The Nimes Aqueduct and the Pont du Gard[242]

The Roman Nemausus (Nimes) is much older than Lyon, founded in the 6th centu­ry BC, even before the conquest of Gaul by the Romans. Nimes was the capital of the Arecomic Volques, a Gallic people often allied with the Romans. Nimes is naturally well supplied with water, from wells and especially from the Fontaine spring, abundant and perennial. Under Augustus the spring was the subject of major construction, including a masonry canal and basin at its outlet. The need for an aqueduct only came later under the pressure of urban development (20,000 inhabitants in the Gallo-Roman Nemausus), with the objective of supplying water to the highest areas of the city above the level of the Fontaine (at an elevation of 51.1 m). Under Claudius, in the middle of the 1st cen­tury AD,[243] the Eure fountain spring is tapped near Uzes at an altitude of 72 m.[244]

Although it is only some 20 km from the source to Nimes as the crow flies, the aqueduct had to wind around the vast scrubland plateau of Nimes, whose elevation is above 100 m (see Figure 6.14). There are numerous obstacles to be crossed on this plateau. These obstacles include ravines that are dry in summer but subject to violent floods, such as the Bornegre ravine with its three-arch bridge; passes, with the two-level arched bridge of Font-Menestiere; and notably the sunken valley of the Gardon, across which the highest bridge-aqueduct of the Roman Empire is built, the Pont du Gard (Figure 6.15). The bridge is about 360 m long, and carries the canal of the aqueduct at an altitude of 65 m, some 48.4 m above the bed of the river. Just upstream of the bridge the canal has a basin provided with a gate and a discharge canal enabling diversion of the discharge of the aqueduct (or its excess) into the Gardon, if necessary. Downstream of the bridge, the canal traverses the rough terrain along the edge of the limestone plateau by means of multiple switchbacks, crossing ravines on small bridges. The canal passes through three tunnels of about 400 m in length, further downstream near Sernhac, then again near 39

Nimes. The terminal point of the aqueduct is the water tower in the city (castellum), at an elevation of 58.95 m.

All of these bridges are designed to handle the strong floods typical of the Mediterranean climatic regime. The bases of the bridge piers are protected by shaped prows on the upstream face. The bridges all leave a very large opening for flood pas­sage; the widths of the openings of the arches of the Pont du Gard are 24.5 m and 19 m.

The castellum of Nimes (Figure 6.17) is one of the rare Roman water towers still conserved in more or less its original state. The aqueduct dumps water into the tower’s

Figure 6.17 The castellum of Nimes: this distribution basin is the terminal point of the Nimes aqueduct. It is visible in the city, on rue de la Lampeze (photo by the author).

circular basin of 5.5 m inside diameter and depth 1.4 m. Issuing from the basin are ten circular distribution pipelines of 0.4 m diameter. Valves enable the isolation of one cir­cuit or another, and drains in the bottom make it possible to empty the basin.

The aqueduct has several slope changes;[245] upstream of the Pont du Gard it is only 38 cm/km on the average, which is relatively small compared to other aqueducts. But downstream of the bridge, the slope is only 8 cm/km along a particularly sinuous seg­ment of more than 10 kilometers length. The theoretical maximum discharge for this slope can be estimated at around 40,000 m3/day. Even though this aqueduct may not attract the same admiration as the Pont du Gard, it is all the more impressive for its incredibly small slope, and the precision of surveying and construction that this implies.

These slope changes have hydraulic consequences that were not well understood or mastered by the Roman engineers. In a canal of constant width, the depth of water is larger when the slope is small; yet the Nimes aqueduct was initially constructed for an essentially constant depth. Early on, it became necessary to raise the canal walls in sev­eral locations (of flatter slope) to avoid overflow.

Another problem is the fact that the water from the Eure fountain is very calcareous. Over the years, the useful cross-section of the aqueduct’s canal becomes considerably reduced by deposits, effectively reducing the discharge to a value that was probably only about 20,000 m3/day. The canal had to be scoured out on several occasions.