Dujiangyan

Dujiangyan

This UNESCO World Heritage Site honors a flood control and irrigation system created without any dams that was built around 200BC.  It is so famous that Onions remembers learning about it in school, although when pressed, he really had no clue what it was about, just that he had heard about it.  You know me, I'm ALL ABOUT this kind of stuff, so I read about it before hand and spent the entire time trying to figure stuff out.

If you care, you can follow along.  The issue is that the river comes rushing down from the mountains into the plains and there were regular floods. The engineer who designed this, Li Bing built an artificial island in the river which, right here at Fish Head, splits the river into an inner channel on the right (40% of water flows into it at regular water level and 60% in times of flood), and the outer channel on the left.  This occurs because it takes advantage of the natural curve in the river AND he dug in the inner channel deeper.

The inner channel in the foreground.  The manmade island separates it from the outer channel.  The Fish Head is to the right outside the photo.

The outer channel is currently dry, because there has been a dam built in modern times. at the Fish Head to divert all water to the inner channel.

So down here at the eastern end of the island are our next features.  Near the bottom of the picture you'll see a little inlet of water.  Li Bing, using a technique of heating the mountain rock and then cracking it with cold water, excavated this area out of the mountain.  Why?  Because at high tide, the water would create a whirlpool and rocks and sediment would naturally divert into the channel that connects to the outer channel in the background.  This is part of the Flying Sand Weir.  The dam is also modern.

Looking upstream along the inner channel from the Flying Sand Weir.  Another technique to keep water where he wanted it, Li Bing raised the height of the base of the conduit connecting to the outer channel so it only accepted water when the level in the inner channel was high.  Flood control for downstream.

The narrow inner channel flowing to the left of that temple is now, rock and sediment free as well as controlled in the consistent flow of water.

The Flying Sand Weir and the inner channel.

Downstream along the inner channel, it is split again.

From right (east) to left, the Fish Head, the island, and the Flying Sand Weir.  Engineering at it's finest.