The Chengdu Plain, now known as the "Land of Abundance", was a place prone to severe floods and droughts in ancient times. In the famous poem "The Road to Shu Is Difficult", Li Bai's laments and misery, "There are thickets of silkworms and fishtails, how confused the founding of the country is" and "People may become fish and turtles" are true portrayals of that era. This situation is caused by the "harsh" natural conditions of the Minjiang River and Chengdu Plain. The Minjiang River is a major tributary of the upper reaches of the Yangtze River, and the western Sichuan Basin it flows through is a rainy area in China. It originates from the southern foot of Minshan Mountain at the junction of Sichuan and Gansu. It is divided into east source and west source. The east source comes from Gonggaoling and the west source comes from Langjia Ridge. The two sources meet at Wuba in Zhangla, Songpan territory. It flows southward through Songpan County, Dujiangyan City, and Leshan City in Sichuan Province, and joins the Yangtze River in Yibin City. It has a total length of 793 kilometers and a drainage area of ??133,500 square kilometers. The average slope is 4.83‰, and the total water volume averages about 15 billion m3 per year. The Minjiang River originates from Gonggaoling and Langjia Ridge in the Minshan Mountains, with a total length of 735 kilometers and a drainage area of ??140,000 square kilometers. The height of the entire river is 3,560 meters, and the hydropower resources are more than 13 million kilowatts. The Minjiang River is the tributary with the largest water volume in the upper reaches of the Yangtze River. The upper reaches above Dujiangyan are mainly used for driftwood and hydroelectric power generation; the section from Dujiangyan City to Leshan is the middle reaches, flowing through the Chengdu Plain area, and together with the Tuojiang River system and many artificial river networks, it forms the Dujiangyan Irrigation District ; Below Leshan is the downstream, mainly shipping. The Minjiang River has more than 90 tributaries, including the Heishui River and Zagunao River in the upper reaches; the Heishi River, Jinma River, Jiang'an River, Zouma River, Baitiao River, and Puyang River in the Dujiangyan Irrigation Area in the middle reaches; and the Qingyi River and Dadu River in the lower reaches. , Mabian River, Yuexi River, etc. The main water source comes from the steep right bank of the mountain. The large tributaries overflow from the gaps in the mountains on the right bank. Rainfall is mainly concentrated in the rainy season, so the water of the Minjiang River rises and falls rapidly and the water is turbulent. The Minjiang River flows out of the Minshan Mountains and flows southward from the west side of the Chengdu Plain. It is an authentic above-ground river that hangs over the entire Chengdu Plain, and it is very dangerous. The entire terrain of the Chengdu Plain slopes toward the southeast from the Yulei Mountain at the mouth of the Minjiang River, with a very steep slope. Dujiangyan is 50km away from Chengdu, and the drop is as high as 273m. In ancient times, whenever the Minjiang River flooded, the Chengdu Plain was a vast expanse of ocean; when there was a drought, the land was barren for thousands of miles, with no harvest. Floods in the Minjiang River have long affected Xichuan, devouring fertile farmland, intruding on people's livelihood, and becoming a major obstacle to the survival and development of the ancient Shu Kingdom.
In the fifty-first year of King Zhaoxiang of Qin (256 BC), Li Bing, the prefect of Shu County in the State of Qin, and his son learned from their predecessors' experience in water control and led the local people to preside over the construction of the famous Dujiangyan Irrigation Project. The overall plan of Dujiangyan is to divide the water flow of the Min River into two streams, one of which is introduced into the Chengdu Plain. This can not only divert floods and reduce disasters, but also divert water to irrigate fields and turn harm into benefit. The main project includes the Yuzui diversion dike, Feishayan spillway and Baopingkou water inlet. 1. The construction process of Baopingkou First of all, Li Bing and his son invited many farmers with experience in water control to conduct on-site surveys of the terrain and water conditions, and determined to drill through Yulei Mountain to divert water. Since gunpowder had not been invented at that time, Li Bing burned the rocks with fire to explode them, and finally carved a pass 20 meters wide, 40 meters high, and 80 meters long in Yulei Mountain. Because its shape resembles the mouth of a bottle, it was named "Baopingkou", and the pile of stones separated from Yulei Mountain was called "Lidui". The reason why Baopingkou needs to be built is that only by opening up Yulei Mountain so that the Minjiang River can flow smoothly to the east can the flow of the river in the west be reduced, so that the river in the west will no longer flood, and the drought in the east can also be relieved. Let the surging river water flow into the dry areas and irrigate the fertile fields there. This is a key link in flood control and the first step in the Dujiangyan project. 2. Construction process of water diversion Yuzui. After the Baopingkou water diversion project was completed, although it played a role in diversion and irrigation, due to the high terrain in the east of the river, it was difficult for the river water to flow into Baopingkou. In order to enable the Minjiang River water to flow eastward smoothly and maintain A certain flow rate, and giving full play to the flood diversion and irrigation functions of Baopingkou, the builder Li Bing decided to build a water diversion weir in the Minjiang River after excavating Baopingkou to divide the river into two parts: one goes down the river , the other one was forced to flow into the mouth of the bottle. Because the front end of the diversion weir is shaped like the head of a fish, it is called the "fish mouth". The construction of Yuzui divided the river flowing upstream into two parts: the west side is called the Outer River, which flows down the Minjiang River; the east side is called the Neijiang River, which flows into the Baopingkou. Because the inner river is narrow and deep and the outer river is wide and shallow, the water level is low in the dry season, and 60% of the river water flows into the inner river with a low river bed, ensuring water for production and living in the Chengdu Plain; when floods come, due to the high water level, As a result, most of the river water is drained away from the outer river, which is wider. This design of automatically allocating the water volume of the inner and outer rivers is the so-called "four-to-six water divide". 3. In the construction process of Feishayan, in order to further control the amount of water flowing into Baopingkou, play the role of flood diversion and disaster reduction, and prevent the water volume in the irrigation area from being fluctuating and unable to remain stable, Li Bing also constructed the Yuzui Diversion Bank. At the end of the river, close to the mouth of Baoping, a flat water channel for flood diversion and a "Feisha Weir" spillway were built to ensure that there is no disaster in the inner river. There is a curve in front of the spillway, and the river water forms a circulation. When the river water exceeds the top of the weir, the flood will The entrained mud and rocks flowed into the outer river, so that they would not block the inner river and Baopingkou waterway, so it was named "Feisha Weir". The Feisha Weir is built with pebbles packed in bamboo cages. The top of the weir is set to a suitable height, which plays a role in regulating the amount of water.
When the water level of the Inner River is too high, the floodwater will flow over the Feisha Weir and flow into the Outer River through the flat water channel, so that the amount of water at the mouth of the bottle will not be too large when entering, protecting the Inner River irrigation area from floods; at the same time, the flood will flow over the Feisha Weir and into the outer river. The water flow of the river creates swimming vortices, and due to centrifugal action, mud, sand and even boulders will be thrown across the Feisha Weir, thus effectively reducing sediment deposition around the mouth of the bottle. [1] In order to observe and control the water volume of the Neijiang River, Li Bing carved three stone statues and placed them in the water to determine the water level based on the principle that "dry water does not cover the foot, and flood does not cover the shoulder". A stone horse was also carved and placed in the middle of the river as a standard for scouring the beach during the minimum water flow every year. [2] Under Li Bing's organization and leadership, people overcame numerous difficulties, and after eight years of hard work, they finally completed this historical project - Dujiangyan.