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Origin and history

The Watershed of the Air Plat is an emblematic stretch of the Gier's waterworks, built under the Roman Empire to supply Lugdunum (Lyon) with water. With its 86 km, it is the longest of the four Lyon aqueducts and one of the best preserved. Its layout, studied in the 18th century by scholars such as Guillaume Marie Delorme and Paul de Gasparin, reveals various Roman techniques: covered trenches, tunnels, bridge-siphons and aerial passages on arches. The section of the Air Plat in Chaponost is distinguished by its 72 stone and brick arches, adorned with a rare opus reticulatum trimming in France, and by its hunting tank linked to the siphon of the Yzeron.

The dating of the aqueduct has long divided historians. Originally attributed to Claude (I century) due to stamped lead pipes, it was then associated with Hadrian (II century) after the discovery in 1887 of the Stone of Chagnon, a terminal prohibiting crops near the work. In 2018, wooden formwork dated by dendrochronology of year 110 confirmed a construction under Trajan, possibly completed under Hadrian. The Air Plat, classified as a Historical Monument since 1900, has benefited from recent restorations (2009–2010) to preserve its masonry and its waterproof tile-based mortar (opus signinum).

The Gier's lake, protected by terminals such as Chagnon's, also illustrates the technical challenges of Rome: its siphon of the Yzeron, 2.6 km long with a 123 m fall, used 12 lead pipes to withstand a pressure of 13 bars. At Chaponost, the airway alternates walls and arches, while sights (now partially localized) allowed its maintenance. Although disused, the work remains a major testimony of ancient hydraulic engineering, studied since the 16th century by antique dealers such as Pierre Sala or Jacob Spon, then systematized by Germain de Montauzan in 1908.

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