By Cameron Stuart
*All photos produced by the author unless otherwise stated
Visiting Rome allows one to see countless breathtaking creations of the ancient world. From the Colosseum to the Pantheon, people today are still amazed by what Romans were able to make nearly 2,000 years ago. And while these sights define what we think of Rome, Rome has always been defined by water. From the Tiber defining Rome’s mythological creation story and initial water supply to ingenious aqueducts traveling miles above and below ground, to the fountains that still run continuously today, Rome is intrinsically connected to water.
When thinking of the water supply in Rome, people may only consider the Tiber River and the aqueduct arcades such as that in Figure 1 below. These are both iconic sights, yet they only make up a small piece of the complex water infrastructure of ancient Rome.
Dug through the hills, waterproof tunnels continued water flow between the arcades. Collection systems allowed for sediment settling and distribution to various sources with differing pressure. Public baths, toilets, and fountains ran constantly, and waste was carried off through a sewer system. In this piece, we will cover each piece of the aqueducts and distribution system to fully understand the complexity of ancient Rome’s water supply.
History and Background
The Tiber River
Rome’s geography is defined by the great Tiber River, as well as its history. The twin children Romulus and Remus were said to have been left to drown in the Tiber but were saved by a wolf and founded their own city in 753 BC on the banks of the river . From the founding of Rome—named after Romulus, after he killed his brother and became the first king—the Tiber was an important source of water along with wells and cisterns [1, 2]. For over 400 years the Romans acquired water by these more traditional means, although they quickly became polluted by waste which produced the need for alternate sources of water .
Today, the river is still an iconic part of the Roman experience. In the summer you can find markets along the lower banks or take a food and drink cruise through past famous neighborhoods. You can cross and observe it from any of the myriad of bridges along its length. These days you can do so much in and around the river aside from drink from it. For centuries, the river has been a critical part of Rome from water source to tourist attraction.
The city of Rome was the recipient of 11 aqueducts which spanned over 420 km in total . The first of which was the aforementioned Aqua Appia constructed in 312 BC and the last being the Aqua Alexandrina constructed in 226 AD , both of which can be seen in Figure 3 below.
In the roughly 500 years following the construction of the Aqua Appia, aqueducts were built across the entire empire, eventually totaling 1,600 . This discussion will focus on the city of Rome itself, yet it is important to understand the scope of Roman aqueducts and how much the Empire relied on them to sustain its growth across the world. In Figure 4 below, you can see this indicated by the points showing the locations of aqueducts.
At its peak, it is estimated that this system could deliver around 144 L of water per resident per day to the city of Rome . In comparison, Americans consume roughly 300 L of water per resident per day . This estimate for ancient Rome includes all water (private and public) on a per capita basis, so this number would be smaller than stated. Later sections will discuss how water was allocated after arriving in the city.
Aqueduct Design and Hydrology
As mentioned earlier, the aqueducts served the purpose of delivering water from the mountains to the city. This meant traversing distances of anywhere from 16 to 90 km with only slight elevation changes relative to the distance . Each aqueduct consisted of a string of arcades, tunnels, and siphons to achieve a steady downward slope from the source to the city.
A glance at Figure 5 above quickly shows how each of these components were used depending on the terrain. Imagine the complex planning required to construct an aqueduct like this example across the long, winding terrain such as that traversed by the Aqua Claudia (see Figure 3). While some only imagine the impressive arcades as aqueducts, the entire aqueduct consisted of this system of interconnected parts which relied solely on gravity to deliver water to the city, where it was then distributed to a variety of destinations.
The aqueduct arcades are the most visible pieces of Ancient Rome’s water infrastructure as many miles of them are still standing and are quite an impressive display. These arcades consist of a series of arches atop which runs a covered channel for the water to flow (see Figure 1). Because the slope of the water channel needed to be gradual and constant, arcades were built to raise the surface on which the channel rested to meet the elevation of the outflow of a tunnel, or the spring itself. See Figure 5 above for a visual representation of how the arcades maintained the continuity of the aqueduct and Figure 6 below for an image of the supported channel across an arcade.
When visiting these in person, it is nearly impossible to tell that they gradually slope downward towards Rome at less than a 1% grade . It’s impressive to imagine how this was measured and executed two millennia ago. The arcades still standing serve as a testament to the ancient Romans’ expertise with construction materials and techniques.
Unfortunately, these beautiful structures were the most troublesome components of aqueducts. A pillar that happened to sit over softer ground would sink slightly, thus slowing or stopping the flow of water altogether. To resume flow a footer would need to be installed to serve as a foundation to support the pillar on unstable ground. An example of one of these footers can be seen below in Figure 7.
Additionally, the exposed arcades were very easy targets for either theft or attack. We will discuss the distribution system later, but Romans were very particular about how the water was allocated. Some private residents could pay for a direct source, but most of the supply was for public works such as baths. As in any society some people attempted to skirt this and tapped the aqueducts by diverting water from the arcade and creating their own personal supply without paying .
More dire was also the prospect of enemy armies destroying parts of the arcades to cut off water from the city to lay siege to it, as the Barbarians did in the 5th century . Considering this and general wear and tear, the arcades required relatively constant oversight. Looking back at Figure 6 you can see the right arch in the image is supported by smaller, more uniform bricks that stand out compared to the large stones the arcade was initially constructed from. This is just another example of later work required to maintain these structures over the centuries.
The underground work which went into the aqueducts usually remains unknown to the public who admire the grand arches that remain all over Europe. However, the importance of the tunnels cannot be understated. In fact, most of the length of the aqueducts were underground, with only about 20% of the aqueducts traveling above the surface on arcades . Although they were carved through the ground, the tunnel walls in Figure 8 below are clearly smooth. This is because Romans coated the walls with a special concrete called “opus signinum”. It was like other forms of Roman concrete except that it was fully waterproof . This made it perfect for lining the aqueduct tunnels to prevent water seeping out through the ground.
In Figure 8 you can see the interior of one of these tunnels. In the background is a person, standing at full height. This is not a coincidence, as the aqueducts were designed to accommodate people so they could perform maintenance rather than designing for a specific volumetric flow rate. Dimensions varied slightly, but one could expect an aqueduct channel to measure around a meter wide and two meters tall on average [2, 11]. In Figure 9 below you can see an example of a maintenance shaft which would have been used to both dig down to the correct elevation to dig the tunnel as well as service it when necessary.
Maintenance was crucial to ensure smooth operations because Rome has hard water sources, meaning there is a high presence of limestone in the water . This is generally a harmless occurrence, but this led to a buildup of solids along the sides of the aqueduct channels. Therefore, easily accessibly tunnels and human-sized flow areas were necessary, as every year several millimeters of limestone could accumulate within these channels . To get a sense of how much this truly is, one can examine the aqueduct in present day Nimes, France, which accumulated almost half a meter of limestone over the course of 200 years . Limestone buildups in aqueducts were undesirable for flow conditions, yet it had one unintended benefit which will be discussed later.
Like arcades, siphons were a method of keeping the water flowing above ground over lower terrain. The difference with siphons was that siphons were used when the drop in elevation between two points where tunnels were used was too great to build arcades. Siphons were instead used, or perhaps it would be more accurate to refer to them as “inverted siphons” because they dip down initially and form a “U” shape. This component of the aqueducts is less known and preserved, yet perhaps had the most complicated technicalities to consider.
Siphons consisted of a header tank on the uphill side and a receiving tank on the other. These tanks are simply where water was collected to enter or leave the siphon and were connected to regular flow channels on either end. The connection between these tanks was a series of pipes which either crossed the ground or a small arcade of their own, as seen in Figure 10 below.
It is known that a flowing fluid if diverted down will be able to return to its initial height, which the Romans also knew. However, they also knew that this was not realistic and due to friction within the pipes there was significant head loss between one end of the siphon and the other . This was remediated by simply lowering the receiving end by a substantial amount—this is noted as the hydraulic gradient in Figure 10 and is the amount of head loss between each end of the siphon. The longer the siphon, the more head loss would occur as the water would experience friction with the pipe for a longer duration. This is not a desirable occurrence as it requires a drop in water level, but it was often the only way to continue the flow across a large valley where building an arcade was not safe or feasible.
Unlike the other parts of the aqueduct, siphons channeled fast flowing water as it traveled steeply downhill. The arcades and tunnels had rather slow water with their gradual slope, which helped to prevent damage to the infrastructure. The siphon pipes could not avoid dealing with high speeds and therefore high pressure which they needed to accommodate. The pipes therefore had to be built differently and it was after the header tank where water entered pipes to traverse the valley, seen in Figure 10. These pipes were roughly 25 cm in diameter and often experienced pressures of up to 12 atmospheres, with later experiments finding they failed around 18 atmospheres .
In Figure 10 you can see two other labeled components, the geniculus and the venter. The venter refers to the small bridge pictured which was built to reduce some of the elevation drop of the water, therefore reducing the pressure in the pipes and the head loss . The geniculus is the bend of the pipe which experienced the most forces due to the water changing direction after falling downhill.
Romans had a fair understanding of how pressure impacted these pipes, as the venter reduced some of this stress as well as practices they had when performing maintenance. Their engineers knew to turn off the water supply slowly to avoid water hammer, which would cause a shockwave up through the siphon and damage the pipes and header tank . Similarly, they would turn the water on slowly to prevent higher than normal pressure from being exerted on the geniculus which could cause cracks and leaking. Overall, siphons are perhaps one of the least known components of the aqueducts yet demonstrate some of the Romans’ greatest understanding of hydrology.
With siphons essentially being the end of an aqueduct, the water system shifted from transport to distribution. These components are still quite impressive, as water would undergo a basic treatment in large tanks before flowing through a network of pipes to various destinations. The settling tanks, or castella, and flow through the pipes were all still powered by gravity alone.
Once water reached the city it was collected, underwent a very basic treatment process, and was piped out to its various destinations. These collection points were called castella, an example of which is shown in Figure 12, and showcase another incredible feat of ancient Roman engineering. Water entered the city from the aqueducts at a high elevation which would allow them to continue to use gravity to move it throughout the city .
The castella had pipes set up to ensure some sources would receive constant flow, such as the outflow holes visible in Figure 12. These would likely have sent water to public fountains and baths as the Romans prioritized public sources. You can also see the other pipes along the rim of the castellum base which would have taken water to other sources like private homes.
Additionally, castella served as a very basic form of water treatment—primary settling. The basic concept of primary settling applies the same today as it did 2,000 years ago. Water was allowed to flow slower than the general particle settling velocity such that suspended particles would precipitate out before entering the pipes.
When leaving a castellum, devices called calices were installed to manipulate the flow rate , a schematic of which can be seen in Figure 13. Romans at the time did not have a full grasp of flow rates as they measured them in a unit called quinaria which was effectively a unit of area . Regardless, the concept the Romans employed was the relative difference in flow rates so they could deliver different amounts to a given source . Various sources will disagree on the approximate flow rate of water ultimately entering the city from the 11 aqueducts due to lack of accurate measurements or documentation of the calices.
The final step in bringing water from mountain springs to public areas and houses was lead pipes. A simple concept still used today, pipes left the castella (seen in Figure 12) and allowed water to flow throughout the city to fountains, baths, private homes, and anywhere else it was needed. They could also be found all over the empire, and Figure 14 below shows an exposed piece of pipe which can be seen in Pompei.
Today, in Rome itself, one will not find exposed pipes like this as they are buried below several feet of dirt due to sedimentation. They are visible in places like Pompei because it has been unearthed—in this case, from ash—and preserved.
And to acknowledge the elephant in the room—yes, lead pipes are not conducive to good health and today we monitor lead contents in our drinking water to keep it below what are defined as safe levels. There have been several theories that lead pipes caused Romans to go crazy, and ultimately led to the fall of the empire. As discussed in the section about aqueduct tunnels, the hard water would deposit calcium carbonate as it flowed. Ultimately, most pipes would quickly become coated with a thin layer of calcium carbonate which kept water out of contact with the lead, therefore reducing the likelihood of lead poisoning .
The most important part about bringing water in from miles away in the hills is for people to be able to use it. As one would expect from a culture with grandiose architecture, their water use was no different. Some people had water delivered directly to their homes, yet that was not the norm. Only wealthy individuals paid for private water access. The public retrieved water from the plethora of fountains and could enjoy trips to the baths for a variety of water related activities.
Some of Rome’s most famous sights are fountains for their design incorporating creative water displays and incredible architecture. They were (and still are) always running, supplied by the constant source of incoming water. The fountain in Figure 15 is one many will recognize as the Trevi Fountain, and it appears to be a display piece that is putting water to no practical use. Today, the fountain functions as any other and cycles its water, but in ancient times it served as a source of drinking water for public access.
As with the aqueducts, Roman baths can be found across Italy and Europe as they were very popular during the height of the Empire. Baths provided several options for water use, from cold to warm baths as well as swimming pools and shared toilets (see Figure 16 below). Shared toilets are an odd idea by today’s standards but show that Romans could separate water uses and drainage within the same facility.
The bath houses themselves were incredible displays of architecture and water use, as can be seen in Figure 17 below. These buildings were massive, sometimes two or more stories, and they contained multiple rooms with large pools of water at various temperatures for various uses. The caldarium was, as the name suggests, a warm bath like a hot tub, the tepidarium had tepid water, and the frigidarium was a cold pool .
These baths are often regarded for the complexity of the structure, but many overlook the implications for water delivery. Water must be brought in from a higher elevation, separated, and heated or cooled appropriately. The handling of water in these structures is a marvel and shows how proficient the Romans were at applying these principles to a place of leisure.
Today, the public can similarly enjoy constant access to water throughout the streets of the city. Nasoni, or water spigots, can be found every few blocks, and like the fountains of old are constantly running clean water anyone can use to rinse or drink. The tradition continues of allowing unfettered public access to water, which is especially appreciated on a hot summer day.
And finally, the water must leave after its long journey through miles of aqueduct to a castellum, filtered, and diverted around the city. Part of this system is the Cloaca Maxima. The Cloaca Maxima was constructed in the 6th century BC as an open sewer, and later covered in the 2nd century BC  which is what we see today.
In Figure 18 the large arch which signifies the outlet of the Cloaca Maxima. It now lies along the bank of the Tiber and fills partially with water when the river is high. Looking closely, inside the arch, you can see a line along the back wall between whiter bricks on the bottom and darker on the top. This is the line remaining from the previous high water.
It was initially built to drain floodwater and connected to an expanding canal system that evolved into a sewer system over time, much of which is still used today . There is a longer, messier history (pun intended) regarding this sewer system, and you can check out the following link for more information: http://www.thehistoryblog.com/archives/21511.
Despite nearly 2,500 years and several regimes between its construction and the present day, the Cloaca Maxima still functions in some places and the outlet in Figure 18 still allows wastewater to flow into the river. Like the Tiber River this piece of ancient Rome’s initial water infrastructure has changed over time while remaining in use and an important landmark.
We began discussing the Tiber River and its significance in founding and sustaining Rome for hundreds of years. Now we have tracked water through the centuries and kilometers back into the river. From its conception to the present day, Rome has been a city of water unlike any other. Its rich history still stands or lies beneath, and one can visit sites across all of Europe and appreciate the technological wonder of their water infrastructure.
The ancient Romans were known for their expertise at building and left us with countless sites to wonder at. Among these are the aqueducts, both the arcades that remain as a visual reminder and the many other components which are overlooked by the public. Through a complex series of meticulously engineered works, Romans brought water from the mountain springs into their homes and public fountains and baths with a constant and steady supply. Their understanding of hydrology was not limitless but far surpasses what one may expect given their “ancient” existence. Hopefully a brief glimpse into a critical piece of ancient Rome’s infrastructure allows you to realize what made their civilization so powerful and long-lasting. Their society was fueled by the constant presence of water across a vast empire.
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