Tunnels of Claudius

History

Lake Fucino was once the 3rd largest lake in Italy and the area is a basin completely enclosed by mountains and with no natural outlet. It is fed by snow meltwater and the seasonal ancient water level variation is estimated to be typically 12 m.

The Marsi, the local inhabitants, had made petitions to Rome to control the unstable level of the lake which very often flooded villages because of the frequent cloggings of the only natural swallow-hole at Petogna near Luco dei Marsi. In summer because of the receding waters, the lands surrounding the inhabited areas often became marshy causing serious health problems due to malaria.

The first plan to drain the lake was of Julius Caesar (r. 49-44 BC) but he died before it could be advanced.

In 41 AD Emperor Claudius resumed the ambitious plan and, thanks to substantial public financing, entrusted a Roman enterprise with the works. The easiest plan was to dig the shortest tunnel through the low Cesolino hill and into the Salto River and eventually into the Tiber. This was discarded because it would be a flood threat to Rome. The second, more difficult, plan was therefore adopted with a longer, deeper tunnel to lead the water into the Liri River

The aim was to only regulate the water level and not drain the lake completely. The tunnel had to be designed to be the shortest compatible with a very shallow slope and in a compass direction that ensured it entered the Liri river at a level lower than the lake, a remarkable feat considering the instruments of the time. Once the direction was decided, the required inclination then determined the location of the tunnel ends.

The emissary has a variable cross-section of 5 to 10 m2 (54 to 108 sq ft), an average flow rate of 9.1 m3/s (320 cu ft/s) and an average gradient of 1.5 m/km (7.9 ft/mi).

The cunicola: the secondary service tunnels and wells. …