How did the Colosseum fill with water for naval battles?

The Colosseum is the most famous and largest amphitheater in the world: represents the symbol of Roman civilization and its ability to create large and versatile constructions. Still visitable today, this archaeological monument was built taking advantage of an extensive arch system that develops along the perimeter of the same e radially towards the center of the arena. It was mainly used for combat shows between gladiatorsbut also for re-enact historic naval battles, so-called Naumachie: it is this particular condition of use that represents, even today, one of the greatest curiosities about the hydraulic functioning of the work. How did they fill it with water?

The supporting structure of the Colosseum

The Colosseum is one elliptical plan structurehollow in the central area and with perimeter steps, used by spectators as seats during the events held inside. The steps are supported by arched structures arranged on various levels and such as to also guarantee passage corridors in the perimeter area of ​​the structure. There is clear evidence of these arches on the front facades, while the internal ones are less visible. In fact, the arches themselves have also become the symbolic element of distinction of the work.

The external oval that surrounds the Colosseum has a geometry characterized by two diameters of length equal to 189 m and 156 m. The maximum height of the perimeter wall structures is approximately 50 m. The central arena, however, is a second ellipse, whose diameters are approximately equal at 80 m and 45 m. In essence, therefore, the structure of the Colosseum it develops along this crown delimited by the two ellipsesthe internal internal one and the external one, and rises in height through the support of arched structuresmade using blocks of travertine and tuff and possibly finished with marble parts. Although no longer present, it should be remembered that wooden structures had been built in the summit area of ​​the Colosseum, used to support roofing elements that acted as repairs for rainy events, as well as shading systems for sunny days.

Compared to the original structure, it is today in a different conformation, due to the collapse which this has suffered with the succession of violent earthquakes over the years. Some of these, in fact, have even led to the need to carry out detailed consolidation works, to limit the static problems triggered by structural weakening. The system of arches, however, is still clear in its conformation today, which also made it possible – after sector studies – the archaeological reconstruction of the main elements of the work and the determination of the conditions of use of the same.

The naval battles inside the Colosseum

Historically known as naumachiain Roman times this term identified a show that had the objective of reproduce a naval battle. In fact, the term Naumachia was also often confused with the place where these battles took place: it seems that there are therefore several historical traces that lead back to the use of the Colosseum as a place to host these naval battles. It goes without saying that, to achieve this objective, a filling of water in the central area of ​​the arenaat a level sufficient to float the ships used for the aforementioned battles.

This practice is widely documented in historical literature, although to date it is not yet completely clear what the dynamics were with which the water was made to flow into the area and then removed, to guarantee the restoration of the starting conditions of the Colosseum . From reading the technical literature documentation, it is also clear that they were precisely some branches of theClaudius aqueduct those used to fill support cisterns, whose stored water was then used to fill the arena.

With these purposes, technical doubts arise as to how it was possible to have such a structure at that time flexible to be used at the same time both for naval battles and for combats between Gladiators, for example: how long did it take to fill up? How long did the emptying operation take? How was its hydraulic seal guaranteed to preserve these quantities of water?

Filling with water

From the historical and technical studies carried out on the base parts of the arena, it emerged that the original structure underneath the area, where today we find a series of walls, was actually previously formed by removable wooden structureswhich therefore supported the arena in condition ordinary. To confirm this theory, there is clear evidence of the presence of particular holes in the basal partwhich served precisely as a temporary housing for these poles. In fact, therefore, the piling was dismantled when the Colosseum was to host naval battles, the structure was filled with water and, subsequently – once emptied – the piling and the support surface of the arena were reassembled. In addition to this, the bottom of the base was made using a finish of particular mortars that they guaranteed waterproofing of the central surface, i.e. of the ellipse of the arena: the tightness of the flooring, therefore, was guaranteed.

Considering the dimensions of the previously mentioned ellipse and a height of the water head equal to 1.5 m in the maximum filled condition, the volume of water necessary to carry out these naval battles was equal to approximately 17000 m³or 17 million liters of water. From the archaeological studies conducted on the internal tunnel systems of the Colosseum, as well as in relation to the proximity of the aqueducts already used at that time to serve the different areas of the city, it was identified a possible system of pipelines that can be used to fill the arena. In particular, it is assumed that the same pipes used to guarantee a distribution of water inside the amphitheater (already in use to supply water to spectators during the shows, also and usually carried out during daytime hours, perhaps during periods of the year with high temperatures) could also be used to fill the arena for the volumes of water indicated above. In these dynamics, the present pipeline system guaranteed, according to simulations carried out, filling the arena for a time ranging from 2 to 5 hours.

The water was taken from the city’s aqueduct system, with particular reference, in this specific case, to the aqueduct near the Colosseum, with a possible cistern located near Colle Celio. In fact, one of the branches of Nero’s aqueduct passed through here, that is, one of the secondary structures of the main Claudian aqueduct.

It was established from numerical analyzes that the conditions for emptying the arena of the water present involved times oscillating up to approximately 6-7 hoursalthough the first two hours already guaranteed an overall emptying of more than 80% of the filling. The drainage of the accumulated water occurred through four channelsarranged at the ends of the main axes of the internal ellipse of the arena. These channels had approximate dimensions of 1m (width) x 1.5m (height). All four conveyed the water towards a conduit external to the Colosseum and approximately as large as the entrance canals. It should be underlined that the drainage system was still necessary to remove the rainwater that accumulated inside during rainy events. However, the important size of these channels found in the diametric areas of the arena identifies a clear signal of use of the Colosseum for purposes different from the usual ones, or in any case those historically remembered perhaps due to assonance with similar structures found in other areas of Italy.