The Iron Gate () is a sixth-century dam which is located in the mountains to the east of Antakya (ancient Antioch) in Turkey. It is one of the earliest known arch dams and is now the oldest to survive. It was built in the mid-sixth century on the order of the Byzantine emperor Justinian I to collect and slow the waters of the Parmenios stream, which in the rainy season could cause torrential floods endangering Antioch's city centre. Previous attempts to channel the floodwaters into underground passageways had not fully solved the problem.
The Iron Gate is one of the few ancient structures for which a contemporary technical report of its construction, provided by the Byzantine historian Procopius in his On Buildings, survives. The structure integrated a former aqueduct and bridge into its construction, and additionally carried part of Antioch's city wall. Its innovative arched structure redirected the water pressure, in times of flood, into the rock cliffs on either flank. The structure also included a city gate on one side of the dam—strongly fortified, it was probably the source of the name "Iron Gate".
In the medieval period, the Iron Gate was damaged by a flood too big for the existing outlets to handle; the waters overtopped the dam and caused the collapse of the city wall. The reconstructed upper fortification, with additional outlets, is what is visible today. As Antioch declined in importance, the structure was gradually abandoned. Looting of building materials caused the collapse of most of the side structures, leaving only the dam and some of the city wall intact on top. Today, the Iron Gate is vulnerable to further erosion or even total collapse; nevertheless, it is still functioning as a dam.
Background Antioch
The city of Antioch was founded in 300 BC by Seleucus I Nicator, one of the successors of Alexander the Great and the founder of the Seleucid Empire. Located at a strategically important location on the Orontes River in Syria, close to trade routes and with access to the Mediterranean Sea, Antioch became the most important city in the Eastern Mediterranean. It was the capital of the Seleucid Empire between 240 BC and 63 BC, and thereafter a regional capital of the Roman Republic and empire. Its peak population has been estimated at approximately 500,000 people. Antioch, often threatened by the Sassanid Persians, was conquered by the Muslim Umayyads in 640 AD. The city was the capital of the Principality of Antioch during the Crusader era, between 1098 and 1268. Damaged by sieges, earthquakes, and changing trade routes, Antioch became less important under the Mamluk Sultanate and the Ottoman Empire. Its ruins are now located under the modern city of Antakya.
One significant drawback of Antioch's location was its vulnerability to hydrological processes. On the western side, the Orontes flooded each year during the rainy season, spreading debris and silt along its banks. At the same time of year—between October and April—the torrential rains would cause large amounts of runoff from the mountains Silpius and Stauris on Antioch's eastern flank. Much of the city was therefore inundated on a yearly basis by soil and mud. Attempts to corral the water through a complex network of drains, conduits, aqueducts, and dams would fail over centuries; the deposition of alluvial material became so great that an island in the Orontes River disappeared and the ancient city has been buried deep. The situation was further worsened by the location's tectonic instability. Antioch sits atop the Al-Chab fault near the Marash triple junction of the Anatolian plate, the African plate and the Arabian plate. The earthquakes of 115, 526, and 528 AD caused huge devastation, but even minor shifts damaged structures, especially aqueducts.
The Parmenios stream The most threatening mountain stream was the Parmenios (), today called the Haci Kürüş Deresi, which carved out the gorge between Silpius and Stauris and then flowed down into the Orontes. Its colloquial name was the Onopniktes (), or "Donkey-Drowner", because in addition to flooding the city center, its flash floods could easily drown pack animals in its ravine, which was occasionally used as a road to Apamea and eastern Syria. By the 4th century AD at the latest, and possibly as early as Hellenistic times, two huge masonry vaulted tunnels had been constructed to conduct the Parmenios under the area of the Forum of Valens and Antioch's main street.
Nevertheless, as described by the sixth-century writer Procopius, the problem was not solved:
Initial structures
There had long been a structure spanning the Parmenios at the site of the Iron Gate, the only location where crossing the ravine without being threatened by floods was possible. The first structure, probably built in the Seleucid era, was an wide arch bridge for foot travellers and beasts of burden, who could traverse the valley floor below over a brick arch wide founded on the rocks at each side of the ravine. Following Pompey's capture of Antioch for the Roman Republic in 64 BC, this route would have cut travel times to the booming town of Laodicea by several hours. Around 300 AD, this bridge was converted to carry an aqueduct—the road was likely then rebuilt on top—transporting water to Diocletian's imperial construction projects on the Orontes island, from the springs which also supplied water to the suburb of Daphne. However, after an earthquake in 458 AD severely damaged both a bridge across the Karakiz Deresi gorge further up the aqueduct's course and the imperial residence, the aqueduct was abandoned.
Antioch suffered many disasters during the sixth century, including a great fire in 525; the two devastating earthquakes in 526 and 528; its capture in 540 by the Sassanid ruler Khosrow I, who pillaged the city and deported its population to build a new city, Weh Antiok Khosrow, in Mesopotamia; and the impacts of a great epidemic of plague in the 540s. In the aftermath, the emperor Justinian I initiated a construction programme to restore Antioch, now called Theoupolis ("City of God") in an attempt to win back divine favour. Particularly relevant was the consolidation of the city walls, which were deemed unfeasibly long and vulnerable in the aftermath of the Persian sack. Justinian's wall excluded Antioch's northern quarter and the Orontes island, and diverted part of the river to run alongside the western walls to act as a moat.
On the opposite side of the city, the settlement of Epiphaneia on Mount Staurin was also excluded, meaning the city walls traversed the ravine of the Parmenios. The already-present arch bridge was reinforced with a new arch, also wide, on its outer side. The city wall, built using both the opus mixtum and opus vittatum techniques, measured wide and tall. Its width necessitated that the old aqueduct bore part of its weight, and so the road atop the aqueduct became too narrow for pack animals. A new road was thus built on the slopes of Staurin, with a gate through the wall. This strongly-fortified opening, probably the source of the name "Iron Gate",, was wide and high. It survived until at least 1939, although it appears to have been partially closed off to offer attackers less of a weak point.
Construction and design
A few years after the modification, it was decided to construct a dam underneath the arch bridge to help prevent the floods of the Parmenios. Procopius, who had a special interest in hydraulic engineering, provides one of the few contemporary technical reports available for any ancient infrastructure project:
Procopius' identification of the location of the dam relative to the wall with the phrase , translated above as "before that part of the circuit-wall", caused some dispute among historians of Antioch. The scholar Richard Förster, writing in 1897, understood this to mean that the dam was built outside the new wall; the mid-20th-century historian Glanville Downey argued the opposite. The archaeologist Gunnar Brands, writing in the early 21st century, notes that Downey's proposed arrangement would have been useless in practice as the Parmenios would have first encountered the city wall, which was not built to allow controlled drainage, before building up at the purpose-built dam. Förster's arrangement is more practical, but only if there was a very short distance between dam and wall—otherwise, the same issue with water striking the city wall would reoccur, with the added issue that the dam would be vulnerable to attack. "Under these circumstances", Brands argues, with the help of archaeological investigation, " should be understood as an addition directly connected to, i.e., as a single structural unit with, the city wall."
The first known arch dam is the Glanum dam, built 600 years earlier in France, but the Iron Gate is the only known surviving ancient structure of the type. The hydraulic engineer Matthias Döring, terming it a precursor of modern arch dams and thus fourteen centuries ahead of its time, notes that the dam is not just surviving, but is still active. The location of the Iron Gate, at a narrow span above a cliff, was critical for its conversion into an arch dam, allowing for the redirection of water pressure into the lateral flanks by using a horizontal arch. Döring has calculated that 30–50% of the water pressure behind the dam was redirected at right angles into the rock flanks, while 50–70% dissipated tangentially through friction. This innovative design not only reduced construction time and materials usage but also, relative to traditional gravity dams, increased the potential capacity of the reservoir behind the Iron Gate.
The dam, wide in total ( wide under the arch bridges and with an additional thickness on the outside edge), was constructed using opus caementicium (Roman concrete) faced with small ashlars on the facades. To strengthen the bonding of the concrete infill, layers of ashlars and multi-layers of bricks were included in the structure. Remnants of the initial arch bridges, which were incorporated into the lower dam structure, are clearly visible on the city-facing side of the Iron Gate. The dam had a maximum height of around . The structure had two outlets for the water of the Parmenios. The one at the bottom allowed smaller floods to pass with no need for damming. Meanwhile a square spillway (sides long) at the height of the former aqueduct allowed increased flow in the case of severe floods which filled the dam's reservoir to capacity. Manual human intervention was never required, as is also the case with modern retention basins. The original name of the dam is unknown.
The two main risks to the dam were earthquakes and floods. The former, if they did not result in immediate collapse, might weaken the dam's structural cohesion. Floods, on the other hand, threatened to cause immediate damage: if the small outlets were blocked by debris, as commonly happened, floodwaters would have quickly filled the reservoir (which had a surface area) behind the dam wall and begun exerting pressure on the unprotected city wall.
Medieval history
An incident of this nature happened during the medieval period—a large flood proved too much for the bottom outlet and single spillway to handle, and, overtopping the dam section which remained undamaged, caused the collapse of a portion of the city wall, in addition to the aqueduct and path. Parts of the city-side facade of the dam were also damaged. The flood may have been caused by a blockage in the barrel-vaulted bottom outlet, which may have contained a grate to protect against attacks, but which debris or mud could rapidly adhere to.