Obviously this wouldn’t relieve the issues caused by low snowpack and decreasing precipitation, but it could supplement the water used for everyday living or agriculture during times of drought right?
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Well, you have a few problems to contend with, and some of them don’t have good solutions – plus you have an issue of scale.
First of all, you’ll be driving a tunnel through land that’s geologically active. The ground moves, and not all of it is going in the same direction. Over small scales, like a subway or a city aqueduct, that’s not much of a concern since (barring things like earthquakes) movements are small and generally along the same axes. But a transcontinental tunnel will be put under a lot of strain by ground shifting – and that drift can be in all three axes. But, really, this is the least of our worries.
A bigger problem is the geology you’ll have to deal with, particularly mountain ranges. In North America, for example, you have the Rocky Mountains. Hard rock, loads of pressure, and as an added bonus a lot of geological strain energy which leads us back into the first problem of ground shifting and snapping your pipeline (or at least cracking it and causing leaks). You’ll also have to contend with ground unsuitable for load bearing (which means shoring up your pipeline to keep it from sinking under its own weight and causing a break) and ground hydrology causing problems (saturated soils causing buoyancy or loss of support, depending on the strata involved). Even drilling a subway involves dealing with most of these problems, and that’s only over the area of a city, not several thousand miles of terrain. Sure, you could try to sidestep the problem by only drilling through bedrock, but that’s going to add to the project cost by a magnitude of hundreds.
You’d have to source water for the pipeline, of course, and that’s another problem. Moving water from where it’s wet to where it’s dry sounds sensible when you’re looking at footage of floods and droughts (just move one to the other!) but in reality that floodwater isn’t captured. It’s all runoff. Building a system to capture that water would be an enormous headache on lots of levels, to say nothing of actually making water that badly contaminated by runoff usable. And lest we forget, just because an area isn’t in lake-drying levels of drought doesn’t mean it’s flush with water, either.
Finally, though, you have a massive problem of scale. In order to make any kind of dent in a water shortage like you see in the American Southwest, you’d need a transcontinental pipeline a kilometer across or more just to move enough water. A normal aqueduct the size of a subway tunnel won’t move enough water to do more than fulfill the water needs of a neighborhood or two at most. Shifting enough water to actually cure – or even substantially alter – a severe water deficit just isn’t practical; you’d need the largest tunnel diameter ever built, and the engineering of the tunnel alone, even ignoring the problems with the land it’ll travel through, would be immense. Factor in all the other issues and building a hundred desalination plants is a financial and engineering snap by comparison.
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