The Romans constructed numerous aqueducts to serve any large city in their empire, as well as many small towns and industrial sites. The city of Rome had the largest concentration of aqueducts, with water being supplied by eleven aqueducts constructed over a period of about 500 years. They served drinking water and supplied the numerous baths and fountains in the city, as well as finally being emptied into the sewers, where the once-used gray water performed its last function in removing waste matter.
The first Roman aqueduct was the Aqua Appia, built in 312 BC during the Roman Republic. The methods of construction are described by Vitruvius in his work De Architectura written in the 1st century BC. His book would have been of great assistance to Frontinus, a general who was appointed in the late 1st century AD to administer the many aqueducts of Rome. He discovered a discrepancy between the intake and supply of water caused by illegal pipes inserted into the channels to divert the water, and reported on his efforts to improve and regulate the system to the emperor Trajan at the end of the 1st century AD. The report of his investigation is known as De aquaeductu. In addition to masonry aqueducts, the Romans built many more leats — channels excavated in the ground, usually with a clay lining. They could serve industrial sites such as gold mines, lead and tin mines, forges, water-mills and baths or thermae. Leats were much cheaper than the masonry design, but all aqueducts required good surveying to ensure a regular and smooth flow of water.
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"The extraordinary greatness of the Roman Empire manifests itself above all in three things: the aqueducts, the paved roads, and the construction of the drains."
The combined length of the aqueducts in the city of Rome is estimated between 490 to a little over 500 miles. However, only 29 miles (47 km) were above ground, as most Roman aqueducts ran beneath the surface of the ground. Building underground helped to keep the water free from disease (the carcasses of humans would not be able to get into the aqueduct) and helped protect the aqueducts from enemy attack. The longest Roman aqueduct was that of Constantinople (Mango 1995). "The known system is at least two and half times the length of the longest recorded Roman aqueducts at Carthage and Cologne, but perhaps more significantly it represents one of the most outstanding surveying achievements of any pre-industrial society". Perhaps the second longest, the Zaghouan Aqueduct, is 57.5 miles (92.5 km) in length. It was built in the 2nd century to supply Carthage (in modern Tunisia).
Arches are often used to depict an aqueduct but should not be confused with the aqueduct itself. These arches, sometimes on several tiers, together with tunnels, were constructed to maintain the pitch of the aqueduct, and the flow of water, over irregular terrain, for the long course to its destination.
Roman aqueducts were extremely sophisticated constructions, built to remarkably fine tolerances; for example, the gradient of the Pont du Gard is only 34 cm per km (3.4:10,000), descending only 17 m vertically in its entire length of 50 km (31 mi). Powered entirely by gravity, they could carry large amounts of water very efficiently. The Pont du Gard could transport up to 20,000 cubic meters — nearly 6 million gallons — a day, and the combined aqueducts of the city of Rome supplied around 1 million cubic meters (300 million gallons) a day. The volume of water actually transported depended on the catchment hydrology – rainfall, absorption, and runoff – and the quality of maintenance. As a comparison, the maximum capacity of Rome's aqueducts is 126% of the current water supply of the city of Bangalore, which has a population of 6 million. Sometimes, where depressions deeper than 50m had to be crossed, gravity pressurized pipelines called inverted siphons were used to force water uphill (although they almost always used venter bridges as well); four inverted siphons crossed river valleys in the Aqueduct of the Gier, one of four supplying Lugdunum (Lyon). Modern hydraulic engineers use similar techniques to enable sewers and water pipes to cross depressions. In addition to the expertise needed to build them, Roman aqueducts required a comprehensive system of regular maintenance to repair accidental breaches, to clear the lines of debris, and to remove buildup of chemicals such as calcium carbonate that naturally occur in the water. Lead pipe inscriptions provided information on the owner to prevent water theft.
The methods of building aqueducts and the surveying needed to ensure a regular water supply is described by Vitruvius in Book 8 of his De Architectura. The work specifies the tests needed to ensure that the water is potable, and he warns against lead pipes for their toxicity, recommending either masonry channels or clay pipes. He suggests a low gradient of not less than 1 in 4800 for the channel, presumably to prevent damage to the structure. This value agrees well with the measured gradients of surviving masonry aqueducts, but many temporary aqueducts, such as those used for laundry, for example at Dolaucothi in Wales and Las Medulas in northern Spain, are much higher. At Dolaucothi, the gradient of the main 7 mile long structure is about 1:700, considerably higher than those of the permanent masonry aqueducts. Vitruvius also describes the construction of inverted siphons and the problems of blow-outs where the pressures were greatest. Aqueducts were built to supply water mills, the most famous excavated example being at Barbegal on the supply system for Arles. The engineering here is impressive, with a single aqueduct driving 15 overshot mills linked together in series. They were used to carry the water into the city.
The aqueducts required very careful planning before building, especially to determine the water source to be used, the length of aqueduct needed and its size. Great skill and training were needed to ensure a regular grade so that the water would flow smoothly from its source without the flow damaging the walls of the channel. As the need for water grew, extra sources would be utilized, very often making use of existing structures as with the Aqua Claudia and Anio Novus in Rome. The problems of aqueduct building and use are described by Vitruvius and Frontinus, the latter producing a long report on the state of the aqueducts of Rome in the last years of the 1st century AD.
Several surveying tools were used to facilitate construction. Horizontal levels were checked using a chorobates, a flatbedded wooden frame fitted with a water level. Courses and angles could be plotted and checked using a groma; this relatively simple apparatus was probably displaced by the more sophisticated dioptra, precursor of the modern theodolite.
Many aqueducts were built to supply water to industrial sites, such as gold mines, where the water was used to prospect for ore by hydraulic mining, and then crush and wash the ore to extract the gold. They usually consisted of an open channel dug into the ground, with a clay lining to prevent excessive loss of water and sometimes with wooden shuttering. They are often known as leats. However, they were built just as carefully as the masonry structures, but often at a higher gradient so as to deliver the greater volumes needed for mining operations.
The large quantities of water supplied by the aqueducts were used for prospecting for ore-bodies by stripping away the overburden, and for working the ores in a method known as hushing. The technique was used in combination with fire-setting, which involved creating fires against the hard rock face to weaken the rock and so make removal much easier. These methods of mining survived into Medieval times until the widespread use of explosives. The water could also be used to wash ores, especially those of gold and tin, and probably to work simple machines such as ore-crushing hammers and water wheels.
The remains of such leats are visible today at sites like Dolaucothi in south-west Wales, and at Las Medulas in northwest Spain. These sites show multiple aqueducts, perhaps because they were relatively short-lived and deteriorated rapidly. There are, for example, at least seven major leats at Las Medulas, and at least five at Dolaucothi feeding water from local rivers direct to the mine head. At Dolaucothi, they used holding reservoirs as well as hushing tanks, and sluice gates to control flow, as well as drop chutes for diversion of supplies. The palimpsest of such channels allows the mining sequence to be inferred.
Some aqueducts were used for mills, such as the dramatic site of Barbegal, where one aqueduct fed sixteen mills arranged in two columns down the side of the hill. There was a similar arrangement on the Janiculum at the terminus of Aqua Traiana, the highest aqueduct of the many feeding Rome.
There are a number of other sites that were fed by several aqueducts but have not yet been thoroughly explored or excavated, such as those at Longovicium near Lanchester south of Hadrian's wall. It appears that the water supplies may have been used to power stamp mills for forging iron.
With the fall of the Roman Empire, although some of the aqueducts were deliberately cut by enemies, many more fell into disuse from the lack of an organized maintenance system. The decline of functioning aqueducts to deliver water had a large practical impact in reducing the population of the city of Rome from its high of over 1 million in ancient times to considerably less in the medieval era, reaching as low as 30,000. The massive masonry aqueducts and the many other visible remains, such as the Pantheon, Coliseum, and Baths of Diocletian, were to inspire architects and engineers of the Renaissance in order to build more amazing inventions. The first of the ancient aqueducts to be restored in renascent Rome was the Acqua Vergine: Pope Nicholas V renovated the main channels of the Roman Aqua Virgo in 1453.[2] Previously Pedro Tafur, a Spanish visitor in 1436, unconsciously revealed that the very nature of the Roman aqueducts was popularly misunderstood:
Through the middle of the city runs a river, which the Romans brought there with great labour and set in their midst, and this is the Tiber. They made a new bed for the river, so it is said, of lead, and channels at one and the other end of the city for its entrances and exits, both for watering horses and for other services convenient to the people, and anyone entering it at any other spot would be drowned.[3]
On the other hand, many aqueducts elsewhere in the empire continued in use, such as the aqueduct at Segovia in Spain, a construction that shows advances on the Pont du Gard by using fewer arches of greater height, and so greater economy in its use of the raw materials. The skill in building aqueducts was not lost, especially of the smaller, more modest channels used to supply water wheels. Most such mills in Britain were developed in the medieval period for bread production, and used similar methods as that developed by the Romans with leats tapping local rivers and streams.
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