The Fontenay aqueduct, also known as Aqueduc du Cher, is a largely underground aqueduct that helped supply water to the ancient city of Caesarodunum (now Tours), alongside springs, wells, and another intramural aqueduct. It owes its most common name to a locality near Bléré, where its starting point was located, some 25 kilometers east-southeast of Tours, close to a main spring; the water table from which it springs is still exploited in the 21st century. The aqueduct was also fed by several smaller springs along the way.
The aqueduct's route followed the Cher valley along its left bank, totally or partially embedded in the limestone hillside, except a few engineering structures required to cross valleys, almost in a straight line between Bléré and Saint-Avertin, a commune bordering Tours from which it is separated by the Cher. Although a large part of its route is attested, it has not been possible to identify its course beyond Saint-Avertin, in an area that has been profoundly altered by urbanization, which still casts doubt on its final destination and even on the premature interruption of its construction.
It was built during the Early Roman Empire, probably in the 1st century AD, at the same time as the main public monuments of Caesarodunum. It underwent regular maintenance and repairs until it was abandoned at an uncertain time that seems to predate the High Middle Ages. Only a few easily accessible remains remain, such as at the entrance to the Véretz municipal campsite (a section of canal exposed after having been moved) or along a roadside (piers and pipe). In many cases, the aqueduct is only revealed by anomalies in the terrain (elongated slope), which are not very suggestive of its presence. The fragility of the hillside supporting the aqueduct has led to the destruction of a significant proportion of the masonry by landslides. A section of the vaulted canal, cleared of the backfill that covered it, is located on a private estate in the commune of Athée-sur-Cher and is protected as a listed historic monument.
Water in Caesarodunum
Importance of the city The ancient city of Caesarodunum covered an area of around 70 hectares, within which the density of urbanization seems to have been greatest on the banks of the Loire but is still very poorly known as of 2015. It therefore seems impossible to estimate the population residing in Caesarodunum during the Early Roman Empire. Archaeologists and historians suggest population densities for cities in Roman Gaul ranging from 150 to 225 inhabitants/ha, and these estimates are uncertain. Most of Gaul's medium-sized cities, including Caesarodunum, had a population of no more than 5,000 to 6,000.
Water supply systems Caesarodunum's water supply systems are not well understood. In the 19th century the existence of the Fontenay aqueduct and its ancient origins were known, but the hypothesis that it could have supplied water to Caesarodunum was rejected by some historians as no remains had been found in its terminal section. Other authors, on the other hand, were convinced that the aqueduct did indeed lead to Caesarodunum. Later, in the early 1980s, the presence of individual wells exploiting a shallow water table was revealed, as well as the equipping of rare domus with water collection basins (impluvium).
In 2003, an excavation determined the location of the left bank of the Loire during the Early Roman Empire, and an aqueduct perpendicular to the Loire, heading south and towards the center of the city was discovered, fed by a noria drawing water from the river. The noria seems to have had a relatively short life span, on the order of a few decades in the 1st century; it may have been a source of non-potable water for the town's artisanal activities. However, nothing is known about the water distribution networks inside the city from the aqueducts.
Water needs The water supply systems in Caesarodunum were designed to meet the needs of, in descending order of priority, public fountains, public thermal establishments, artisanal activities, and some private residences. This order of priority, apparently very old and widespread in the Roman Empire, was taken up by Agrippa and then cited by Frontin; it is attested by the archaeological remains of Pompeii and Nemausus (now Nîmes). Only two public thermal establishments, a few domus equipped with private baths, and probable craft activities are attested. This very fragmented view makes it impossible to estimate the city's water requirements, as the variability is already very great for cities much better known than Caesarodunum: an inhabitant of ancient Rome had at his disposal around 1,100 liters per day, while that of Pompeii was half that amount. By way of comparison, the water requirements of a modern city, taking all public and private uses into account, are around 400 liters per inhabitant per day.
Architecture and layout
Construction techniques
Canal The aqueduct is usually buried on the hillside on the left bank of the Cher. A trench is dug into the slope, after leveling the ground over a width corresponding to the aqueduct's right-of-way (phases 1 and 2). A layer of clay mixed with gravel, designed to level the bottom of the trench, is then deposited at the bottom of the trench (phase 3). The mortar invert is then shaped and leveled at the bottom of the trench; it temporarily supports a formwork of planks used to cast the aqueduct's pedestals (side walls), also built in mortar (phase 4). A vault of limestone or siliceous stone, cemented with lime mortar, probably mounted on a temporary longitudinal truss in the shape of an inverted "V", caps the canal; the same mortar is also applied to the inside and outside of the vault (phase 5). The trench is then backfilled until the aqueduct vault is completely or partially concealed (phase 6). At the time the canal was built, no waterproof mortar was used on the inside of the invert and pedestals, as the rapid development of limestone concretions seems to have taken its place; the waterproof tile mortar (opus signinum) that covers the walls and invert on certain sections was applied during a repair campaign. The canal (or specus) is approximately 0.50 m wide and 0.60 m high, with an arch height of 1.30 m; these are average dimensions, but they vary little along the aqueduct's length.
Engineering structures At 18 sites along its route the aqueduct is aerial, with the pipes carried for dozens of metres by a continuous wall or by arches supported on piers (pont-aqueduc). Six pont-aqueducs have been documented, the largest being 110 meters long. One load-bearing wall has been identified, 60 meters long. In the last eleven cases, the use of a crossing structure was made compulsory by the local topography, but its nature cannot be specified due to a lack of remains; the longest of these structures crosses the Cher Valley, over a distance of at least 1,000 meters. The state of preservation of the remains does not allow us to describe the canal supported by the bridges or walls, which may have been built on the same model as the buried canal in terms of its invert and pedestals, with a series of stone slabs or tiles replacing the masonry vault in a commonly adopted arrangement. Numerous landslides along the valley sides have certainly buried many bridge remains.
Bridge-aqueduct construction begins with a face of small, regular units with no brick bases (opus vittatum), enclosing a blocking core of stones embedded in mortar (opus caementicium). In some places, a layer of mortar covers the whole, but the masons took care to draw patterns on its surface simulating a masonry unit. In a second phase, perhaps in the second or third century, the intrados of the vaults were strengthened by the veneering of a second, lowered vault in the masonry, using terracotta elements. At the same time, some piers were fitted with buttresses; others were completely rebuilt, perhaps following the collapse of a first bridge undermined by leaking water from the aqueduct.
The remains of three piers (photos of piers 1 to 3 in figure 4) remain in the Chandon valley (Athée-sur-Cher). By measuring the distance between these piers, Cyril Driard was able to calculate the span of the arches of the bridge-aqueduct that crossed this valley over a length of around 60 meters, and to propose a restitution (photo 4 in figure 4). The attested location of a recently lost fourth pier confirms these calculations. The height of the bridge was estimated based on the altitude of the canal remains upstream and downstream of the valley.
Building materials and construction time All the materials used to build the aqueduct could be found or produced on site. Sand and gravel used to make mortar were most likely taken from the Cher riverbed. The limestone and masonry flints used for the engineering structures and the canal vault were extracted from the hillside, which also provided the basic material for the manufacture of lime, certainly produced in small kilns close to the current construction site. The quantity of materials required for the construction of the canal alone, over the recognized length of the aqueduct, is estimated at 20,000 cubic meters of mortar and 12,500 cubic meters of stone; a similar estimate cannot be made for the engineering structures, due to the fragmentary nature of their remains. The duration of the work, impossible to specify in the absence of data on the number of workers involved, must have been no less than one year.
Aqueduct gradient and flow rate Over its entire recognized route, the aqueduct has an average gradient of 1.17 m/km, varying from 0.50 to 4.40 m/km, giving a total difference in gradient of around 25 meters. These values are not exceptional: parts of the Nîmes aqueduct have a gradient of only 6.59 cm/km, while the Yzeron aqueduct has a gradient of 16.8 m/km. The slope estimate made in 2003 differs from the data obtained in previous studies, which only provided values for sections of the aqueduct. The estimated flow rate of the aqueduct varies between 2,300 and 5,400 m3/day, or 27 l/s to 62 l/s, below Caesarodunum's daily requirements; the flow rate is estimated using Bazin's formula, taking into account the average slope of the aqueduct channel, its cross-section and the traces left on its walls (either in relief due to the accumulation of deposits, or in hollows due to water erosion at the rare bends), which allow us to appreciate the water level in the channel.
Route The route of the aqueduct has been identified in six communes on the left bank of the Cher, successively and from east to west, following the slope of the aqueduct: Bléré, Athée-sur-Cher, Azay-sur-Cher, Véretz, Larçay, and Saint-Avertin. The sites where the aqueduct has been identified since the mid-nineteenth century, even if some have been the subject of works leading to the destruction of the remains or have been swept away by natural landslides on the hillside, are sufficiently numerous and geographically close to one another that a large part of the aqueduct's route can be established with a fair degree of precision. Some oral traditions, such as the claim that a castle tower was built on the site of an aqueduct pier, have not been verified.