The Humblest of Roman Aqueducts (part II)
The absence of a catchment basin is worth a comment. All but three of the 11 Roman aqueducts had catchment basins as collection points for their multiple sources of spring water. The basins allowed mineral-rich water to drop some of its sediment reducing build-up of deposit in the aqueduct channels and they also allowed the amount of water entering the aqueduct channel to be controlled by a sluice. Neither of these benefits was necessary for the Alsietina. A catchment basin was, in any case, out of the question given that the intake was deep under water and cut into the sloping rock wall of the lake.
I now turn to the curious phenomenon of the changing water level of Lake Martignano. If, as we suppose, the intake we visited was indeed that of the Alsietina the water level of the lake must have dropped at least 12 metres since the time of Augustus. The evidence of Frontinus suggests that the water level probably started dropping well before his time as Water Commissioner. Why else would it have been necessary to augment a once massive supply from Lake Martignano with an extra feed from Lake Bracciano? Frontinus says that in his time the water delivered by the Alsietina, including the supplement from the Lake Bracciano branch, was a mere 392 quinariae, equivalent to 16,228 cu metres in 24 hours. This must have been a small fraction of its original capacity a hundred years earlier in 2BC. One could argue that the needs of the Naumachia were so huge that the aqueduct may by itself have partially drained the lake, acting like a plughole in a bath. But this would not account for the later drop in the water level below the floor of the intake. Recent underwater explorations of the lake bed have discovered masonry and a petrified oak forest indicating that later-on the water level dropped to as low as 29 metres5 below the bottom of the intake, then rose back again to its present level. Such wild fluctuations could only be explained by geological phenomena such as an earthquake. What then can we deduce from all of this? Perhaps the Roman engineers in 2BC fell down on their homework and overlooked the propensity of Lake Martignano’s water to rise and fall. Perhaps they did recognise this but failed to make sufficient allowance for it, setting the level of the intake too high. What is certain, with hindsight, is that they made an error of judgement in choosing Lake Martignano as the aqueduct’s source. They would have done better to choose Lake Bracciano. A hundred years after its inauguration the Alsietina was approaching obsolescence and was a shadow of its former self. It was a byword for unwholesomeness and insignificance. It is not surprising that the short section of the channel which was supported above ground on arches completely disappeared once the aqueduct became redundant.
Finally, I would like to offer some thoughts on the construction methods used to create the intake. The first point is obvious; that to draw water from the lake the intake had to be constructed below the water line. If the Roman engineers knew that the water level of the lake was liable to fall they would have cut the intake as deep as possible below the water line to allow the maximum margin of comfort. This would have provided the additional benefit of stronger water pressure in the aqueduct channel from the head of water above it. The challenge was how to carve a large tunnel in the rock face of the crater slope several metres under water. We know that the Romans were well able to meet this challenge from the Roman writer Vitruvius6, who wrote about building methods and architectural design in the 1st Century BC. He described the principle of the coffer dam which is widely used today for building underwater supports for bridges and harbours. A coffer dam is a box which sits on the river, sea or lake bed and extends upwards to above the water level. Water is evacuated from the box to enable construction to take place inside the box in the dry. Vitruvius writes that:
"a coffer dam, with its sides formed of oaken stakes with ties between them, is to be driven down into the water and firmly propped there; then the lower surface inside, under the water, must be levelled-off and dredged, working from beams laid across"
In more difficult situations he adds:
" A coffer dam with double sides, composed of charred stakes fastened together with ties, should be constructed in the appropriate place, and clay in wicker baskets made of swamp rushes should be packed-in amongst the props. After this has been well packed-down and filled-in as closely as possible, set up your water screws, wheels and drums, and let the space now bounded by the enclosure be emptied and dried. "
In the case of the Alsietina a three-sided, double walled dam would probably have been needed, anchored against the rock face. The bottom of the poles would not have sat easily on the sloping rock at the base of the dam and the potential for leakage from the base would have been high. To reduce the risk of workmen drowning it may have been decided to excavate the whole of the aqueduct channel right up to, but not into, the lake and to save the breakthrough into the coffer dam until the very last. In any event, the integrity of the coffer dam was crucial for the safety of the excavators working underground. If it failed and released prematurely a high pressure torrent of dirty lake water into the channel, the prospects for anyone trapped along its 33Km length do not bear thinking about.
To summarise, the Aqua Alsietina was something of a poor relation to the other ancient Roman aqueducts. It was the lowest, the dirtiest, the least spectacular and probably the shortest-lived of the aqueducts. Aqueduct experts have relegated it to a position of relative unimportance. For the future, the challenge to aqueduct hunters lies in exploring deep into its intake. This humblest of aqueducts may yet prove to have some surprises for the intrepid.
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Annamaria Liberati Silvero, Il Trionfo Dell’Acqua 1986, page 74
Vitruvius, “Ten Books on Architecture”, Book 5 Chapter 4
Translation by Morris Hicky Morgan.
Published in 1914 by Harvard University Press.