Dujiangyan, or Dujiang Weir, refers to an ancient irrigation and flood-control system in Dujiangyan City in Sichuan Province, which is one of the must-sees for tourists.
The system constructed around 256 BC is still working today, irrigating over 6,600 square kilometers of land and controlling floods. The Dujiangyan Weir 都江堰, the Zhengguo Canal 郑国渠 in Shaanxi Province and the Lingqu Canal 灵渠 in the Guangxi Zhuang Autonomous Region are collectively known as the “three great hydraulic engineering projects by the Qin Kingdom.”
Although famed as “the land of abundance,” Sichuan Province was not always in that way. With a sharp altitude drop from Dujiangyan to Chengdu, the Chengdu Plain was prone to flooding, as the Minjiang River, a major tributary of the Yangtze, rushes down from the Min Mountains and slows abruptly after reaching the plain, filling the watercourse with silt. The plain was often underwater in the rainy season and a thousand-mile barren patch in drought.
And in 256 BC, Sichuan governor Li Bing 李冰 and his son, both engineers, led the construction of Dujiangyan, with support from King Zhao of the Qin Kingdom in the Warring States Period (476-221 BC), who thought highly of the strategic position of Sichuan in uniting China.
Rather than a simple dam, Li and his son planned to harness the river by channeling and dividing the water. Three projects were made to work in harmony, as a safeguard against flooding and to keep the fields well supplied with water. They were the “Fenshui Yuzui” 分水鱼嘴 (Fish Mouth Levee), the “Feishayan” 飞沙堰 (Flying Sand Spillway) and the “Baopingkou” 宝瓶口 (Bottle-Neck Channel), with 100,000 taels of silver from the king and a team of tens of thousands of workers.
The Yuzui Levee (A) divides the Min River into inner river (right) and outer river (left). The Feishayan Spillway (B) further divides the inner river into inner and outer streams, and carries away excess water and sediment by natural swirling flow. The Baopingkou Channel (C) was carved in the Yulei Mountain to deliver the water into the channel network (D) on the Chengdu plain. Arrows indicate the direction of water flow.
Fish Mouth Levee 分水鱼嘴
With careful study of the landform and water conditions, Li proposed building an artificial levee to redirect a portion of the river flow and cut a channel through the Yulei Mountain to discharge the excess water of the Minjiang River into the dry Chengdu Plain beyond.
The Fish Mouth Levee 分水鱼嘴 with its conical head resembling the mouth of a fish is the key part of Li’s plan. It is an artificial levee that divides the water into the Inner Stream toward the plain in the east and the Outer Stream in the west joining the Yangtze River eventually.
The Inner Stream 内江 is deep and narrow, while the Outer Steam 外江 is relatively shallow but wide. This special structure ensures that the Inner Stream carries approximately 60 percent of the river flow into the irrigation system in the dry season.
But in flood season, this amount decreases to 40 percent of protect people and farmland from flooding. In addition, the wide and shallow Outer Stream drains away the rest, flushing out much of the silt and sediment.
The levee was constructed with zhulong 竹笼 (bamboo basket) — long sausage-shaped baskets of woven bamboo filled with stones, which was widely used for water conservancy projects in fast flowing currents.
The Inner Stream then flows through the Bottle-Neck Channel 宝瓶口 gouged through the Yulei Mountain 玉垒山. Cutting a channel through a mountain was especially difficult in the time before gunpowder was invented.
Li used a combination of fire and water to heat and cool the rocks until they cracked and could be removed. It took eight years to finally complete the 20-meter-wide, 40-meter-high and 80-meter-long channel.
Flying Sand Spillway 飞沙堰
The 200-meter-wide Flying Sand Spillway connecting the Inner Stream 内江 and the Outer Stream 外江 is a third important gear to guide water.
With a narrow entrance like a bottle neck, the channel is expected to work as a check gate, creating the whirlpool flow that carries away the excess water over the third construction — the Flying Sand Spillway 飞沙堰. A modern reinforced concrete weir has replaced the original weighted bamboo baskets.
The Weir, also made of zhulong 竹笼, was set about 2 meters higher than the bed of the Inner Stream. It ensures against flood by allowing the natural swirling flow of the water to drain out excess water from the Inner to the Outer Stream. In addition, the swirl also drains out the silt and sediment that failed to go into the Outer Stream, which contributed to its name “flying sand.” Facing big floods in ancient times, it might crack naturally and lead most water back to the Minjiang River.
Apart from the three important gears, Li also set up an alarm system and maintenance principles to keep the system working in long term. Li set a pair of stone horses onto the river bed as markers for the depth of the water level.
This was replaced with three stone figures of about 2 meters high in the river as a reference for water levels in the Eastern Han dynasty. The water surface should not be lower than the statue feet in dry season, and not higher than the shoulders in rainy season. Otherwise, there would be draught or floods.
The three statues had long been missing until rediscovered in the river bed in 1974, 1975 and 2014. One of the statues is believed to be Li Bing, one is suspected to be an assistant of Li, while the third one remains unknown with its head missing.
Even if the Dujiangyan system helps drain out much silt, it is still inevitable for the silt and sediment to accumulate in the riverbed. In every draught from winter to spring, the local government organizes people to remove the mud from the bed and strengthen the levees and weir.
Li buried a stone horse set in the middle of the Inner Stream, which marks the proper height of riverbed. The mud removing work is not completed until the back of the stone horse uncovered. The function of the stone horse was later replaced by huge wotie 卧铁 (lying iron rods) in the Ming Dynasty (1368-1644).
Four iron rods are still working under the riverbed, respectively buried there in the Ming Dynasty, the Qing Dynasty (1644-1911), 1927 and 1994. Replicas of the four rods are exhibited at the Lidui Park 离堆公园.
After completing the Dujiangyan system, Li kept on leading the residents in building irrigation networks, yet he eventually broke down from overwork. To commemorate the great governor and his son, a temple named Erwang Temple 二王庙 (Two Kings Temple) was built on the riverbank at the foot of the Yulei Mountain.
The levee and weir were reinforced by concrete in recent years. And a scenic spot was created around the system, attracting thousands of visitors.
Visited in Dec 2012