The bridge has three tiers of arches made from
Shelly limestone and stands high. The aqueduct formerly carried an estimated of water a day over to the fountains, baths and homes of the citizens of Nemausus (Nîmes). The structure's precise construction allowed an average gradient of in . It may have been in use as late as the 6th century, with some parts used for significantly longer, but lack of maintenance after the 4th century led to clogging by mineral deposits and debris that eventually stopped the flow of water. After the Roman Empire collapsed and the aqueduct fell into disuse, the Pont du Gard remained largely intact with a secondary function as a toll bridge. For centuries the local lords and bishops were responsible for its upkeep, with a right to levy tolls on travellers using it to cross the river. Over time, some of its stone blocks were looted, and serious damage was inflicted in the 17th century. It attracted increasing attention starting in the 18th century, and became an important tourist destination. A series of renovations between the 18th and 21st centuries, commissioned by local authorities and the French state, culminated in 2000 with opening of a new visitor centre and removal of traffic and buildings from the bridge and area immediately around it. Today it is one of France's most popular tourist attractions, and has attracted the attention of a succession of literary and artistic visitors.
Route of the Nîmes aqueduct to
Nemausus (Nîmes) passes over the Pont du Gard, and many other significant bridges (not to scale). The location of
Nemausus (Nîmes) was somewhat inconvenient when it came to providing a water supply. Plains lie to the city's south and east, where any sources of water would be at too low an altitude to be able to flow to the city, while the hills to the west made a water supply route too difficult from an engineering point of view. The only real alternative was to look to the north and in particular to the area around
Ucetia (Uzès), where there are natural springs. The Nîmes aqueduct was built to channel water from the springs of the Fontaine d'Eure near Uzès to the
castellum divisorum (repartition basin) in Nemausus. From there, it was distributed to fountains, baths and private homes around the city. The straight-line distance between the two is only about , but the aqueduct takes a winding route measuring around . This was necessary to circumvent the southernmost foothills of the
Massif Central, known as the . They are difficult to cross, as they are covered in dense vegetation and
garrigue and indented by deep valleys. It was impractical for the Romans to attempt to tunnel through the hills, as it would have required a tunnel of between , depending on the starting point. A roughly V-shaped course around the eastern end of the Garrigues de Nîmes was therefore the only practical way of transporting the water from the spring to the city. The Fontaine d'Eure, at above sea level, is only higher than the repartition basin in Nîmes, but this provided a sufficient
gradient to sustain a steady flow of water to the 50,000 inhabitants of the Roman city. The aqueduct's average gradient is only 1 in 3,000. It varies widely along its course, but is as little as 1 in 20,000 in some sections. The Pont du Gard itself descends in , a gradient of 1 in 18,241. The average gradient between the start and end of the aqueduct is far shallower than was usual for Roman aqueducts – only about a tenth of the average gradient of some of the aqueducts in Rome. The reason for the disparity in gradients along the aqueduct's route is that a uniform gradient would have meant that the Pont du Gard would have been infeasibly high, given the limitations of the technology of the time. By varying the gradient along the route, the aqueduct's engineers were able to lower the height of the bridge by to above the river – still exceptionally high by Roman standards, but within acceptable limits. This height limit governed the profile and gradients of the entire aqueduct, but it came at the price of creating a "sag" in the middle of the aqueduct. The gradient profile before the Pont du Gard is relatively steep, descending at per kilometre, but thereafter it descends by only over the remaining . In one section, the winding route between the Pont du Gard and St Bonnet required an extraordinary degree of accuracy from the Roman engineers, who had to allow for a fall of only per of the conduit. river It is estimated that the aqueduct supplied the city with around of water a day that took nearly 27 hours to flow from the source to the city. The water arrived in the
castellum divisorum at Nîmes – an open, shallow, circular basin 5.5 m in diameter by 1 m deep. It would have been surrounded by a balustrade within some sort of enclosure, probably under some kind of small but elaborate pavilion. When it was excavated, traces of a tiled roof, Corinthian columns and a fresco decorated with fish and dolphins were discovered in a fragmentary condition. The aqueduct water entered through an opening wide, and ten large holes in the facing wall, each wide, directed the water into the city's main water pipes. Three large drains were also located in the floor, possibly to enable the nearby amphitheatre to be flooded rapidly to enable
naumachia (mock naval battles) to be held. The spring still exists and is now the site of a small modern pumping station. Its water is pure but high in dissolved
calcium carbonate leached out of the surrounding
limestone. This presented the Romans with significant problems in maintaining the aqueduct, as the carbonates precipitated out of the water during its journey through the conduit. This caused the flow of the aqueduct to become progressively reduced by deposits of
calcareous sinter. Another threat was posed by vegetation penetrating the stone lid of the channel. As well as obstructing the flow of the water, dangling roots introduced algae and bacteria that decomposed in a process called biolithogenesis, producing concretions within the conduit. It required constant maintenance by
circitores, workers responsible for the aqueduct's upkeep, who crawled along the conduit scrubbing the walls clean and removing any vegetation. Much of the Nîmes aqueduct was built underground, as was typical of Roman aqueducts. It was constructed by digging a trench in which a stone channel was built and enclosed by an arched roof of stone slabs, which was then covered with earth. Some sections of the channel are tunnelled through solid rock. In all, of the aqueduct was constructed below the ground. The remainder had to be carried on the surface through conduits set on a wall or on arched bridges. Some substantial remains of the above-ground works can still be seen today, such as the so-called "Pont Rue" that stretches for hundreds of metres around Vers and still stands up to high. Other surviving parts include the
Pont de Bornègre, three arches carrying the aqueduct across a stream; the Pont de Sartanette, near the Pont du Gard, which covers across a small valley; and three sections of aqueduct tunnel near
Sernhac, measuring up to long. However, the Pont du Gard is by far the best-preserved section of the entire aqueduct.
Description of the bridge production. The blocks were precisely cut to fit perfectly together by friction and gravity, eliminating the need for mortar. The method of construction is fairly well understood by historians. The patron of the aqueduct – a rich individual or the city of Nîmes itself – would have hired a large team of contractors and skilled labourers. A surveyor or
mensor planned the route using a
groma for sighting, the
chorobates for levelling, and a set of measuring poles five or ten
Roman feet long. His figures and perhaps diagrams were recorded on
wax tablets, later to be written up on scrolls. The builders may have used templates to guide them with tasks that required a high degree of precision, such as carving the standardised blocks from which the water conduit was constructed. The builders would have made extensive use of cranes and
block and tackle pulleys to lift the stones into place. Much of the work could have been done using simple
sheers operated by a
windlass. For the largest blocks, a massive human-powered
treadmill would have been used; such machines were still being used in the quarries of Provence until as late as the start of the 20th century. equivalent to 50 years' pay for 500 new recruits in a Roman legion. Although the Pont du Gard is renowned for its appearance, its design is not optimal as the technique of stacking arches on top of each other is clumsy and inefficient (and therefore expensive) in the amount of materials it requires. Later aqueducts had a more sophisticated design, making greater use of concrete to reduce their volume and cost of construction. The
Aqueduct bridge of Segovia and the
Pont de les Ferreres are of roughly similar length but use far fewer arches. Roman architects were eventually able to do away with "stacking" altogether. The
Acueducto de los Milagros in
Mérida, Spain and the Chabet Ilelouine aqueduct bridge, near
Cherchell,
Algeria utilise tall, slender piers, constructed from top to bottom with concrete-faced masonry and brick. == History ==