Quelli che definiamo emissari “maggiori” dei Colli Albani, cioè dell’edificio del Vulcano Laziale situato pochi Km a sud di Roma, furono scavati per regolarizzare il livello dei laghi di Albano e Nemi, bacini privi di emissari naturali e quindi soggetti a variazioni di livello, soprattutto a seconda delle precipitazioni atmosferiche. Contrariamente a quanto avvenuto per gli specchi d’acqua minori (vedi “Gli emissari minori” in questo stesso numero) i laghi di Albano e Nemi non furono completamente prosciugati sia in ragione della profondità (170 m Albano e 33 m Nemi) sia per mantenere un’ampia conserva d’acqua con possibilità di sfruttamento del rivo ottenuto per l’irrigazione o per il funzionamento di mulini, opifici, etc.
La grande antichità, le difficoltà affrontate e risolte con mezzi limitati, la funzionalità rimasta intatta sino al recente, rapidissimo abbassamento del livello dell’acqua nei due bacini che ha posto gli emissari fuori uso, collocano queste opere tra le più importanti del nostro passato. Scarica i PDF dei contributi speleologici pubblicati Gli Emissari Maggiori dei Colli Albani L’emissario del lago di Nemi indagine topografico strutturale Topografia dell’Emissario di Nemi Nemi nuove evidenze per l’antica storia dell’emissario Emissario Nemi tratto da www.lambertoferriricchi.it per gentile concessione
The artificial outlet channels of the lakes of Albano and Nemi are among the great engineering works of antiquity in the mediterranean area. Their purpose was the regulation of the level of the water basins, lacking natural outlets. The water flows so obtained had several uses, such as moving-power for mills, factories and watering systems. Their epoch of construction is the beginning of the IV century b.P.E. for the Albano outlet, as reported by Livius, and at least the V century b.P.E. for the Nemi outlet; for this latter channel there is no historical record, a fact that suggests a pre-roman origin.
Both tunnels were excavated starting from the two extremities, whose positions were established by means of the “coltellatio” technique, based on a straight line of vertical poles conducted above the ridge. The direction was reported inside the excavation by means of shafts close to the entries; when digging from the downstream entry, the light beam guided the work, with the help of the “waving” shown in figure 5. Once close to the meeting point, the workers were guided by the noise in the search for the contact. The Albano outlet is 1450m long, in origin of rectangular form: one m wide, 2.5 m high, a shape altered at present by concretions and collapses.
The gradient is of about 0.14 per thousand. The explorations have not been many, since the channel has been full of water until a short time ago, with points where the water reached the ceiling. In figure 6 the planimetry and the cross-section are given, according to the surveys by Cardinale et al. (1978), Castellani and Dragoni (1991) and Castellani (1999). Livius states that the outlet was built in two years, a fact that is judged as very likely by Castellani and Dragoni (1991), on the basis on the work progress in every shift as testified by the signs on the tunnel walls. At present the channel is no more filled with water owing to the noticeable lowering of the lake level since the ‘90s, but still, the large amount of bad smelling mud which, for various reasons, covers the floor discourages further investigations. The exit of the tunnel is in the village Mole di Castegandolfo, in the past giving water to a public washing-basin, and going on flowing into the Fosso di Vallerano, a tributary of the Tiber river. Along its course, various mills and factories made use of its stream.
The Nemi outlet, like the Albano outlet, is no more carrying water for the lowering of the lake level. Its construction followed the scheme described for Albano, but the history of the Nemi tunnel appears more complex, with interventions in various epochs (see Castellani et al. 2003 for an exhaustive discussion). In figure 13 one finds the layout and the section, that show a first phase of construction with the expected two shafts close to the mouths, a tunnel with a regular trapezoidal shape starting from the lake side (“discenderia”). Its vault joins smoothly the main channel, while the bottom of the latter is located about two meters below the level of the discenderia. As seen in figure 13, the present entrance through which water entered the outlet till the years ‘80s is given by a side channel that lowered the lake level of about two meters below what foreseen by the first project. The first part is formed by three chambers with walls in big peperino ashlars (sort of control rooms), followed by a tunnel rather roughly planned and executed; it joins the main body of the outlet at the arrival of the discenderia. The outlet goes on straight for 800 m, where we find a deviation on the right in a tract of crumbly rocks that has been lined with bricks during the works in 1930. It is very likely a work of restoration after that a collapse had blocked the passage.
The situation appears more complex at about 1100 m from the entrance. Here we find a series of deviations, called second bypass, shown in figure 15. The problem was likely caused by the presence of an extremely hard bank of lavic rock, that the workers tried to avoid in various way (see Castellani et al. 2003 for a detailed discussion). It is even possible that one of the tunnels (3’ in figure 15) acted as a first version of the outlet until Valle Ariccia. The point is that, just after the troublesome turning around the lava block, the tunnel meets again hard rocks, which appear dug with great regularity. In fact the signs on the walls are circulars, quite different from those left by a pick and suggest the use of new instruments and methods (figure 16). After another 200 m, at 1300 m from the lake, there is the meeting with the tunnel from Valle Ariccia, with an error in height of about 2 m. After we have again hard rock, till almost the end, where collappses in a crumbly soil seem to have destroyed the last tract. At present, the exit is given by a short passage through the right wall, showing on the outside the Chigi coat-of-arms.
The Nordic Institute of Rome has recently performed important explorations regarding the Nemi outlet, during their excavations in the great roman villa located on the lake shore. The discenderia has been partially cleared, showing that almost certainly it carried water, in an epoch between the end of the VI century and the beginning of the V century b.P.E. The first entrance was filled up about in 300 b.P.E. (on the basis of the ceramics found in the soil), and the second (side) entrance was dug. When around 50 b.P.E. the villa was built, the final part of the outlet was inserted into it, assuming the present appearance (Guldager Bilde, 2006). The outlet from Nemi is part of a hydraulic system that goes on till the Thyrrenian sea. In the Valle Ariccia the water flows in an open channel, crossing the border of the Valley with another tunnel of about 600 m and 12 shafts (figure 18). After, the water flows in the ditch Fontana di Papa and enters another tunnel, about 300 m long (Dobosz et al., 2003) (figure 19). The river goes on with various names, till it reaches the sea as Fosso dell’Incastro.
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