The Aqueduct of the Gier
by Aldo Borlenghi
At the foot of Mont Pilat, around ten kilometers from Saint-Etienne, an aqueduct catches the waters of the Gier river, a tributary of the right bank of the Rhône, waters that have travelled a distance of some 86km, providing a potential debit of 15,000 m3 per day to supply the colony of Lyon, formerly known as Lugdunum, founded in 43 B.C. by Munatius Plancus, settled on the hill of Fourvière.
In Lyon the penury of local hydraulic resources – with all of the sources located only halfway up or at the bottom of the hillside – quickly forced the Romans to find solutions to supply the summit and the upper slopes where the majority of the dwellings were located. Because of the difference in altitude, and to limit the risks due to pollution, it was impossible to use the waters of the Saône which flowed at the foot of the hill, nor those of the Rhône, located even further away. Urban development quickly outstripped the use of the many wells and cisterns that were used to collect rainwater, with their presence having since been archeologically verified on the site, as they were unable to ensure the supply to fountains and public buildings such as the local hot baths, which consumed a massive quantity of water on a large scale. The only resources capable of providing an abundant supply of pure water were located in the mountain ranges which surrounded the colony: the Mont d’Or to the north, the Monts du Lyonnais to the west and the Pilat to the south.
Beginning in Augustan times, four large aqueducts supplied the roman city with water. The geographic area where the whole, or at least a huge part, of the path travelled by the four aqueducts was located, coincided with the western part of the territory covered by the Roman colony of Lyon which belonged, before Jules Cesar conquered Gaul, to the Gallic people of the Segusiavi. Even though the borders of the colony continue to be a subject of some discussion among researchers, it is possible to define a territory which is more or less rectangular, bordered to the west by the Mont du Lyonnais, to the north by the foothills of the Beaujolais, to the south by the valleys of the Ozon and the Gier, and to the east by the edge of Crémieu island. Within the area of the colony that contains the aqueducts one can find a number of villas and farms dedicated to polyculture, like the Roman villa of Goffieux in Saint-Laurent d’Agny. To the west is located the territory of the city of the Segusiavi which corresponds to the current region of the Forez with its capital Forum Segusiavorum, known today as the town of Feurs. A road runs along the Saint-Bonnet pass, entering the Loire at Saint-Martin-Lestra, linking the town to Lyon.
In the hinterlands of the colony, the monumental vestiges of the aqueduct of the Gier constitute, in great part thanks to their state of conservation, an exceptional testimony to the hydraulic science of antiquity. Though the demands of urbanization and the exploitation of the piers and the arches as quarries for construction materials for centuries have destroyed numerous areas, the aqueduct remains a permanent feature of the current landscape, marking not only the countryside, the valleys and the wooded areas but also the urban landscape, where the ruins run alongside roads, are surrounded by buildings and factories, and are incorporated into local parks or built into private houses and properties.
The aqueduct and its vestiges have continued to attract the attention of researchers since the Renaissance. Among those scientific studies, the 126 drawings by G.-M. Delorme, dating back to 1760, that were rediscovered in 2003 following their disappearance during the Revolution, show in great detail the state of 37 different aqueducts in the 18th century. The architect’s records were employed in the project to reuse an ancient Roman aqueduct as part of the solution of the problem of supplying water to Lyon. We continue to ignore the reasons why the use of the aqueduct was abandoned, we do know however that it ceased functioning in the 4th century.
The technique used to build aqueducts, the opus reticulatum, typical of central Italy provides a unique testimony, rarely seen in Roman Gaul. The arches, the piers and the walls show a completely smooth surface, composed of local sandstone which, when cut into the form of small pyramids and placed at 45° to the horizontal, looks like a kind of netting: these elements are sunk into the block-work, the core of the construction which is made up of a mix of stone and mortar. Parallel to this, the twin eaves of 60cm long bricks or stone slabs, are regularly inserted into the piers: the top of the arches are also composed of stone slabs, sometimes alternated with bricks.
The creation of the aqueduct represents the final act in a series of preliminary operations, which include the development of a project which takes into account the search for a source, an examination of natural obstacles and restrictions and the exploitation of the material found on the site. Though the goal is to draw the straightest line possible between the place where the water is captured and the basin used for urban distribution, the length of the canal nonetheless depends on the topography of the ground to be crossed and the difference in altitude between the two end points. Water flows according to gravity, the canal should have a relatively constant gradient, not too much so as to avoid damage to the conduit and not too little to avoid slowing the water flow too much: the aqueduct of Gier has an average gradient of a 1.1m drop per kilometer.
The topography of the route forced Roman architects and engineers to achieve an extremely precise level of mastery of their technology. For example, the construction of four inverted siphons can be noted, crossing valleys which were too large and too deep to be avoided or circumvented, or even crossed by bridges: thanks to the principle of connected vessels, the water moves from a free flowing state to a state of pressurized flow. The channel reaches a reservoir where it then emerges under pressure through ten or so lead pipes, which descend along the surface of the valley, passing over a bridge and then climbing the opposite side, with a slight loss in altitude, to a tailrace: here the water continues to flow freely in the channel.
Even though the aqueduct possesses over 40 bridges, 5 overground substructures and 8 lines of arches, 90% of its structure lies underground. Its inspection is ensured by manholes which, placed regularly along the canal archway, at an average distance of 77m from each other, allowing maintenance, cleaning and repairs to be done. The canal, with its interior of around 0.60m wide and 1.50 – 1.60m high, is made up of blocks of stone and mortar.
The aqueduct’s path begins near Izieux, at an altitude of around 400m: having diverted the waters of the Gier thanks to a dam, the canal circumvents the village of Saint-Chamond, crossing a number of valleys via bridges.
After a winding path along the valleys that break up the plateau, the aqueduct reaches the valley of Durèze, which, being almost 100m deep and 700m wide, constitutes a significant obstacle to its progression: a unique example of its kind, a siphon, with its forward reservoir and a few piers of the bridge still visible, crosses the valley in a straight line while at the same time, a buried channel goes around it, extending its path by over 11km. It is along this loop that one might find, way below the village of Chagnon, the “Cave du Curé”, a local name given to a tunnel which is 70m long with a height of 2m. in 1889 on the same loop, a landmark was discovered which, along with the one that recently came to light in the commune of Saint-Joseph, attests to the existence of a surrounding space that had been established all along the route of the channel: an inscription indicates that, by the authority of the emperor Hadrian “no one has the right to till, sow or plant in this space whose land is dedicated to the protection of the aqueduct”.
Near to the village of Génilac, the canal continues its torturous route, crossing other tributaries of the Gier. As it leaves the valley of Bozançon, on the edge of the Plateau Lyonnais, the channel pursues a more regular path thanks to a more even terrain: the presence of rivers and a large stream required the construction of bridges, sometimes spanning 9 arches. Under the village of Mornant a tunnel shortens the path by around 3 kilometers. After the overhead channel of Merdanson at Saint-Laurent-d’Agny, the last in the channel’s route, the aqueduct starts to head towards Lyon in the straightest possible path with the help of some very imposing structures, and certain among them have been very well preserved, allowing a regular and constant drop in level. In Soucieu-en-Jarrest the Grandchamp wall, conserved over a distance of 160m, along with a long line of arches supports the canal all the way up to “La Gerle”, and the reservoir of the siphon, which crosses the valley of the Garon, 1200m wide and 11m deep: a monumental siphon-bridge with 23 arches, with the remains of the piers and arches still visible at the extremities, was built at the bottom of the valley to help the flow of water reach the next reservoir.
On the plateau of Chaponost, the aqueduct, running alternately above and below ground, displays more spectacular vestiges, first the arches and walls of “La Colombe”, and in particular the long row of arches of the “Plat de l’Air” which, spanning a distance of over 600m, recall the typical landscapes that can be found on the outskirts of Rome, with its long rows of arches of aqueducts that supplied the capital of the Empire. The monumental aspect of the area is reinforced by the presence of the reservoir and the climbing arches of the siphon that cross the valley of Yzéron, 3km wide and 140m deep. The imposing remains of the siphon-bridge of Beaunant, almost 300m long, classed as a historical monument in 1875, represent a precious testimony thanks to their state of conservation: as with the siphon-bridge of the Garon, the piers, over 18m in height, are either full and massive or pierced by transversal openings.
Once the valley has been crossed, the aqueduct, which follows the ridge-line of Sainte-Foy, running along a path which moves alternately above and below ground, reaches the fort of Saint-Irénée, where the last arches and piers connected to the reservoir of the fourth siphon can be found. Being shorter than the others (575m), it allows the pressurized pipes to pass through the saddle of Trion, a final obstacle before reaching the hill of Fourvière, where it connects to, at an altitude of 300m, the reservoir that was placed at the summit of the Cardinal-Gerlier, and which was destroyed in 1846. the piers of the rue Roger Radisson constitute the last vestiges of the aqueduct, whose terminus, that is to say the distributing water tower that, thanks to the height of its position, supplies water to the whole of the Roman colony, remains unknown to us, even today.
Translated by Derek Byrne
Aldo Borlenghi is a senior lecturer in archeology at the University of Lyon 2. He belongs to the ArAr laboratory and is the head of the regional archeology group Ville et territoire : de l’oppidum à la colonie.