Roman Aqueducts, An Outstanding Urban Water Supply System


Romans, besides being great empire builders, were also known for their engineering feats. They constructed magnificent structures either to commemorate their victories or to serve the population of their vast empire. These structures continue to awe visitors even today, by their size, scale and ingenuity. These structures also testify to the intellect, perfection, engineering and architecture skills of the Romans. It is indeed surprising that many of these structures have withstood the ravages of nature and have managed to outlive a number of present day structures built with modern materials and advanced engineering techniques. The triumphal arches, bridges, roads, aqueducts, amphitheatres, elaborate sewage systems and baths in the Roman cities provide evidence of the construction skills of the Romans and their liking for an orderly life. The remnants of these structures can still be seen in parts of Europe, Asia and Africa. As the central authority fell apart in the 4th and 5th centuries, the systems established by the Romans also deteriorated.
A prominent example of the engineering skill of Romans is the construction of aqueducts. The Romans built aqueducts throughout their empire and the arches of these grand structures can still be seen in Greece, Italy, France, Spain, North Africa and Asia Minor. Although the sight of these huge structures sprawling across the landscape leads one to believe that these were built by Romans to meet their aspirations for constructing monumental structures as symbols of their authority, the reality is quite different. These structures were built to meet specific needs of the cities in the empire and formed an important part of the infrastructure systems set up by the Romans. The purpose for building these monumental structures is hidden in the word designated for them. The word "aqueduct" was derived from two Latin words "aqua" and "ductus", meaning water and channels. Thus, as indicated by the words, these megastructures were constructed to supply the enormous amounts of water over large distances to the Roman cities, which needed it for their survival and expansion. The Roman society was known to be extravagant in its use of water and hence required a reliable supply of large quantities of water for their day to day needs. Besides, the water was needed to maintain hygiene in the cities and to meet the needs of citizens of their vast empire. The water from the aqueducts was thus supplied to public baths, lavatories, fountains and households and was also used for mining activities and milling of grains. One of the major factors that dictated the size and sprawl of the aqueducts was the need to transport water from distant sources to the cities since the Romans had a tendency to establish their cities on dry plains where the only sources of water were the distant mountain springs. Occasionally pollution of water reserves, as seen in ancient Rome, necessitated the dependence of water on distant sources. In case of Rome, the Tiber River, one of the important sources of water supply, was polluted by continuous dumping of waste in the river and hence the water needs of the city had to be met through the water supplied from the aqueducts. Without the water, the Romans could not have built the cities as big as they did. Besides, without the availability of large quantities of water, the city of Rome itself could not have been kept as clean as it was reported to be during the period. The sewers of the city were designed to take the aqueduct overflow and flush the refuse into the river, which damaged the river, but kept the city of Rome clean. The extent of cleanliness of the city was such that the visitors to the city were awestruck, as evidenced by the records of accounts of such visitors. Thus the water from the aqueducts was instrumental to maintaining the sewage system of the city.
A remarkable feature of these large structures designed to convey a vital resource to the cities was that the water was conveyed through them using gravity alone and no pumps or devices were used to transport the water. It was precisely for this reason that the aqueducts had to be constructed with a downward gradient with conduits of stone, brick or concrete. Most of the aqueducts were buried beneath the ground and followed its contours. Wherever mountains were encountered they were either built by circumventing these or by tunnelling through these, but wherever lowlands or valleys were encountered the conduits were elevated over bridge

Sumeet Agarwal,
Senior Manager,
Sustainable Environment and Eco-Development Society,
New Delhi
works and it is these bridge works which are visible today in various parts of the world. In addition to the systems for conveying the water, a notable feature of the system was the arrangement made in the system to regulate supply and ensure cleanliness of supplied water. Most aqueduct systems included sedimentation tanks, sluices and distribution tanks to regulate supply at need and to allow dissolved impurities to settle down. A large part of the aqueduct system was kept underground not only to protect them from enemies but also to prevent contamination of water from animals, garbage etc. Aqueducts became a fundamental part of Roman city life as they supported a population of over a million. The structure of the aqueduct can be understood from the fact that these were essentially man-made streams conducting water downhill from natural sources to the destination. To tap water from a river often a dam and a reservoir was constructed to create an intake for the aqueduct that would not run dry during periods of low water. To capture water from springs, catch basins or springhouses could be built at the points where the water issued from the ground or just below them, connected by short feeder tunnels. Having flowed or filtered into the springhouse from uphill, the water then entered the aqueduct conduit. Scattered springs would require several branch conduits feeding into a main channel.
Most of the aqueducts were of flat bottom, arch section conduits which ran 0.5m to 1.0m below ground surface with inspection or access covers at regular intervals. The conduits were built either using ashlar stone masonry or brick masonry. They were designed in such a way that a person could enter the space for cleaning. They consisted of conduits carried on arches, masonry, brick or concrete. Thus the development of the Roman arch also helped to build Roman aqueducts. However, Arches were used in the building of aqueducts only when they were required to be built at a height of more than five feet. The construction of arches helped to save material and was not disruptive to the landscape. For aqueducts less than five feet in height walls were built. Thus arches were not regular features in aqueduct construction. However, the construction of arches did impart a certain amount of monumentality and aesthetic quality to the aqueducts. Two important features are immediately noticed in an aqueduct structure. The first of these is their size, which was guided by the terrain and which was rendered particularly imposing by the regular archways, which in some cases reached as high as 30m. The second is the precision required to maintain a constant downward slope and flow in the absence of pumps. It is this latter aspect where the skill of the Romans becomes most apparent. They were faced with the challenge of getting the gradient right since an excessive slope would mean scouring of the surface of the conduits, and lack of slope would bring points of stagnation. The aqueducts were hence built to remarkably fine tolerances and had a gradient of only 34cm per kilometre, descending only 17m vertically in its entire length of 50 km. Powered entirely by gravity, the aqueducts could carry enormous amounts of water. Some aqueducts are known to have carried as much as 6 million gallons of water. Although Pumps existed during the Roman times, they certainly weren't capable of handling the volumes of water in required quantity to be carried to the Roman cities. If water was brought in from some distance, then care was taken to survey the territory over which the aqueduct would run to ensure that it would flow at an acceptable gradient for the entire distance. Roman aqueducts typically tapped springs in hilly regions and hence it was important to ensure a sufficient fall in elevation over the necessary distance. The terrain and the decisions of the engineers determined this distance. Generally, the conduit stayed close to the surface, following the contours of the land, grading slightly downhill along the way. At times, it may have traversed an obstacle, such as a ridge or a valley. If it encountered a ridge, then tunnelling was required. If it hit a valley, a bridge would be built, or sometimes a pressurized pipe system, known as an inverted siphon, was installed. Initially all aqueducts conveyed water through a gravitational system where water was carried in an open conduit down from the slope. In the later period water was brought from a high mountain source under pressure in closed and sealed stone pipeline. In order to relieve the excessive pressure in such a closed system, technically a siphon, three pressure towers were incorporated in the stretch of aqueduct arcade. For construction and maintenance purposes the vault of the conduit was pierced periodically by vertical manhole shafts along its path. Upon arrival at the city's outskirts, the water reached a large distribution tank called the main castellum. From here, smaller branch conduits ran to various districts in the city, where they met lower secondary castella. These branched again, often with pipes rather than masonry channels, supplying water under pressure to local features, such as fountains, houses, and baths. The pipes used for distribution were generally made of lead.
In order to ensure proper gradient, which was crucial to the proper functioning of aqueducts, several surveying tools were used, chorobates being one example of such instruments. The chorobates were used to level the terrain before construction. The chorobate consisted of a wooden frame made in the form of a beam which was fitted with a water level, and two supports at the end of the beam. It is believed to be the instrument that was used to level the Roman aqueducts. The beam had a plumb bob at each end so that it could be placed square on the ground. A groove was made in the top of the beam for use as a sightline, and the beam was parallel to the water level. It enabled levels to be found between two points, and hence a series of levels when planning and surveying the path for an aqueduct. Another tool used for surveying was the groma. Gromas were used to measure right angles and consisted of stones hanging off four strings perpendicular to one another. The instrument which is the forerunner of the modern theodolite was known as the dioptra and was used to measure vertical angles. The dioptre was a sighting tube or, alternatively, a rod with a sight at both ends, attached to a stand. If fitted with protractor, it could be used to measure angles. Screw turns on several different parts of the instrument made it easy to calibrate for very precise measurements.
Source Of Water
Besides maintaining the gradient of aqueducts, ensuring proper quality of water supplied was also essential. Hence the source of water was also carefully chosen by the Romans. There were thumb rules and long traditions of where to look for water, which guided the Romans in their endeavours. Vitruvius, a Roman author, architect and engineer, wrote about factors to be considered for finding water. Although his writings contained some obvious advice like looking for mists or lakes and checking the water quality, it also contained recommendations like talking to people around the water source and checking their complexions. If they looked healthy, it was inferred that their water source was pure and worth tapping.
Water Quality
The water quality was also determined on the basis of appearance. Water that looked brackish was considered unfit for use. Hence the water from streams and lakes posed a problem at times. However, despite all efforts put in for selection of the source, sometimes, the water that arrived in towns would be muddy due to a storm in the countryside. To some extent, the Romans had the capability to clean up the water but when it was too dirty to be cleaned up; it was diverted for industrial use or for irrigation. The prized waters in Rome came from springs which were harder to find, because they often lay underground. The Romans used certain techniques to discover the springs like looking for patches of green grass in the dry season or looking for a certain type of vegetation. Sometimes they would dig down to the water table and build an underground tunnel to begin the aqueduct. The tunnels were sometimes 10 to 20 feet below the surface of the earth. Systems were also planned to ensure cleanliness of water carried by the aqueducts. Although unlike modern times they did not have chemicals to clean the water, they had other means like settling basins. These basins were like pools which would slow the water down so that impurities like sand dropped out of the water. One of the aqueducts had zigzags built into it instead of settling tanks. These zigzags also slowed the water down to unload impurities. The water in the aqueducts was also subjected to a process of aeration like modern times. The aqueducts were also physically cleaned at times. In order to allow access to the aqueducts for physical cleaning, a cleaner would climb down into a tunnel through a well hole or an inspection chamber. Hand and footholds were carved into the walls of the shaft which would sometimes go 30 or 50 feet. Slaves would shovel out impurities which were hauled to the top in buckets.
The masonry used to construct the aqueducts could take many different forms, of which the principal ones were bricks and mortar or stone. Local materials would be used wherever possible and this implies a great variety of architectural results. The mortar itself was quick drying and was probably made of pure chalk rather than common lime mortar, which is slow to dry. The aqueducts were constructed using stones or bricks. The famous aqueduct of Segovia in Spain is a typical example of an aqueduct constructed using stone. The aqueduct is amazing not only because of its size, but also because of the quality of the workmanship. The stone used to construct it has been perfectly chiselled in order to create a perfect match with the masonry surrounding it. In this way no mortar was actually required to bind it and the aqueduct still stands. In some cases brickwork was mixed with stone. The foundations would be of stone, the pillars of stone or brick, the arch of stone and the parapet possibly of brick. Availability of materials was all important. Brick tended to make the job easier and faster, but the greater durability of rock was always borne in mind. In order to construct pillars to support the arches of the aqueducts, the Roman engineers probably used a mix of experience coupled with graphical means, which ensured that the thickness of supporting pillars matched the size of the arch above and sufficient to carry the weight channelled through it. The pillars would be constructed on top of deep foundations which preferably rested on a solid rock or heavily compacted under-bed. It was not very common for the foundations to reach several metres underground. They would normally be constructed out of solid rock blocks, possibly bound together by iron keys. The greatest difficulty was met when such pillars had to be constructed to cross rivers. In these cases, wooden skirts rendered impermeable with clay and terracotta would allow dry access to the river bed for proper construction of the foundations. A cursory glance at an aqueduct will show that the supporting pillars often, if not always, have a collar of jutting masonry towards the top, before the arch section begins. The purpose of this collar was to provide a ledge on which to support wooden trusses around which the arch would then be built. Once the arch was completed on top of its supporting pillars the truss could be safely knocked down. The trusses were made of wooden beams, which on the outer edge of the curve (the one in contact with masonry) actually fitted into holes in the masonry. These holes in the masonry remained visible and could be reused if the arch section required scaffolding for repair work. In areas where wood was scarce, like in North Africa, trusses were made of terracotta and were reused. The ceramic sections were tubular and could be made like water pipes so that one section could be inserted into another to create a longer section. These terracotta sections were extremely versatile as they allowed many different forms and shapes to be assembled rapidly.
Notable Examples
The combined length of all aqueducts built in ancient Rome is about 800km out of which only about 47 km is above ground. Many of the most impressive remains of aqueducts are not in Rome itself, but are further away, in former provinces, like PontDu Gard in Southern France. Rome's first aqueduct Aqua Appia dates back to 312 BC. By the beginning of 2nd century AD, the capital was served by nine aqueducts which supplied a total of nine million cubic metres of water. Variations on the standard design of the aqueducts were also seen. The double arched gate in Rome known as Porta Maggiore carries two aqueducts, the Aqua Claudia and Aqua Novus. The best preserved textbook example of Roman Aqueduct is the Pont Du Gard. This Aqueduct brought water to Nimes in Southern France from a source 30 miles away. The 3-tiered arches of the structure cross the valley of river Gardon at a height of 150 feet. Water was carried at a slope of 1:3000 in an open conduit on the uppermost level. Like many other aqueducts, this aqueduct also served as a bridge over the river valley and a road was carried over its lower arches. The stone parts of aqueducts were lined with a special type of concrete to prevent water loss and a steady reliable flow of water was maintained along its entire length. A sophisticated system of sluice gates could be used to empty sections for maintenance and water periodically made stops in sedimentation tanks to remove impurities. The aqueducts required a great deal of maintenance and constant inspection to run smoothly and as a result many fell into disrepair after the fall of the Roman Empire. Many of these were also destroyed by attacking armies.
Uses of Supplied Water
A notable fact about the water supplied by aqueducts is that the supply was restricted only to public baths, fountains and public water fountains. It was not supplied directly to individual houses except in case of the extremely rich who could pay for individual pipe connections. Further, in addition to drinking, bathing, cleaning etc., the aqueduct water was used for industrial purposes also. Many systems were built to supply water to industrial sites, such as gold mines, where the water was used to prospect for or 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 Medulasin, north-west Spain. These sites show multiple aqueducts, presumably 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. The palimpsest of such channels allows the mining sequence to be inferred.
Decline of Aqueducts
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. On the other hand, many others, elsewhere in the empire continued in use, such as the aqueduct at Segoviain, Spain, a construction which 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. 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.
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