"Walk the beach deep and build a weir low." "When things are right, your heart will be drawn, but when you encounter bends, you will be cut off." This famous saying engraved on the stone wall of Erwang Temple in Dujiangyan is reasonable.
Analysis:
If you go deep into the beach, you can build a weir at a low level; ——"Title on the Wall of Erlang Temple in Dujiangyan"
"Deep scouring of the beach" means that the silt deposited on the bottom of the river must be excavated deeper to prevent the inner river from being too small for irrigation; "low cultivation" "Weir" means that the top of the Feishayan weir should not be built too high to avoid poor discharge during flood seasons and endangering the Chengdu Plain.
Li Bing was an outstanding hydraulic engineer during the Warring States Period in my country, the designer and organizer of the construction of Dujiangyan. The well-known Dujiangyan Irrigation System at home and abroad is located in the middle reaches of the Minjiang River in central Sichuan Province. The entire project consists of three main projects: Fenshui Weir, Feisha Weir and Baopingkou. It is large in scale, suitable in location, and reasonably laid out. It has three functions: flood control, irrigation, and navigation. It is also a rare miracle in the history of world water conservancy projects. For more than two thousand years, it has been playing a huge role in drainage and irrigation, ensuring local agricultural production.
Li Bing’s hometown, birth and death dates, and the construction of Dujiangyan at that time cannot be examined in detail. I only know that in about the fifty-first year of King Zhao of Qin (256 BC), Li Bing was appointed governor of Shu County. After Li Bing arrived in Shu County, he saw with his own eyes the severe local disasters: the Minjiang River originates from the Minshan Mountains in the northern part of the Chengdu Plain. The mountains along both sides of the river are high and the valleys are deep, and the water flows rapidly; A large amount of sediment brought from the upper reaches is also easy to accumulate here, raising the river bed and exacerbating floods. Especially in the southwest of Guan County, there is a Yulei Mountain, which blocks the eastward flow of the river. Every summer and autumn flood season, often causing east Drought and flood in the west. Soon after taking office, he began to carry out large-scale water control work.
Li Bing and his son Erlang conducted on-site inspections along the banks of the Minjiang River to understand the water regime, topography and other conditions, and formulated a plan to manage the Minjiang River. In order to allow the water of the Minjiang River to flow eastward, a twenty-meter-wide opening was first cut out of Yulei Mountain, which was called "Baopingkou". The end of the separated Yulei Mountain is shaped like a pile of rocks, which is called "Lidui" by later generations. In addition, the method of building a water-dividing weir in the center of the river was also adopted to divide the river into two parts and force one of them to flow into the mouth of Baoping. In the process of building the water diversion weir, after the failure of using stones from the center of the river to build the weir, Li Bing found a new way and asked bamboo workers to weave a large bamboo cage three feet long and two feet wide, fill it with pebbles, and then sink it one by one to the bottom of the river. Finally, they overcame the rapids of the river and built a dike to divert the water. The front end of the embankment looks like a fish head, so it is named "Fish Mouth". It faces the upper reaches of the Minjiang River and divides the surging river water into east and west streams. The one on the west side is called the Outer River, which is the main current of the Minjiang River; the one on the east side is called the Neijiang River, which is the main canal of the irrigation canal system. The head of the canal is the Baopingkou. It flows through the Baopingkou and is divided into many large and small ditches and rivers, forming a criss-crossing network. The fan-shaped water network irrigates thousands of miles of farmland in the Chengdu Plain. Large pebble berms are built on both sides of the water diversion weir. The one on the inner river side is called the Inner Jingang Dike, and the outer side of the river is called the Outer Jingang Dike, also known as the "Golden Dike". After the construction of the diversion weir, the Chengdu Plain irrigated by the Neijiang River will have fewer floods and droughts.
Later, in order to further control the amount of water flowing into Baopingkou, a flat water tank for flood diversion and a "Feishayan" spillway were built at the end of the Yuzui diversion dike. Feisha Weir is also built with bamboo cages filled with pebbles, and the top of the weir is made to a suitable height. When the water level of the Inner River is too high, floodwater will flow over the Feisha Weir through the flat water channel and into the Outer River to protect the Inner River irrigation area from flooding. At the same time, due to the vortex of the water flowing over the Feisha Weir and into the Waijiang River, the sediment deposited before and after Baopingkou is effectively washed away. There is a certain proportion of water in the fish mouth. During the spring plowing season, the water volume of the Inner River accounts for about 60%, and the water volume of the Outer River accounts for about 40%. During the flood season, the Neijiang River exceeds the amount of water required for irrigation, and the Feisha Weir overflows on its own. Baopingkou is the gate that controls the water volume of the Neijiang River. In order to control the flow of the inner river, Li Bing and his son built a stone man standing in the river as a ruler to observe the water level, requiring that the water level "never be exhausted, and the water level should not be exhausted".