The backbone of Roman hydraulics and water systems is surely the Cloaca Maxima – the Great Drain – which ancient sources attribute to the mythical reigns of two of Rome’s seven kings: Tarquinius Priscus (616-579BC) and Servius Tullius (579-535BC).
Much of the early knowledge required for this work was in fact learned from the Etruscans (Tarquinius being of Etruscan extract himself). The purpose of the earliest works was that of draining the marshy areas between the Palatine and Esquiline hills by channeling the water into one main stream which was diverted into the Tiber. It wasn’t until later that these streams were covered over by the expanding city.
Part of the Cloaca maxima is still in use today. The outlet of a secondary artery of it is still visible near the Ponte Rotto, Pons Aemilius, described above. When walking in the Forum there is a padlocked metal gate through which you can hear the water still gurgling through underground.
Drains and sewers ranged in size. The main drains of the Cloaca Maxima are big enough to allow a horse and cart through for maintenance inspection. Minor urban drains were rather smaller and were literally holes in the brickwork covered over by an inverted V-shaped arch made from two terracotta tiles.
The Romans themselves went extremely proud of their sewers so much so that Pliny calls them “Opus omnium maximum” – the greatest work of all. He tells us how Agrippa (son-in-law of emperor Augustus and builder of the Pantheon) joined seven streams to meet underground into one main channel: “….with such a rapid current as to carry all before them that they met within their passage. Sometimes when they are violently swelled with immoderate rains, they beat with excessive fury against the paving at the bottom and on the sides….and yet the works preserve their old strength without any sensible damage“.
A testimony of Roman pride for their sewers and the majesty of this underground piece of civil engineering is the famous Mouth of Truth in the nearby church of Santa Maria in Cosmedin. This circular sculpture depicting a river god was actually a drain cover of the Cloaca Maxima.
As has already been mentioned in the section about aqueducts one of the chief problems in the water systems was that of maintenance. The water of Rome is “hard” and likely to deposit vast quantities of calcium (although to a small degree this effect could be used to provide an added water proofing of the channel). Added to this there is the inevitable problem of silt and debris ranging from dead animals and logs through to dust and dirt. In order to obviate these problems the Romans built complex sets of tanks which would allow the water dirt to decant before flowing on. These decantation chambers were regularly cleaned up. Larger materials were stopped by barriers which acted as gross filters.