Kaner, Yijing Cave, is a unique irrigation system in desert areas. Common in Turpan, Xinjiang, China. Turpan basin is rich in groundwater resources. Coupled with the large slope of the ground, it constitutes the possibility of digging karez under natural conditions. Karez consists of four parts: shaft, underground passage, surface passage and flood prevention dam. Firstly, the wellhead is laid on the ground from top to bottom to collect groundwater. Then a culvert is built at the bottom of the well to lead the groundwater to the destination and then the water to the ground. This ensures that groundwater will not be evaporated or polluted due to high temperature and strong wind. The waterlogged dam stores water for human use. The whole Dujiangyan hub can be divided into two systems: weir head and irrigation water network, in which the weir head includes three main projects: fish mouth (water diversion project), flying sand weir (flood discharge and sediment discharge project) and Baokou mouth (water diversion project), as well as ancillary buildings such as inner and outer Jingang Dike and herringbone Dike. Dujiangyan project is mainly based on water diversion irrigation, which has the comprehensive functions of flood control and sediment discharge, water conveyance and urban water supply. "Fishbone" is a water diversion project in Dujiangyan, which is named after it looks like a fishmouth. It is located in the middle of the river and divides the Minjiang River into two rivers. Waijiang River is located in the west, also known as "Jinma River", which is the main stream of Minjiang River and is mainly used for flood discharge. Neijiang, located in the east, is the main canal for artificial water diversion, mainly used for irrigation, also known as "river irrigation". The fish mouth determines the diversion ratio of the inner and outer rivers, which is the key to the whole Dujiangyan project. The intake width of Neijiang is 150m, and that of Waijiang is 130m. Using topography, the river is divided in proportion at the mouth of the fish. When there is little water in spring, 40% will flow into Waijiang River and 60% into Neijiang River, so as to ensure water for spring ploughing. In the spring and summer flood season, the water level rises and overflows the fishmouth, 60% of the water flows directly to the outer river, and 40% flows into Neijiang, so that the irrigation area is protected from floods. This is the so-called "divided into four or six, flat and poor." In addition, in ancient times, the fork was used to artificially change the diversion ratio of internal and external rivers. Chacha is an interceptor with several logs as the skeleton, covered with bamboo mats and filled with sediment. When necessary, as long as a few piers are placed on one side of the river, the water quantity on that side can be reduced. Generally, this method is used to intercept the outer river in spring when there is little incoming water, so as to increase the water supply of Neijiang. After the spring ploughing ended and the water level rose, the dam was cut to make the water flow return to normal. After 1974, a permanent sluice will be built in Waijiangkou to replace the pier to realize cofferdam flood discharge. The cooperation between the bottle mouth and the flying sand weir has the function of controlling the water flow, which is the key to control the inflow of Neijiang. Neijiang water flows in from the bottle mouth to irrigate a large area of farmland in Chengdu Plain. During the flood, when the water level in Neijiang rises above the flying sand weir, the flood will enter the outer river and flow away. In addition, the restriction of the bottle mouth on the water flow plays a role in flood control. After the inland river flows into Baokou, according to the topography of high northwest and low southeast, it is continuously diverted along the large and small aqueducts, forming a self-flow irrigation canal system, which irrigates more than10 million mu of farmland on Chengdu Plain. Sha Fei Weir, formerly known as "Shi Lang Weir", was built in Longshuo, Tang Gaozong (66 1-663). It is an overflow discharge channel in Neijiang, hence the name. Feisha weir is a gap at 7 10 meters in the lower section of Jingang levee, which is located on the other side of Hutouyan, with a width of 240 meters and a crest 2 meters higher than the riverbed. Sha Fei weir discharges the river water outside the irrigation area to the outer river to prevent flooding in Chengdu-Chongqing Plain. A large amount of sand and gravel trapped in water can also be discharged from here to the outer river by centrifugal force to avoid silting up Neijiang, Baokou and irrigation areas. The flying sand weir follows the principle of "low weir", that is, the top of the low weir is the same height as the standard dam abutment on the other side, so that the inner river water exceeding the upper limit of the bottle mouth flow overflows the weir top and flows into the outer river. If there is a serious flood, the weir will burst its banks on its own and let the river return to the positive flow of Minjiang River. When the water level in Neijiang reaches 14 stroke, the flow rate is 385 cubic meters per second, which is enough for spring ploughing in irrigation area. If the crest height of Sha Fei Weir is equal to this water level, when the Neijiang River exceeds 14, excess river water can overflow from Sha Fei Weir to Waijiang River. In addition, the bottle mouth near the downstream of Sha Fei weir has good water control function. Even if the flow rate of Neijiang is as high as 3000 cubic meters per second, the water in the bottle mouth is about 700 cubic meters per second. The rapids descending along the Jingang levee are blocked by the flow restriction at the mouth of Baokou, forming a backwater outside the mouth, that is, Fulongtan. Coupled with the guiding effect of Hutouyan, which extends into Neijiang across the Sha Fei weir, on water flow and sediment, the flood will take a lot of sediment over the Sha Fei weir and discharge into the Waijiang River. According to contemporary measurements, when the flow of Minjiang River exceeds 65,438+0,000 cubic meters per second, 40% of the flood and 98% of the sediment are discharged from Sha Fei weir. The adjustment of the top height of Feisha weir, the restriction of the bottle mouth flow and the guidance of Hutouyan can ensure that there is enough clear water in the water diversion area and there is no threat of flood.