How did Roman Aqueducts work?

Let's explore the aqueducts of Rome. How do they work? What was their function in the Roman city? These are incredible symbols of Roman engineering. They are some of the most iconic features of the Roman Empire. And of course, there's so much that goes into this construction. The serving, the planning, and the execution for the benefit of the citizens of Rome. Let's explore the aqueducts of the Roman Empire. Before the aqueducts, where did Romans get their water from? Well, they collected rainwater into cisterns. They drew water from some of the local springs available to them, and they drew water from the Tiber. But it all started to change in 312 BC with the construction of the first aqueduct, the Aqua Apia, coincidentally coming into being alongside the first major paved road of ancient Rome, the Via Apia. And from there, things really took off. But how exactly does it work? An aqueduct works by gravity flow. You must find a water source at a higher elevation than your city. Then you direct the water from springs or a river or a lake into a man-made channel. Basically, you're making a new conduit, a man-made river, into the city. And sometimes there are obstacles. You have to drill through hills or mountains, or sometimes there's a valley, and you have to prop up that channel with a bridge. This is essentially what an aqueduct is, and therefore the majority of the construction was underground. So for all those incredible miles of arches of various aqueducts throughout the Roman Empire, including Rome itself, those beautiful arcades represent a fraction of the overall length of a given aqueduct line. So with the progression of time and the construction of 11 aqueducts, the Romans would go further and further afield to find better sources of water at a higher elevation, thereby ensuring the water would eventually get to all the seven hills of Rome. We have fantastic resources from antiquity. We have Vitruvius in his 10 books of architecture. He writes about aqueducts in book eight. We have Frontinus, active in the reign of Nerva, who was the curator aquarum. He was in charge of the aqueducts, and he wrote about each aqueduct, the amount of water they brought into the city of Rome, and how they were maintained. So we have fantastic sources from antiquity as our guide. So you start off with identifying a source, which could be a spring, a river, or a lake, but pretty much the Romans preferred the springs because the water quality was so much better. And from that original source at a higher elevation from the city, you had to establish the gradient. You had to bring that water through gravity flow into the city. Well, how steep was that slope? Vitruvius says it had to be one in 200, or half of a percent, but there is some discussion that the Latin is corrupt in the passage in Vitruvius. Now Pliny the Elder says it should be 0.02%. That's one meter drop out of 4,800 meters. But really when we start to look at aqueducts throughout the empire, we see that it's usually between a 0.2 and a 0.03% slope. And that means a one meter drop in 3,350 meters. That's an incredible amount of precision. How did the Romans do it? They had a number of tools. Now we know one was the groma. This was an instrument used as a basic tool for horizontal alignment. And the groma was used to establish a straight line or right angle. It's a surveying tool for constructing roads as well as aqueducts. Vitruvius discusses three tools. The dioptera, essentially an ancient theodolite, is a quite complex instrument detailed by Hero of Alexandria. It was used both for horizontal and vertical measurement and was used to measure distance. Another tool was the corrobates, a water level acting like a modern bubble level. It consisted of a six meter long beam of wooden legs with a series of plumb bobs for establishing the angle of the slope. He also mentions another water level known as the Libra. The majority of this, the spacus, the channel through which the water flowed was constructed in the natural landscape. Here we're seeing portions of the aqua claudia channel. So most of it was just literally carving through limestone in this case and sealing it over with waterproof cement to make sure that the water didn't seep out. Here's an example of the aqua marcha. We're actually in the spacus. We're looking at the base. Look at the sidewalls. We have the exposed opus igninum, the waterproof cement with crushed peaches of ceramics that would ensure the water would not seep out as it flowed through. But it was hard water. So calcium eventually gets deposited on the walls of the spacus. We can see that deposit still today right here. As the man-made water channel approached the city, it had to be propped up on a series of arcades to maintain that gravity flow. And of course, by the time those channels arrived in the city of Rome, they came at a great height in the imperial period. And one important place where we can observe that majesty of the aqueduct channels is at Porta Maggiore on the Esquiline Hill. In fact, eight of the 11 aqueducts of ancient Rome entered Rome at this high point so that you can maintain the great height of the aqueducts coming into the city. Now here we can see in the Claudian Gate that celebrates the arrival of aqueducts, two spacus channels, the lower one for the aqua claudia and the upper one for the annius novus. Nearby, you have the remains of the spacus channels of three earlier aqueducts. At the bottom, the aqua marcia of the second century BC, piggybacking on top from a distinct source, the aqua tepola, and on top of that, dating to the time of Agrippa, the aqua iulia. As the water flowed closer to the city of Rome, propped up on the arcades, there were a number of opportunities to purify the quality of the water. And in fact, you have the construction of a series of castella. Now here's one from Segovia, the famous aqueduct in Spain, and it's an opportunity then for the impurities to drop out into this basin on the way that the water flowed into the rest of the city. Also coming into the city, you have a culmination point of the Castellum aquae. Now this was a structure where you actually then began to funnel out the water into distinct channels. Some would go to the baths, some would go to the public fountains, some would go to the homes of private individuals that would pay for it. And a great example we see here in the Castellum aquae of Pompeii, but even more important, more famous, is this one in the city of Nimes, attached to the Pont du Gard aqueduct. And we can see how complex the distribution was of the water into various channels. And when we think about the piping, what was it made out of? It could be made of stone, terracotta, but the Romans principally used lead because lead was abundant and cheap, obtained through the refining process of silver production. So you had all this lead, it had a low melting point, and you were able then to produce this amazing piping all throughout your city. Modern estimates of flow rates are very difficult to assess. Now it's been determined that it's around one to 1.5 meters per second, that's an average speed of 3 1⁄2 to 5 1⁄2 kilometers an hour, that would have ultimately brought to Rome between 320,000 cubic meters of water a day. Some estimates put it as high as one million cubic meters of water a day. Now against these modern estimates though, we have the ancient sources. Frontinus' unit of measurement is the quinaria, and it's the smallest lead pipe made. And the term comes from five fingers, five digits that formed in the diameter of a given pipe. So Frontinus is measuring the flow rate by the pipe diameter. This measurement has caused quite a bit of difficulty to reconcile modern assessments with the ancient calculations. And the reality is that the water, coming mostly from springs, would have been frequently, constantly depositing calcium and other minerals that would have served to finally restrict the flow of water in a given spacus. Now there are slaves to maintain the water channels, to maintain the arcades, and clean out this accumulated material called sinter. But the reality is when we look inside a number of spacus channels, we see quite a heavy amount of deposit of calcium. In fact, one of the most striking examples we have of calcium deposit is here in the spacus at the top of the Pont du Gard aqueduct. The amount deposited is impressive. So we can imagine then even with the estimates of water flows in antiquity, even modern estimates today, a huge impact would have been on the restriction of that water flow over the centuries. What were the aqueducts used for? Well, they're built from the 4th century BC into the 3rd century AD, parallel to Rome's increased population. Up to a million people were sustained in part because of the abundant supply of water from so many aqueducts. But it was also geared for public enjoyment, especially the baths. So the aqueducts were a great public amenity and antiquity in Roman cities throughout the empire. Now Pliny the Elder tells us that Agrippa formed 700 wells in addition to 500 fountains and 130 reservoirs, many adorned with statues of marble and bronze and 400 marble columns. All this for the aqueducts that he constructed in the reign of Augustus. Moving into the 4th century AD, we have a catalog that details there were 19 aqueduct lines feeding 1,352 public fountains, 15 nymphaea, big public fountains, 11 imperial thermae, bath complexes like the Baths of Caracalla, and 856 public baths and 254 reservoirs. Now when the water came into the city, a portion of it was already diverted outside the city limits for private use, mostly private villas, says Frontinus. Within the city, Frontinus specified three clearly distinct groups for water consumption. First primarily was for public baths and for other lines for the special privilege use of the Roman emperor himself. The second group consisted of public use for the military, official establishments, theaters, public fountains, and private baths. The third section was for private users, bringing lines directly into one's home and also for industries such as the Fullers, the Philonica. Water from the aqueducts was also used to flush out the public latrines, frequently located in public baths and near the Forum. By the fourth century, they were documented as being 144 public toilets in the city of Rome. When we look throughout the empire, we'll find spectacular examples of all these activities that underline the importance and the prestige associated with having an aqueduct in your city. From the monumental underground Basilica of Constantinople to the sprawling aqueduct on the beach by Caesarea Maritima in Israel, to the Pont du Gard in southern France, to the sprawling aqueduct of Hadrian of Carthage, to the elegant arcades of the Aqueduct of Segovia in Spain, the narrow arcades of Los Milagros Aqueduct, which is for Merida in Spain, fed from a reservoir called Proservina Dam that still exists today. Back in Rome, we enjoy the channels of the ancient aqueducts as impressive ruins in the Park of the Aqueducts, as well as some of the lines that were put back in use in the papal period, the most famous of which is the Acqua Virgo, still flowing from its natural springs all the way into the heart of Rome, the Campus Martius, flowing to other fountains in the center, including those in Piazza Navona. Thanks for tuning in, and be sure to subscribe to our YouTube channel and get our newsletter at ancientromelive.org for more free lectures. See you soon.