Evolution of aqueduct techniques

The Cretan, Greek, and later the Hellenistic aqueducts primarily use terra-cotta pipes. The Romans, following the Etruscan heritage, build their aqueducts as masonry canals, usual­ly rectangular in section, and covered over by a vault or stone slabs. The aqueducts are fit­ted with openings at regular intervals (from tens to hundreds of meters apart) to facilitate inspection and maintenance. We have seen earlier that the Aqua Appia, the first Roman aqueduct built in 312 BC, is nearly entirely underground. This practice is surely inherited from the Etruscan techniques, but also has a military dimension, since Italy is still far from being pacified at this time — this is only eleven years after the death of Alexander.

The Romans stay with this concept throughout the long period of aqueduct construc­tion in the Empire, i. e. from the 4th century BC to the 3rd century AD. A canal (specus) is, depending on the local topography, sometimes laid on the surface or on a supporting wall, or sometimes buried underground, occasionally passing through true tunnels, and, when necessary, supported on arched structures. The longitudinal profile of an aqueduct is driven by two constraints: first to maintain sufficient slope to convey the water, as regular as possible to avoid local break points; and second to deliver water at a high enough elevation to supply the city’s water tower (castellum) and thus enable distribu­tion of water to the highest areas of the city. Thus the arch structures that one often sees near the cities are needed to keep the canal at a sufficiently high elevation. And it is of course these arch structures, still visible today in numerous locations of the Roman Empire, that one commonly associates with the Roman aqueducts. However one should not forget that these structures represent only a small fraction of the total length of an aqueduct. The slopes of the canals are quite variable, as we will see in examples pre­sented further on, but most often are of the order of one meter per kilometer. Occasionally, where there is a need to drop to a lower elevation, there are local chutes comprising short sections of steep slope, or even true cascades.

Let’s once again listen to Vitruvius:

“Water is conducted in three ways, either in streams by means of channels built to convey it, in leaden pipes or in earthen (terra cotta) tubes (that is, according to the Greek process cited earlier), according to the following rules. If in channels, the structure must be as solid as pos­sible, and the bed of the channel must have a fall of not less than half a foot to a length of one hundred.[222] These channels are arched over at top, that the sun may strike on the water as lit­tle as possible.

“(….) If hills intervene between the city walls and the spring head, tunnels under ground must be made preserving the fall above assigned; if the ground cut through be sandstone or stone, the channel may be cut therein, but if the soil be earth or gravel, side walls must be built, and an arch turned over, and through this the water may be conducted.”[223]

A masonry canal is not inherently watertight. Therefore it is necessary to plaster the useful (or “wetted”) walls of the canal, both to minimize leakage and to protect the masonry walls themselves from infiltration damage. The Roman plaster (opus signinum) is of very high quality. It is a mortar of crushed tile solidified with thick lime, also con­taining crushed brick and pottery shards, and sometimes other additives.[224]

The most imposing and important structures are those necessary to carry the aque­ducts across valleys. These are the famous bridge-aqueducts such as the Pont du Gard near Nimes (47.8 m high), or the bridges of Segovia and Tarragona in Spain or, similar­ly, the bridge of Chabet Ilelouine on the Cherchell aqueduct in Algeria, all three some thirty meters high. There are also the inverted siphons echoing the technology of the Hellenistic world. These expensive siphons are reserved for valleys more than 50 m deep, a depth beyond which the cost of a bridge becomes prohibitive. Further on we describe the siphons of Lyon, the largest ones in the Roman world, but it is first useful to provide a glimpse of this Roman technology.

Roman inverted siphons begin with a head tank, receiving water from the aqueduct’s canal, then distributing the water into one or more parallel lead pipes leading out of the basin. These pipes descend to the bottom of the valley and cross it on a bridge-siphon, and then come back up on the other side to an exit, or escape, basin. This exit basin then delivers water into the continuation of the aqueduct (Figure 6.7). Vitruvius emphasizes the need to provide a rather long and rectilinear length of siphon at the low point of the valley (the “venter”):

“(…) when it arrives at the bottom, let it be carried level by means of a low substruction as great a distance as possible; this is the part called the venter, by the Greeks coelia. When it arrives at the opposite acclivity, the water therein being but slightly swelled on account of the length of the venter, it may be directed upwards.

“If the venter were not made use of in valleys, nor the level substruction, but instead of that the aqueduct were brought to an elbow, the water would burst and destroy the joints of the

pipes. Over the venter long stand pipes (collivaria) should be placed, by means of which, the

violence of the air may escape. Thus, those who have to conduct water through leaden pipes,

may by these rules, excellently regulate its descent, its circuit, the venter, and the compres — 1 8

sion of the air.”

One can appreciate the value of a rectilinear “venter” (usually implemented as a bridge-siphon) in reducing the hydrodynamic force that would be concentrated on a small-radius elbow at the valley floor. But the reasons given by Vitruvius (i. e. to avoid the excessive “swelling” of the water) somewhat miss the mark. There has been consid­erable questioning of what is meant by the colliviaria. Some have interpreted them as purges to eliminate air. According to Henning Fahlbusch,[225] [226] [227] they are towers supporting an elevated intermediate reservoir, acting like a modern surge tank to purge air from the siphon’s “venter”. Such towers are useful when, in an inverted siphon, there are elbows or high points favorable to the formation of air pockets. The Tourillon de Craponne, on one of the aqueducts of Lyon that we describe further on, could serve as an example. Possible remains of others can be found at Aspendos, in the south of Anatolia.

Table 6.1 presents a synthesis of data for a number of known aqueducts in the Roman Empire. It shows that the aqueducts could reach quite significant lengths, often up to 50 or 100 km. But the length of an aqueduct is not, by itself, a good indicator of its grandeur. The construction of bridges, arcades, tunnels, etc. in effect reduces the length of an aqueduct, limiting the numerous detours that would be necessary if the structure had to follow the local topography along the ground surface. Certain aqueducts are actu­ally shortened during renovation, thanks to the construction of such projects. This is notably the case for the Aqua Marcia, one of the aqueducts of Rome:

“But now, whenever a conduit has succumbed to old age, it is the practice to carry it in cer­tain parts on substructures or on arches, in order to save length, abandoning the subterranean 20

loops in the valleys.”