Stopping the fan alluvial
Water’s an amazing thing. Combine it with dirt and suspended silt and gravity, and it’s even more astounding. It etches and carves and reshapes, and it can do all of that remarkably quickly.
Take a walk after a big rain, and you can see what water does quite clearly, even on paved roads with gutters: the speed of the water suspends soil, gravel and silt and carries it downstream until the water slows, and then it spreads a broad fan of its loot in an organization set by progressively descending weight — something called an alluvial fan.
On a gravel path in a wooded park, it’s not surprising after a
heavy rain to find scores of such fans reaching their fingers out from the trail into the surrounding woods.
Sometimes, fast-moving water carrying sediment and stone stops more abruptly, such as behind a dam or other impediment, and the sudden drop in speed means the water’s ability to carry materials stops so suddenly that the carried materials drop out before they can build deltas or sand bars or barrisways.
Instead, they fall to the bottom and start to fill the pond or pool where the water is caught.
Fishing on a stream or a river, you can find places where the bottoms of pools fill in with tons of small rock every year, where there’s only occasionally enough storm-fuelled flow to gouge out the accumulated stones, gravel, sand and fines and restore the pool closer to its original shape.
For big dams, that can create a series of problems. Every year, the water in a reservoir to generate power can shrink, reducing the amount of stored energy behind the dam and the amount of energy its turbines can produce. The Three Gorges dam on the Yangtze River in China — the world’s largest hydroelectric development — is a poster-child for what happens when huge volumes of silt begin dropping out of entrainment in the water column: silt is piling up behind the dam, not only affecting future power levels, but removing the silt from being able to supply needed nutrients to farming operations below the dam.
It’s fine to write about the scale of the problem. But pictures are truly worth thousands of words.
Six years ago, the White Salmon River’s Condit Hydroelectric dam in Washington state — which only produced a maximum of 14.7 megawatts — was intentionally breached, its base blown out. Online, you can see video of that breaching in this column, and it’s shot in a way that’s deceptively beautiful.
What’s interesting to me about the video — besides the blast itself — is something that comes at about 1:29 into the time-lapse video, when the camera moves from the water gushing out of the dam to the reservoir itself. It’s the massive amount of soft silt that has piled up behind the dam in 100 years or so.
The silt is a giant, chocolate-pudding-like gelatinous mass, one that slumps down with the shrinking reservoir and then travels downstream with it. All of it material that, without the dam, would have continued on its river-ride towards the sea years ago.
And that’s the silt caught and carried in just one small tributary of the Columbia River, one that supplied a tiny fraction of the power of Muskrat Falls.
All of that material, dropping out of solution so slowly that that wouldn’t be visible to the naked eye, piling up year after year after year. The alluvial fan interrupted, delayed perhaps, but not stopped.
We think we’re winning over nature.
It’s only because the speed of the losing is so incredibly, glacially slow.