The arches are the exception
Picture a Roman aqueduct and you’ll probably see a line of arches crossing the countryside. Rome’s first aqueduct had almost none. The Aqua Appia, built in 312 BCE by the censor Appius Claudius, was 11,190 Roman paces long, about 16.5 kilometres, and 11,130 of those paces ran underground. Only about sixty paces, near the Porta Capena, stood on substructures and arches. Later aqueducts had more arches, but most of every line still ran underground: of the Aqua Claudia’s 46,606 paces, 36,230 were below ground.
Where a hill stood in the way, the builders tunnelled through it; where a valley opened, they carried the channel across on a wall or on arches. The arches you photograph are the places where the ground fell away and the water could not.
How gentle was the slope?
Very. The aqueduct that crossed the Gardon on the Pont du Gard, in southern France, fell on average 25 centimetres for every kilometre of its 50-kilometre course, less than the length of a school ruler over the distance of ten football pitches. Holding that slope steady over tens of kilometres was the real achievement of Roman water engineers.
The result could be enormous. The Pont du Gard aqueduct delivered between 30,000 and 40,000 cubic metres of water a day to Nîmes, and kept working for about five centuries.
Nine aqueducts and a water commissioner
By the end of the first century CE, nine aqueducts brought water into Rome: the Appia, the Anio Vetus, the Marcia, the Tepula, the Julia, the Virgo, the Alsietina, and the two great works begun by Caligula and finished by Claudius, the Claudia and the Anio Novus. Their story is well documented because in 97 CE the emperor Nerva made Sextus Julius Frontinus curator aquarum, the commissioner in charge of the water supply, and Frontinus wrote a book about it. He gives the source, the length and the builder of each aqueduct, and compares these “indispensable structures” with “the idle Pyramids or the useless, though famous, works of the Greeks”.
Keeping the water clean was part of the job. Along the line, settling tanks slowed the flow so that sand could drop to the bottom: the aqueduct of Segovia still has two of them along its route. Bringing water in was half of the system; the other half was the city’s sewers.
Do any still run?
One does. The Aqua Virgo, opened by Agrippa in 19 BCE, is still running, and its water ends in the Trevi Fountain. Others were reused: in 1585 Pope Sixtus V built the Acqua Felice, which for part of its course runs on top of the Aqua Claudia, and it still works. In Segovia the Roman aqueduct carried water to the city until the last decades of the twentieth century.
So how did a Roman aqueduct work? With a spring higher than the city, a channel that fell a little at a time, and a great deal of patience. Arches were the last resort, used only where the land left no choice, and like the roads, the aqueducts were maintained for centuries by people who kept relying on them.
Where to see it
In Rome, walk the Parco degli Acquedotti on the south-eastern edge of the city, where the arches of the Aqua Claudia and the later Acqua Felice cross open fields, and see Porta Maggiore, where two aqueducts entered the city. Outside Italy, the Pont du Gard near Nîmes and the aqueduct of Segovia, in Spain, are the best preserved.
Sources
- Frontinus, De aquis 1-16 (trans. Bennett, LacusCurtius).
- Pont du Gard official site, “History”; UNESCO World Heritage Centre, “Pont du Gard (Roman Aqueduct)” (344).
- Turismo de Segovia, “Aqueduct” and “History of the aqueduct”.
- Turismo Roma (Roma Capitale), “Parco degli Acquedotti”; “Fontana di Trevi”.
