Dujiangyan is an irrigation infrastructure built in 256 BC during the Warring States period of China by the State of Qin. It is located in the Min River in Sichuan, China, near the capital Chengdu. It is still in use today to irrigate over 5,300 square kilometers of land in the region. The Dujiangyan along with the Zhengguo Canal in Shaanxi and the Lingqu Canal in Guangxi are known as "The three great hydraulic engineering projects of the Qin dynasty."
During the Warring States period, people who lived along the banks of the Min River were plagued by annual flooding. Qin governor Li Bing investigated the problem and discovered that the river was swelled by fast flowing spring melt-water from the local mountains that burst the banks when it reached the slow moving and heavily silted stretch below.
One solution would have been to build a dam but Li Bing had also been charged with keeping the waterway open for military vessels to supply troops on the frontier, so instead he proposed to construct an artificial levee to redirect a portion of the river's flow and then to cut a channel through Mount Yulei to discharge the excess water upon the dry Chengdu Plain beyond.
Today, Dujiangyan has become a major tourist attraction. It is also admired by scientists from around the world, because of one feature. Unlike contemporary dams where the water is blocked with a huge wall, Dujiangyan still lets water go through naturally. Modern dams do not let fish go through very well, since each dam is a wall and the water levels are different. In 2000, Dujiangyan became a UNESCO World Heritage Site.
The Yuzui or Fish Mouth Levee, named for its conical head that is said to resemble the mouth of a fish, is the key part of Li Bing's construction. It is an artificial levee that divides the water into inner and outer streams. The inner stream is deep and narrow, while the outer stream is relatively shallow but wide. This special structure ensures that the inner stream carries approximately 60% of the river's flow into the irrigation system during dry season.
The Feishayan or Flying Sand Weir has a 200 m-wide opening that connects the inner and outer streams. This ensures against flooding by allowing the natural swirling flow of the water to drain out excess water from the inner to the outer stream. The swirl also drains out silt and sediment that failed to go into the outer stream. A modern reinforced concrete weir has replaced Li Bing's original weighted bamboo baskets.
The Baopingkou or Bottle-Neck Channel, which Li Bing gouged through the mountain, is the final part of the system. The channel distributes the water to the farmlands to the west, whilst the narrow entrance, that gives it its name, works as a check gate, creating the whirlpool flow that carries away the excess water over Flying Sand Fence, to ensure against flooding.