Ancient expertise provides answers to Sichuan river project
A major construction project has unearthed a piece of history and thrown light on engineering expertise dating back millennia.
Twenty ancient rock pillar bases, probably used to support a dam, have emerged from the depths near the Precious Bottleneck Channel in the wake of large- scale maintenance of the 2,000-year-old Dujiangyan Irrigation Project.
The project had been suspended for 11 years but was relaunched on Dec 10.
About 40 workers spent hours damming the Minjiang, a tributary of the upper Yangtze River, in Sichuan province.
Of course they had access to modern machinery but some of the methods they used harked back to earlier times.
One of the traditional methods they used was to put stones in bamboo cages, and sandbags around rafts, each made of tree trunks. This ensured an effective dam, much as it did thousands of years ago.
“Although workers used machines, the method of using rafts to dam the river is basically the same as that used a very long time ago,” said Liu Zhenghui, senior engineer of the Dujiangyan city management bureau.
After the river was dammed, its bed emerged allowing workers to clear the waterway.
“Unlike previous maintenance, workers will raise the river bed near the Precious Bottleneck Channel,” said Liu Gang, a water conservancy expert in Dujiangyan.
“If the river bed is not raised, the inner river will have more water in summer and may threaten to flood Chengdu .”
But when the water level dropped, 20 ancient round rocks and boulders were found near the channel.
“The boulders might be building materials for maintaining the levee of the project, while the round stones might be the ‘bedders’ for increasing the stress area of the pillars of large buildings, which had been washed into the river by floods,” said Fu Hao, deputy director of the Dujiangyan cultural heritage bureau.
Experts say the site has many submerged relics.
“To suppress floods, people long ago threw large objects into the water,” Liu said.
In previous centuries, the Chengdu Plain, now one of China’s most important agricultural regions, suffered from incessant flooding of Minjiang in the summer, and withered with drought in the winter.
Li Bing, then governor of Sichuan, decided to harness the Minjiang and started construction of the Dujiangyan Irrigation Project around 256 BC.
He divided the river in two by building a midstream weir. From there, at Fish Mouth, the Minjiang splits into the outer river and inner river, which Li had diverted to a new course to the east.
The inner river was divided at Lidui Hill, a manmade embankment, where the west stream was linked to the outer river through the Flying Sands Spillway, and the east stream squeezes through the Precious Bottleneck Channel to feed a grid of irrigation canals now watering 670,000 hectares in 34 counties of the West Sichuan Plain.
The plain has been more or less spared from flooding and drought for more than 2,000 years, thanks to the project, earning it the name “the land of abundance”.