The Roman aqueducts in Gaul

The growth and prosperity of the period of peace between 97 and 180 AD directly ben­efits the Roman provinces. All Gallo-Roman cities of importance benefit from one or several aqueducts during this period, under the reigns of Nerva, Trajan, Hadrian, Antonius, and Marcus Aurelius, then until 235 AD under Severus. Aix-en-Provence and Lyon have four aqueducts; later on we will examine those of Lyon in detail. At Vienne, where the sources are very near,[232] we know of eleven aqueducts; at Poitiers (Lemonum) and Perigueux (Vesone), three. Some of these aqueducts are quite simple in design, with a very regular slope, as at Perigueux and Rodez. Others have quite a variable slope, most often steeper upstream than downstream, and with quite a variety of hydraulic works: tunnels, bridges, drops, siphons. The average slope itself can be quite variable from one aqueduct to another; it is usually between 0.2 and 1.6 m/km. One notable singularity is the Craponne aqueduct at Lyon, whose average slope is 16 m/km.

The dimensions and discharges of the aqueducts are also quite variable. The aque­ducts of largest cross-sectional area are at Vienne and Nimes. At Lyon and Nimes, the lengths rival those of the aqueducts of Rome itself (50 to 75 km). What is striking from the outset is, as shown in table 6.3, the diversity of configurations. Yet the dates of con­struction, when known, are most often in the 1st and 2nd centuries AD, the period of prosperity, Roman peace, and the concomitant economic climate favorable for develop­ment of the provinces. Samples of the broad panorama of Roman techniques of this period, adapted to local conditions, can be found in Gaul. We too rarely know the his­tory of these aqueducts, histories that are sometimes quite dynamic[233] as conditions change. The needs for water can increase over time, or the discharge available from the sources can decrease. Calcareous water can clog the conduits with deposits.

At Sens (Agedincum), the Fauconderie spring (on the left bank of the Vanne) is ini­tially the sole water source for the aqueduct. But when the spring dries up, four other springs are tapped successively until the flow is restored to, or slightly exceeds, the ini­tial supply. The aqueduct is increased in length from 6.1 km to 14.2 km, so as to reuse both the canal between the former Fauconderie spring and Sens, and the bridge on which the aqueduct crosses the valley of the Vanne.

At Saintes (Mediolanum), after some fifty years the capacity of the aqueduct is reduced due to the increase in roughness caused by calcium deposits. Therefore the aqueduct is completely rebuilt, being replaced by one of larger cross-sectional area but flatter slope, capturing water from new springs and delivering it to the city at a higher elevation. This reconstruction, like that of Sens, was based on re-use of large art works — the two bridges called the Arcs (27 arches, 298 m long) and the Hautmont (62 arches, 400 m long) as well as the tunnel of Neufs Puits. On these bridges, the canal is simply raised. But along underground segments, major construction efforts are devoted to mod­ification of the cross section. However, the water from new sources is even more cal­careous than the original. According to the estimates of Marcel Bailhache, the discharge, which had been increased from 4,000 to 22,600 m3/day by the reconstruction, subse­quently decreases to a value of only about 8,000 m3/day.

At Toulouse (Tolosa), the capturing of supplementary sources increases the dis­charge of the Ardenne aqueduct. But the resulting water depth (0.66 m) ends up exceed­ing the depth of the 40-cm canal lining, thus causing serious degradation of the canal walls.

Water quality is a strong determinant of the longevity of the aqueducts. If the water is calcareous, lime encrustation can reduce the flow area and increase the roughness to the point of reducing the discharge capacity by a factor of two in a few decades. Without maintenance, the canal can become completely blocked. On the other hand, if the water is not calcareous, the canals can have a very long lifetime. The aqueduct of Sens, for example, will be used up until the middle ages.

The most important works are those of Lyon and Nimes.

Table 6.3 A collection of synthesized data on the principal known Gallo-Roman aqueducts, listed in decreasing order of length.[234]

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