The Pergamon Bridge is a Roman substruction bridge, consisting of two parallel pipes measuring 183 and 196 meters in length, running diagonally beneath the former temple precinct and crossing the Selinus River (modern Bergama Çayı) in the ancient city of Pergamon (today Bergama), Turkey. It is the largest structure of its kind from antiquity, designed during the reign of Emperor Hadrian (117–138 AD) to create a passage beneath the large courtyard of the monumental temple complex known as the "Red Basilica." Both pipes, made up of supporting walls covered with cylindrical vaults, remain intact and continue to serve their function to this day.

Due to its extraordinary width, the Pergamon Bridge is often mistakenly identified as a tunnel; however, it should be regarded as the foundation of a bridge, since the entire structure is built above ground. This necessitates the use of construction techniques characteristic of bridges, which differ significantly from those employed in tunnel construction.

Such engineering constructions hold special significance for urban development, as they enable the creation of extensive open spaces in densely populated city areas. This was also the case in Pergamon: the construction of the Serapis temple required covering an entire section of the Selinus River to create sufficient space for a wide platform in front of the temple.

A similar project was implemented in another ancient Anatolian city — Nysa, where the Nysa Bridge, 100 meters wide, supported the front theatrical stage.

The Pergamon Bridge’s design includes two parallel linear cylindrical vaults separated by a continuous partition.

The entrances to the double pipe are at the same level, while the exits on the valley side are spaced 13 meters apart, resulting in a significant difference in length between the western branch (183 meters) and the eastern branch (196 meters). Due to the late addition of a built-in threshold 2 meters high, some sections of the western pipe are silted up today.

The dimensions of both semicircular arches are almost identical: the clear spans are each 9 meters, the rise from the support line to the arch’s apex is 4.5 meters, and the clearance to ground level is 7.5 meters. The vaults are constructed from rubble bound with mortar and rest on a foundation of ashlar stone.

Upstream and downstream from the bridge’s foundation are two other well-preserved ancient bridges over the Selinus River, known as Tabak Köprüsü and Üç Kemer Köprüsü (“Three-Arch Bridge”).

The Pergamon Bridge’s flood capacity has been the subject of hydraulic and hydrological studies. The tunnel gradient is 0.6%, and the maximum flow capacity is 360 cubic meters per second. Exceeding this value creates internal pressure that damages the structure. Considering that the Selinus River is 13.4 kilometers long, with an average gradient of 2.2% and a catchment area of 101 square kilometers, intervals during which floods exceeding the bridge’s capacity may occur have been calculated: statistically, such events happen on average once every 700 years.

Thus, the Pergamon Bridge not only serves as an outstanding example of Roman engineering but also continues to demonstrate how the ancient Romans addressed challenges in urban construction and water flow management.

Sources:

https://en.wikipedia.org/wiki/Pergamon_Bridge

Photo: Zübeyir Talha Cerrah