Analysis:
Deep scour beach, low weir; When it comes to cutting corners, if you are right, you will draw your heart. -"Dujiangyan Erlang Temple City Wall"
"Deep beach scouring" refers to digging the sediment deposited at the bottom of the river deeper to avoid the water in Neijiang being too small to be irrigated; "Low weir" means that the top of the flying sand weir cannot be built too high, so as to avoid poor flood discharge in flood season and endanger the Chengdu Plain.
Li Bing was an outstanding water conservancy engineer and the designer and organizer of Dujiangyan during the Warring States Period. Dujiangyan, which is famous at home and abroad, is located in the middle reaches of Minjiang River in central Sichuan Province. The whole project consists of three main projects: diversion weir, flying sand weir and treasure bottle mouth. Its large scale, suitable location and reasonable layout have three functions of flood control, irrigation and shipping, and it is also a rare miracle in the history of water conservancy projects in the world. For more than two thousand years, it has been playing a huge role in irrigation and drainage, ensuring local agricultural production.
Li Bing's life, hometown, year of birth and death and the construction of Dujiangyan at that time will not be described in detail. I only know that in the fifty-first year of King Wang (256 BC), Li Bing was appointed as the guard of Shu County. After Li Bing arrived in Shu County, he witnessed the seriousness of the local disaster: it originated from the Minjiang River in the northern part of Chengdu Plain, with high mountains and deep valleys on both sides and rapid currents; Near guanxian, entering a Ma Pingchuan, the water potential is huge, and it is often flooded; A large amount of sediment carried by the upstream is also easy to deposit here, raising the riverbed and aggravating the flood; Especially in the southwest of guanxian, there is a Leiyu Mountain, which hinders the eastward flow of rivers. Every summer and autumn flood season, it often causes drought in the east and waterlogging in the west. Shortly after he took office, he began to carry out large-scale water control work.
Li Bing and his son Erlang conducted a field survey along the Minjiang River, learned about the water conditions and topography, and worked out a planning scheme for governing the Minjiang River. In order to make the water of Minjiang River flow eastward, a 20-meter-wide hole was opened in Leiyushan, which was called "Baokou Bottle". The end of the separated Leiyu Mountain looks like a big pile, which is called "left pile" by later generations. In addition, a diversion weir was built in the middle of the river to make the water in the river into two streams, forcing one of them to flow into the bottle mouth. In the process of building the diversion weir, after failing to throw stones in the middle of the river, Li Bing found a new method, which made bamboo workers weave a big bamboo cage three feet long and two feet wide, filled with pebbles, and then sank to the bottom of the river one by one, finally overcoming the rushing river and building the diversion levee. The front end of the levee looks like a fish head, hence the name "fish mouth". It faces the upper reaches of Minjiang River and divides the surging river into two parts. The western share is called Waijiang, which is the positive flow of Minjiang River. Neijiang in the east is the main canal of irrigation canal system, with Baokou at the head, which flows through Baokou and is divided into many large and small canals, forming a criss-crossing fan-shaped water network to irrigate thousands of miles of farmland in Chengdu Plain. Gravel berms are built on both sides of the diversion weir. The inner diamond dike on the Neijiang side and the outer diamond dike on the outer river side are also called "Golden Dike". After the completion of the diversion weir, there are few floods and droughts in Chengdu Plain irrigated by Neijiang.
Later, in order to further control the amount of water flowing into the bottle mouth, a flat trough and a "flying sand weir" spillway for flood diversion were built at the tail of the fishmouth diversion dike. The flying sand weir is also made of pebbles in bamboo cages, and the top of the weir is at a suitable height. When the water level in Neijiang is too high, the flood will overflow the flying sand weir through the flat tank and flow into the outer river to protect the Neijiang irrigation area from the flood. At the same time, due to the vortex effect of water flowing over the flying sand weir into the outer river, the sediment deposition before and after the bottle mouth is effectively washed away. There is a certain proportion of water in the fish mouth. During the spring ploughing season, the water in Neijiang accounts for about 60%, and the water in Waijiang accounts for about 40%. In flood season, when the Neijiang River exceeds the amount of water needed for irrigation, the Sha Fei weir will overflow the Neijiang River. Bottle mouth is the entrance of Neijiang to control water quantity. In order to control the flow of Neijiang, Li Bing and his son stood in the middle of the river like stone men, as a yardstick to observe the water level, demanding that the water level be "inexhaustible and full of shoulders".