Computer modelling is lifting the lid on the secrets of Roman hydraulic engineering. Hitherto mysterious hurdles and holes, it hints, may have smoothed the flow of water.
In the third century AD, Roman engineers built a system of tunnels and tanks to bring water to the city of Aspendos, today in Turkey. Aspendos was a crucial hub of Roman trade in Asia Minor, sitting at the crossroads of important routes with river access to the Mediterranean Sea.
The Roman writer Vitruvius described the now-ruined system for delivering water to a major settlement. But aspects of his account remain obscure to modern readers because the meaning of some of his Latin terms has been lost and because there were probably other features of Roman civil engineering of which no record survives.
So Charles Ortloff and Adonis Kassinos of the private company CTC/United Defense in Santa Clara, California, have tried to make sense of the Aspendos siphon system by calculating how water might have flowed through it.
A pipe made from stone blocks bored with 30-centimetre holes carried water from an aqueduct across a 1.5-kilometre valley, running down its northern wall and across its floor before ascending its southern side. Here water was gathered in a storage tank to supply the city.
The aqueduct to the north was higher than the tank in the south, so an overall downward gradient drove the water through the pipe. But en route across the valley, two arched stone towers took the pipeline up and down again.
Why did the Roman engineers build these apparent obstacles? Ortloff and Kassinos calculate that the arches divided the siphon system into three shorter legs, damping out any sloshing that could have made water supply intermittent or even damaged the pipeline.
The researchers also have clues to a second mystery. Vitruvius says that the key to the siphon's success are colliquiaria - a Latin technical term whose meaning is no longer known. Otrloff and Kassinos believe that the colliquiara may be the little holes, about 3 centimetres wide, that perforate some of the pipeline's blocks.
The pair carried out tests on a scale model, and conclude that the holes probably reduced turbulence in the flow by letting air and water escape.
The researchers also reckon that the operation of the siphon would have depended critically on the texture of the pipeline walls. If they were too smooth, large waves would have formed when the pipe was opened to fill up the receiving tank; too rough, and friction would have slowed the flow.
The ideal texture is more or less that of the Romans' hand-chipped stone blocks.
