The largest dam in the world is the recently finished Three Gorges Dam on the Yangtze River, China. It’s more than 600 kilometers long and holds about 40 cubic kilometers of water.But it also stops an average of 151 million tons of sediment per year from making its way downstream, new research in Geophysical Research Letters shows. The river makes up for some of that in extra erosion downstream. But by the time the Yangtze washes out into the East China Sea, its sediment load is still lighter by 85 million tons per year.
With ocean waves eating away at it faster than new silt arrives, the Yangtze river delta has begun to shrink. For the record, that’s what happened to Louisiana’s barrier islands after we tamed the Mississippi.
Dams are tricky. On the face of it, they seem like a green miracle: They store water and generate clean power. But – as we’ve learned after damming nearly every American river – they block salmon runs, kill rare mussel species and give invasive plants a toehold by eliminating the annual scouring of spring floods. Worse, the massively expensive projects quickly depreciate: they begin to shrink from the bottom up as soon as they start to fill, as silt piles up behind the dam. At the same time, the precious water they’re hoarding drifts endlessly away into the sky.
At present rates, the authors calculate, the Three Gorges Dam will be full of sediment in about 150 years. Not to worry, though. The Chinese are building four more dams upstream, and they’ll keep much of that sediment from reaching Three Gorges.
Good piece, Hugh. Add to your list of difficulties the problem of disposing of the dam once it is completely silted up. There are several dams in the Applachians that are over a hundred years old and have been silted up for some decades now. They are useless for their original purposes and they are deteriorating. Naturally, there is not much interest in appropriating money to repair a useless dam. On the other hand, taking out a silted-up dam is a delicate business. One must be careful not to precipitate a catastrophic failure for fear of wiping out downstream communities in a massive flash deluge of mud. Yet another example of the folly of not thinking in terms of life-cycles.
Thanks – that’s another good example. I hadn’t heard about the completely full Appalachian dams. But there’s a similar problem on a dam outside of Missoula, Montana, on the Clark Fork River. The dam is silting up, no longer necessary, and obstructing fish. But decades of accumulation of mine tailings have left heavy metals in the sediment. No one can figure out how to remove the dam without stirring up the pollutants and washing them downstream to Missoula (and onwards to the Columbia).
Yo H-
They’ve already been at work on the Milltown dam for about a year–check it out at
i was very much intrested in learning about the sillting and scouring in dams i hope it will be usefull to me as well as some images and more information regarding the above matter kindly send this to my id phanikumar1582@gmail.com phanikumar
civil engineering student