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In the limestone rock of the Acropolis Hill, on which the late Ottoman village stands, two tunnels for the water supply were excavated in ancient times. The upper one can be visited in a guided tour. It was the last section of a 170 km long, mostly underground aqueduct built between 90 and 210 AD to satisfy the enormously increased water needs of the Decapolis towns of Adra'a, Abila and Gadara in Roman times. The tunnel system of the later known as Qanat Fir'aun (Arabic: canal of the Pharaoh) is by far the longest built structure of antiquity and one of the most important engineering achievements of that era.
The 380 m long upper tunnel in Gadara was never completed. Therefore the traces of work are clearly visible and one can learn and understand how the Qanat Fir'aun was carved out of the rock. Also some entrance shafts for the construction and scoop holes of older cisterns, which the tunnel crosses, are well preserved (see the photo pages).
The rocky outcrop on which Gadara was built, although strategically located, did not offer any natural springs. The water was provided by cisterns until the late Hellenistic period (ca. 160 - 30 BC), in which rain water was stored, as well as by three lower located springs outside the Akra (fortress). On the settlement hill 75 reservoirs with a capacity of 6 up to 450 cubic meters have been documented.
When in Roman times the population increased, the city area extended and more thermal baths and wells were needed, the cisterns were no longer sufficient and considerable quantities of water had to be brought in from a greater distance.
In order to meet the increased demand, a long-distance water supply system was built from the Ain Turab spring to Gadara, 11 km away, even before the common era. The water probably flowed through clay pipes in a tunnel dug near the earth's surface, called today Qanat Turab. Since it had to be led around the valleys, it was 22 km long and surpassed all tunnels built up to that time. Arriving in Gadara, the aqueduct crossed a valley by means of a bridge and ended in the lower tunnel under the Acropolis, from where it was fed into the urban water supply.
In the second half of the 1st century AD the population of the Decapolis towns of Adra'a, Abila and Gadara had grown to about 50,000 people . The further development required more and more water, especially also because the demand due to public fountains and thermal baths, house connections, etc. increased to the 300 to 400 litres per person/day usual in Roman cities.  Therefore the three cities evidently decided to build a common long-distance water pipeline. It was constructed from 90 to 210 AD in several phases.  In modern times it became known as Qanat Fir'aun (Canal of the Pharaoh).
In order to allow considerable flow rates, the water no longer flowed through pipes, but through a canal or tunnel. Since pumps did not yet exist, an aqueduct had to be led from a higher, more productive reservoir with a precisely calculated, very small gradient over valleys and through mountain ridges.
This gigantic building structure, one of the most elaborate of Roman antiquity, began at a reservoir in Wadi Harier (near the Syrian village of Dille) with two dam walls made of basalt blocks and a capacity of 4 to 6 million cubic metres. From there to the end point in Gadara the distance was 170 km and the height difference about 217 m. The aqueduct ran for about 106 km through a system of tunnels, for the construction of which about 2900 inclined shafts with stairs had to be excavated. Beside the main conduit there were at least 14 secondary lines, including tributaries from the lake Muzarib (today Syria) and various springs.
The Qanat Fir'aun could have been used until the end of the Byzantine era in the 7th century, but at the latest until the devastating earthquake of 747 which destroyed Gadara.
Some explanations of the construction method, on the photo pages.
1 - Information from: Mathias Döring, Wasser für die Dekapolis, 2011
2 - For comparison: in Hellenistic times it was 20 - 40 litres per person/day, and in Germany drinking water consumption per capita/day was about 123 litres in 2017.
3 - The radiocarbon method (14C) was used to date the ground charcoal added to the sealing plaster on the interior walls to make it waterproof.
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Compilation of information, editing, translations, photos: Universes in Universe, unless otherwise indicated