one narrow in different periods is subtle. In the dry season of winter and spring, the water quantity of Minjiang River decreases. Because the Neijiang River is deep, the main river line is near the concave bank of Neijiang, about 60% of the water flows through Neijiang, while the outer river is shallow, and about 40% of the water flows through the outer river, thus ensuring the irrigation water for farmland. On the other hand, in the wet season, the water level of Minjiang River rises greatly, and the main line of the river is relatively straight. Because the Neijiang River is narrow and the outer river is wide, about 40% of the water flows through the Neijiang River and 60% of the water flows through the outer river in rainy season, thus avoiding the farmland from being flooded. This is the famous "46 water diversion". This shows that the functional design of fish mouth still has its magic.

Secondly, in addition to the fishmouth diversion dike, the mouth of Baokou bottle and the "flying sand weir" to its east can also play the role of water diversion and water control. The width and height of the bottle mouth are small, and the amount of water passing through is limited. Moreover, the flying sand weir is actually a dam with relatively low height. When the amount of water flowing through the east side of Feisha weir is small, the water with low water level will flow to Chengdu through the bottle mouth. When the water level rises, part of the water will flow back to the outer river over the flying sand weir, so this is also the magic point of controlling the water quantity by the bottle mouth and flying sand weir.

Finally, a very important point in the construction of water conservancy projects is the function of dredging sand. If the sediment cannot be discharged effectively, once a large amount of sediment is deposited, the operation of water conservancy projects will be affected (Sanmenxia Reservoir is an example). Li Bing skillfully applied some physical principles to make Dujiangyan's desilting function design reveal the magic in nature. According to the law of bend flow, that is, the river with less sediment on the water surface will flow along the concave bank at the bend of the river, while the river with more sediment on the bottom will flow along the convex surface. According to statistics, the proportion of this kind of sediment separation is about 2:8 from Yu Zui diversion levee, that is, 20% of the sediment will go to Neijiang and 80% to Waijiang, which is also commonly known as "28% sediment separation".

Of course, after the sediment enters Neijiang, it does not completely flow to Chengdu or deposit in Neijiang, but because of the existence of the flying sand weir and the rock wall on the left bank (the direction of water flow), the bottom water flow is blocked and the vortex is generated. When the water flow in Neijiang exceeds the upper limit of the bottle mouth flow, the excess water will leak into the outer river through the flying sand weir, and about 15% of the sediment flowing to Neijiang will be discharged from the flying sand weir with the water flow, thus ensuring that,

In addition, any water conservancy project is inseparable from post-maintenance, and Dujiangyan water conservancy project is no exception. In fact, in the dry season of winter and spring, Dujiangyan was renovated, and the silt deposited in Neijiang and Waijiang rivers was washed away as much as possible to strengthen the flying sand weir, which was widely circulated among the people.

It is precisely because of the careful maintenance of Dujiangyan by the people of past dynasties that Dujiangyan has been able to operate continuously and steadily to this day, even after nearly 2300 years of baptism. Dujiangyan is located in Longmenshan fault zone in China, but it has even withstood the cruel test of Wenchuan earthquake in 2008, which shows the greatness of the project.

He also presided over other water conservancy projects in Shu County, built bridges and dug salt wells in Guangdu, and made great contributions to the development of Chengdu Plain and agricultural production. The legend of Li Bing's water control has existed since the Eastern Han Dynasty. In the Tang Dynasty, Li Bing Temple was built in Daojiang County (now Dujiangyan City). The myth that Li Erlang, the son of Li Bing, helped to control water began to spread in the Northern Song Dynasty. In folklore, the prototype of the mythical figure Erlang God is Li Bing's son.

During his tenure in Shu County, Li Bing, the magistrate, also built other water conservancy facilities, presided over the construction of bridges and salt wells, and made great contributions to the agricultural development in western Sichuan and the development of Chengdu Plain.