Obviously to get water from the source to the cities where it’s consumed, I know. But the whole point of an aqueduct is to have a line with a constant slope, to ensure that the water will flow unimpeded - if this requires the line to be raised to a level above that of the surrounding ground, then you need to build an aqueduct.
Yet, the constant slope thing is unnecessary. The law of communicating vessels, which is attributed to Archimedes and thus should have been well known to the Romans, means that, as long as one end of the line lies higher than the other, water will flow from the latter to the former even if the line goes up and down in between. This makes it possible to build a simple ground-level pipeline, eliminating the need for the expensive bridge structures we know as aqueducts.
So why did the Romans bother to build them nonetheless? Did they have problems to make pressurised tight pipes? Was it worry that a ground-level line would be susceptible to sabotage? Or did they simply not make the mental leap of putting a law known to their phycisists into engineering application?
Sometimes they did use pressurized pipes to get across a particularly difficult valley. I’d have to guess that for the other situations, digging a tunnel was just easier.
This. The law of communicating vessels is lovely in theory. Getting it to work properly in the real world is a nontrivial engineering problem. Especially with bronze-age technology.
The nice thing about aqueducts is that they don’t need to be airtight or pressurized.
I’m guessing this. Metal was much, much, scarcer in Roman times. I imagine making pressure-tight pipes out of stone is pretty difficult (at least over any distance).
Also pressure-tight pipes require a lot more maintenance and tend to fail catastrophically, wheras a leaking sloped aqueduct just loses some water.
Pipes were not unknown to the Romans - we get the word ‘plumbing’ from them - but that word in turn derives from the Latin word for Lead (plumbum). Lead isn’t a particularly strong material, so I would imagine pressure inside ordinary lead pipes (at least the kind the Romans used) could be a problem.
Roman sewage plumbing used glazed ceramic tile. my suspicion is that they used aqueducts for water supply not only because of metal shortage, but because of maintenance. Not only don’t aqueducts require no sealing to work, they don’t corrode and are relatively low maintenance – no small thing in maintaining a system over the years.
It also guarantees a pressure “head” without the necessity of having a water tower and a pump system to get it up there.
Lindsay Davis, in Three Hands in the Fountain, one of her Marcus Didius Falco mystery novels set in Titus-era Rome, describes him going into the Roman water system. I don’t know how fully she researched it, but it has a ring of truth – rickety ladders used for access and portions of the water system silted up with sand and soil brought down by the waters. I could see all of that rapidly clogging up a metal pipe system, and being a real bitch to clean out, whereas with an open-air aqueduct you just need some guys with shovels.
I presume you mean that it can be horizontal, and does not have to slope downhill. Surely you’re not suggesting that an uncovered aqueduct can go uphill, as the OP would have needed?
I’m not saying their solution was not functional. I’m just asking if they could not have achieved the same, or at least a lower but still satisfactory, level of functionality more easily or cheaply with ground-level pipes.
Slaves were used extensively, but so were paid laborers and even the legions when they would otherwise be idle. Aqueducts were a huge and endless infrastructure project. It’s hard to make a single generalization stick.
The reason for a constant slope, aside from having a better flow for the user, was I understand to avoid silting. A place where there was a level would tend to silt up over time, or so I’ve read, so you want them to be as few as possible.
By using the legions, you kept them busy doing things instead of plotting how to overthrow the emperor and put their own commander in office to get bonuses (which happened often enough as it as). Military minds love make-busy work.
Too much slope is bad, too. Too little or too much sloping can leave solids behind to cause clogs.
Too much slope might cause the water flow to run to shallow for too long to properly suspend (and move) debris (i.e., if you wish debris/silt to move along).
The Aqueduct of Segovia is 93 ft tall. If they’d used a pressurized pipe instead, the pressure at the bottom of the valley would have been 40 psi. That’s not a trivial pressure to deal with.