The Dujiangyan Weir: Civil Engineering that Lasts —by Jinny Batterson
Civil engineering is not one of my skills. Even trying to understand basic maps, diagrams, and schematics can leave me scratching my head in confusion. So I’m sure that I cannot appreciate, in the same way a trained engineer would, the elegance of the design of the Dujiangyan Irrigation Projects site (also translated into English as the Dujiangyan Weir). A “weir” is a manmade structure to redirect a river’s flow. It is less restrictive than a dam, allowing water to flow over or around parts of it. I’ve visited the site twice during travels in the area—in 2004 and again in 2009. I did, through a guide’s explanation and through direct observation, develop a sense of awe for the crafters, builders and maintainers of this irrigation system built over 2000 years ago. The Dujiangyan weir is now the only remaining no-dam irrigation project in the world. It continues to provide both irrigation water and flood protection despite nearly 2300 years of wear and tear, countless downpours and droughts, and numerous earthquakes, including a very serious one in 2008.
The project was first built starting in 256 B.C., during the Warring States period that preceded China’s unification. It was masterminded by area governor Li Bing and his son, with financial and manpower assistance from the state of Qin (pronounced “chin”), which a generation later consolidated its power over the country we now call China. The aim was to relieve the periodic flooding of the Min River, at the same time providing irrigation water to farms in the fertile surrounding plains. The system includes three main parts: a “fish mouth levee,” splitting the river into two parts at the head of a fish-shaped island, one part a deep, narrow inner channel, the other a wide, shallow outer one; a broad opening, “flying sand weir,” that allows excess water, plus silt and sediment, to swirl from the inner to the outer channel; and a “bottleneck channel” that distributes irrigation water through a narrow aperture toward area farmland, while holding back excess water to prevent flooding. This final channel was constructed before the availability of dynamite. It was hewn out of its surroundings by successively heating and cooling its rock walls, probably through bonfires and drenching with river water, until the walls cracked and could be chipped away. Digging this part of the channel took eight years.
During dry periods, about 60% of the river’s water flows through the inner channel and bottleneck channel, providing much-needed irrigation water. During heavy rains, the proportions are reversed, with about 60% flowing harmlessly down the outer channel, protecting surrounding areas from flooding. Although I do not understand all the engineering involved, I’m impressed at the combination of natural and artificial features that have kept this system working through earthquake, fire, and flood.
Since the project’s initial construction, several surrounding temples have been built to honor Li Bing. Several pedestrian bridges now span the river to make it easier to take in the scope of the project. In 2004, I visited the site with a group tour on our way back from mountainous northern Sichuan. On that trip, our tour bus was partway down the mountainous slopes when lightning started flashing around us. Thunder boomed closer and closer, echoing off the surrounding peaks. Our bus driver hurtled through the gorges upstream of the weir down bumpy roads at the highest practical speed. When we reached the flash-flood-safe area of the irrigation project before the rains hit, everyone heaved a collective sigh of relief. We cheered our driver, who was the most relieved of all. In 2009, a local friend drove me out from her home near Dujiangyan to see the weir. Damage from the 2008 earthquake to the temples and to one of the bridges was still being repaired, but the structures in the river itself were completely restored after only minor impact. The Dujiangyan Irrigation Project is an important tourist destination for both domestic and international tourists.
My lasting impression of the dikes, levees and river are of clear, rushing water, and an engineering feat that continues to show remarkable beauty, practicality, and endurance.