"Basics of the irrigation system - Part II" Dujiangyan Things to Do Tip by mke1963
Dujiangyan Things to Do: 14 reviews and 30 photos
A similar principle applies right next to the bottleneck by the Fulong Temple, where a churning "bowl" of water can be seen. In the flood season, any silt and boulders here are swept over the much newer spillway. The water entering the irrigation system next to the Fulong Temple has had 99% of the silt removed by harnessing the natural actions of the river itself.
Other than the very last spillway, all this was designed in 256BC!
The present sluice-gates are a modern feature intended simply to divert more than 60% of the river into the irrigation system during the dry season, with no risk to the irrigation system. However, it would appear to have had an effect on the enviornment of the main river which is almost dry for much of the year. No idea whether any Environmental Impact Assessment was done!
But how does this all prevent flooding? Well although not well explained on site...the interpretative panels are full of nonsens and non-sequiturs...it is a simple process. The floods are ironically prevented by filling the irrigation system to its maximum. In effect, the irrigation canals, channels and ditches of the Chengdu Plain fil and fill and fill, so storing the floodwater until the floodwaters have receded (flooding rivers never actually flood for more than a few days at a time). By "storing the water" in the irrigation system, it dramatically increases the ability of the river system to hold water. On many rivers in the world there are natural versions of this 'storage' system, most notably in Dongting Lake in Hunan Province which soaks up the excess of the Yangtze River until the floodwater pass, and the water then flows back out of Dongtinghu into the Yangtze. Li Bing, 2250 years ago, created a man-made storage system for the floodwaters of the Minjiang River. He did it with no moving parts. His system is more technically sound than any modern day irrigation system.
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