Aqueduct of the Gier

The Aqueduct of the Gier (French Aqueduc du Gier)^[1] is an ancient Roman aqueduct probably constructed in the 1st century AD to provide water for Lugdunum (Lyon), in what is now eastern France. It is the longest and best preserved of four Roman aqueducts^[2] that served the growing capital of the Roman province of Gallia Lugdunensis. It drew its water from the source of the Gier, a small tributary of the Rhone, on the slopes of Mont Pilat, 42 km (26 mi) south-west of Lyon.^[3]

Following a sinuous path, at 85 km (53 mi) the aqueduct of the Gier is the longest known of the Roman aqueducts. Its route has been retraced in detail, following the numerous remains. Leaving the uplands of the Massif du Pilat, department of the Loire, the aqueduct hugs the surface relief and crosses the department of the Rhone, passing through Mornant, Orliénas, Chaponost and Sainte-Foy-lès-Lyon to terminate at Lyon.

In its extent, it draws upon the whole repertory of Roman techniques of aqueduct building, taking a slope that averages 0.1%, or a meter every kilometer. There are 73 km (45 mi) of covered ditches laid with a concrete culvert 3 m (9.8 ft) high and 1.5 m (4.9 ft) wide, which is sunk as deep as 4 m (13 ft) beneath the land surface. The aqueduct passes through 11 tunnels, one of which, near Mornant, is 825 m (2,707 ft) in extent. Access for cleaning and repairs was through manholes at 77 m (253 ft) distances. There are some thirty stretches in the open air. There are ten stretches raised on walls and arches, which provide the most spectacular visible remains of the aqueduct (illustrations).

Four inverted siphon tunnels cross the particularly deep and wide river valleys of the Durèze, the Garon,^[4] the Yzeron and the Trion on pipe bridges raised on high arches. In these, water filled a sunken tank tower (castellum^[5]) on the brim of a slope. The tank effected a transition between open channel flow and a lead pipeline. From the castellum water was carried, now pressurized, in a set of airtight lead pipes laid side by side, with soldered joints, down the valley slope, across a bridge spanning the river—whose piers and arches are the most notable remains of the system—and up the facing slope, to a tank slightly lower than the head tank, losing just a little hydraulic head in the process. The inverted siphons obviated the bridging of deep valleys with arcade upon arcade of arches, as at Pont du Gard, which marks the limit of such a system.

Date

The Gier aqueduct was built in a single great campaign, since no part of it could have served until it was completed; it must have taken years. The aqueduct of Giers was dated by Germain de Montauzon^[6] to the reign of Hadrian in the early 2nd century AD, but, as James Stephen Bromwich points out, its reticulated stonework (opus reticulatum) was characteristic of the later 1st century BC and the first half of the 1st century AD, rather than of later masonry.^[7] In addition he notes that a recently excavated public fountain on the hill of Fourvières, datable about 50 AD could not have been supplied with water until the Giers aqueduct was complete.

Gallery

Arch of the aqueduct at Saint Maurice sur Dargoire showing opus reticulatum
Chaponost, Plat de l'Air, headertank of the Yzeron inverted siphon
Vestiges of the Roman aqueduct in Chaponost
Aqueduct of Gier at Mornantet

Notes

^ It was called the Aqueduct of Pilat when Aubin-Louis Millin de Grandmaison visited it and inspected its dramatic remains in 1804 (Millin de Grandmaison, (tr.) Travels through the southern departments of France London, 1808:136f; the translator renders Pilat "Pila"
^ The others were the Mont d'Or, the Yzeron and the Brevenne aqueducts (L. Mays, Ancient Water Technologies, 2010;132); the names, of course, are modern.
^ Distance in James Stephen Bromwich, The Roman remains of Northern and Eastern France: a guidebook 2003:420; Bromwich gives detailed instructions for viewing sections of the aqueduct system.
^ Here the siphon drops 21 m, the length of the crossing 208 m (Jean Pierre Adam and Anthony Mathews, Roman Building: materials and techniques, 2003:244).
^ The parts of a Roman aqueduct are illustrated and explained in Peter J. Aicher, Guide to the aqueducts of ancient Rome, 1995 and A. Trevor Hodge, Roman Aqueducts & Water Supply, 2002.
^ Montauzon, Les aqueducs antiques de Lyon, 1909, refers to a Hadrianic inscription (CIL XII 2494) at Chagnon, in effect a "no trespassing" notice to farmers who were in the practice of diverting water for irrigation. Within the limits Roman aqueduct zones were public property (Rabun Taylor, Public Needs and Private Pleasures: water distribution, the Tiber river and the urban development of ancient Rome 2000:57ff.
^ Date revision was suggested by Jeancolas, JEAR 183, noted by Hodge 2002:435 note 10.