The Romans built 258 miles of aqueducts across the Roman Empire, from Germany to North Africa. The engineering was so precise that it was not to be surpassed for 1,000 years, and the word itself is derived from two Latin words: aqua (‘water’) and ducere (‘to lead’).
The Pont du Gard in South France is one of the largest and best preserved examples of a Roman aqueduct. Built around 2,000 years ago, it supplied the city of Nemausus for 300 years.
The Nemausus Aqueduct
The full aqueduct was built to supply the ancient city of Nemausus, today the French city of Nîmes. It ran a course of 50km: from a small village called Uzes to the north of the city.
The aqueduct has long been credited to Marcus Vipsanius Agrippa, the son-in-law of the Roman Emperor Augustus, in around 19 BC. At this time he was serving as aedile, the senior magistrate responsible for the water supply of Rome and her empire.
In Roman times, around 40,000 cubic metres flowed through the aqueduct each day, taking 27 hours from source to the castellum divisorum (the repartition basin) in Nemausus. From there it was distributed to fountains, baths and private houses to supply the 50,000 inhabitants.
A feat of engineering
The spring at Uzes was just 17 metres higher than the basin, allowing a decrease in height of just 25 cm per km. It would have taken around 1,000 workers to labour for 3 years to complete it.
They would have used simple tools to shape the blocks, and the heavy lifting was done by cranes, powered by workers running on a treadmill.
The blocks, some of which weighed 6 tons, were taken from a local limestone quarry. The builders used a technique called opus quadratum. This placed the blocks seamlessly without mortar, and required meticulous cutting. The pillars of the middle and lower stories were aligned to ease the weight borne by the arcade arches.
The exterior of the structure appears rough and unfinished, but the inside channel was as smooth as possible to ensure it would not obstruct water flow. The channel walls were constructed from dressed masonry; the floor was constructed from concrete.
This was then covered with a stucco made of tiny shards of pottery and tile. It was coated with olive oil, and covered with maltha, a mixture of slaked lime, pork grease and juice of unripe figs.
The Pont du Gard is just a small surviving part of this enormous aqueduct, and it crosses the Gardon tributary. The 3 levels of the Pont du Gard were 49 metres high, with 52 arches. The channel is 1.8 m high and 1.2 m wide.
The design of stacking arches on top of one another was inefficient and expensive. Later Roman aqueducts would make greater use of concrete to reduce their volume and cost. Stacked arches were replaced by tall, slender piers, made from concrete-faced masonry and brick.
Decay and restoration
After the 4th century, the aqueduct fell into disuse. By the 9th century it was blocked by silt and used as a footbridge. A new footbridge was built in 1747, although this work weakened the structure and led to further decay.
Napoleon III, who greatly admired all things Roman, visited the Pont du Gard in 1850. He took a close interest in the structure and made arrangements to repair the bridge. Charles Laisné, a famous architect, was employed to complete the restoration during 1855-58 – a project that the Ministry of State funded.
Featured Image: Benh LIEU SONG / CC BY-SA 3.0.