While the Roman aqueduct system deserves a very detailed and extensive treatment, this writeup does not provide one. What it does contain is a short paper I wrote for a Classics course discussing two aspects of Etruscan engineering and how they changed when the Romans showed up. I would recommend that the reader learn more about the aqueducts, but for now he should content himself with this somewhat rudimentary survey. Enjoy.
The Surveying and Construction of Water Management Systems in Ancient Rome and Etruria
The Etruscans were responsible not only for early Roman culture, but also for many Roman technological advances. In this paper we will be examining two aspects of Roman engineering which rely heavily on their Etruscan heritage, and may in fact be two of the most important engineering disciplines in all of Early Rome: the construction of systems to control the flow of water, and the art of surveying, which made this and other construction possible. One might question the wording of the phrase “water management systems” in the title, thinking that perhaps “water supply” would perhaps be more accurate as well as shorter, but the two cultures actually had quite different purposes in mind for their waterways, as we shall see in a moment. For now, let us discover their commonalities from an engineering perspective, as that is the more interesting question.
When one hears the word “aqueduct,” the image which immediately comes to mind is one of high arches and elaborate masonry, with the Aqueduct of Segovia and the Pont du Gard as typical examples. However, these imposing structures are descendents of discreet, utilitarian Etruscan works known as cuniculi, which means “rabbit burrows” (Hodge p. 45). Cuniculi were little more than tunnels dug through the soft rock of northern Italy, with frequent vertical shafts to the surface to allow water to drain into them. The tunnels were normally only a few meters underground, and generally followed the slope of the ground above (Hodge p. 46). Though this sounds simple, the tunnels were often dug from several points at the same time, which sped up construction but made alignment of the separate tunnels difficult. In addition, all underground work has inherent problems with measuring direction and gradient, both of which were extremely important to keep accurate in a cuniculus.
To overcome these problems, the Etruscans used several simple surveying methods. Since the slope of the tunnel followed the slope of the surface, the shafts could be sunk to the correct depth by marking previous shaft heads with poles and sighting down them. Each new shaft would then be dug to the same depth below the resulting reference line (Lewis p. 205). Of course, the tunnel itself must be measured accurately in order to meet the shafts at the correct depth, but the frequency of the vertical shafts would provide regular reference points to judge the accuracy of the tunnel by, as well as making it very easy to track the alignment over long distances. While it is not known precisely what leveling instruments the Etruscans had access to, Lewis postulates that they used similar tools to those of the Persians, whose qanats were the likely inspiration for the cuniculi (Lewis p. 213). If this is so, a very simple yet accurate leveling tool would simply be a sighting tube; a bronze tube hanging from strings. As long as the strings were the same length, gravity would level the tube, and with a little ingenuity it could be used to determine the difference in height between two points. To solve the alignment problem, the Etruscans seemed to have used a groma. The name for this tool was originally Greek, but was transferred to the Romans through the Etruscans (Lewis p. 125). Surveying Instruments of Greece and Rome contains the following description:
A horizontal cross, its arms at right angles, was carried on a vertical support, and from the end of each of the four arms hung a cord or plumb-line tensioned by a bob. The surveyor sighted across one pair of these cords to project a straight line, and across another pair to set out a right angle. He therefore worked only in the horizontal, or more or less horizontal, plane and was not concerned with differences of height (Lewis p. 124).
The Etruscan engineers and surveyors were enormously successful. Not only do many cuniculi survive today (Hodge p.45), but they directly inspired the Romans to build waterways of their own. However, the Roman aqueducts were primarily intended to supply water for mills, irrigation, and civic use (especially baths), while the cuniculi were built to divert water, not supply it (Hodges pp. 6, 46). Regardless of the purpose, it was clear to the Romans that water could be easily controlled in this manner, and they built many channels of their own. In fact, most Roman aqueducts “closely followed the surface of the land, instead of being raised on arches or sunk deep beneath it in a tunnel” (Hodge p. 93) The channel was usually close to the surface, with regular manhole access rather like our modern city sewers. Though the Romans modified what they learned from the Etruscans and regularly built at or above the surface of the ground, they did still dig tunnels on occasion. These were meant to allow an aqueduct to continue through a ridge (Hodge p. 126), and were not self-contained systems like the cuniculi. The Roman tunnelers continued the tradition of using multiple vertical shafts to aid construction, allowing measurements to be checked at intervals and digging to continue from multiple points. These shafts were much further apart than those in a cuniculus, which indicates the greater skill of the Roman engineers (Hodge p. 23), which is probably due to more precise measuring tools. Though the shafts in an Etruscan tunnel were an important part of the finished work, allowing water to drain from the surface and be carried away, Roman shafts were capped and were primarily used for maintenance access after construction was complete (Hodge p. 127).
If tunneling was not a significant part of Roman aqueduct construction, what was built, and how was it done? Though the entire range of Roman water systems engineering is well beyond the scope of this paper, we can touch on a few highlights, including important advances in surveying. The groma remained in widespread use (Hodge p. 203), but became less useful in aqueduct construction, likely because perfectly straight lines became less of a concern once construction moved above ground. Skilled surveyors could also use a dioptra, an expensive and complex instrument for measuring very precise heights and horizontal bearings (Hodge p. 203). A notable modification of an Etruscan instrument is seen in the libra, a Roman tool for measuring gradients. While its exact design and origin are not certain, Lewis explains that it is probably an evolution of the sighting tube which allowed much more accurate measurements, and consequently the construction of aqueducts with very shallow gradients, even as low as 1 in 20,000 (p. 115), though more common examples of gentle gradients are only one in a few thousand.
Gentle gradients are often very desirable in waterway construction, because quickly rushing water leads to greater erosion
of the channel, whether it is a stone tunnel or a raised masonry trough. The Romans also required shallow gradients to transport water over huge distances without much loss of energy, such as the Nimes
aqueduct, which is 49.5 km
long (Lewis p.175, table 9.2). By comparison, the longest known cuniculus
is only 4.5 km long (Hodge, p. 46). In order to maintain a shallow gradient, the Romans were often forced to build enormous arcades
across the plains approaching cities so that water could still be supplied even to buildings on the acropolis
(Hodge p. 161). The particular reasons for building arcades are varied. They were prohibitively expensive, but they did allow free passage across the plain, an important consideration when we recognize that arcades were most often built over long distances near large cities, and cutting the plain into sections would be enormously disruptive to commerce
. Along with the practical considerations, an extensive arcade was an unmistakable indication of a city’s prosperity. These are the most memorable parts of the aqueduct, and though they are extremely impressive, they are only a small part of a complex system involving channels, bridges
, pipes, siphons
, settling tanks
, and tunnels.
One can easily track the progress of the aqueduct from simple Etruscan tunnels used to drain farmland, all the way up to seemingly endless structures which made life in Rome’s famous cities possible. Designing the many parts of the aqueduct and constructing it in so many terrains required multiple feats of engineering and surveying, from learning to tell directions underground, to making a channel which appears to run perfectly level, yet still carries water. Each of these is notable in itself, and together they are of a humbling scale.
A. Trevor Hodge. Roman Aqueducts & Water Supply. London: Gerald Duckworth & Co. Ltd., 1992.
M. J. T. Lewis. Surveying Instruments of Greece and Rome. Cambridge: Cambridge University Press, 2001.
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