The most important elements of a siphon are the transition of an open channel into one or more pipes: the influx basin or header tank, the two bends in between hills and valleys: geniculi in Latin, the substructure to support the pipes in the valley: the so-called venter (belly), and the transition of the pipe(s) into the open channel again: the output basin or receiving tank. Commonly the pipes at the bottom of a valley were laid down on a siphon-bridge so that the river in the valley could pass below without damaging the aqueduct.
Headertank and ramp of the Yzeron siphon at Plat de l'Air in the Gier aqueduct of Lyon (France) with room for 11 (or 12) lead pipes. This tank was the start of the siphon towards the Beaunant venter bridge, 122 m deeper!
The Aspendos inverted siphon (Turkey) looking north, as seen from the acropolis hill. The south tower and northernmost double tier venter bridge are visible in the foreground. The long venter bridge and the south tower can be seen towards the back.

Longest siphons in Roman aqueducts

One way by which natural features such as valleys and depressions could be crossed, was the (inverted) siphon, a technique based on the simple physical principle that 'water finds its own level'. The Romans and even the Greek were well aware of this principle. The basic purpose was to prevent long detours.

Based on own estimate 10% of all Roman aqueducts were equipped with one or more siphons. In the aqueduct literature the nine siphons in the four aqueducts of Roman Lyon (France) are often cited. As far as we know there were even two instances with double siphons among which the Capronne siphon in the Yzeron aqueduct supplying Lyon. The other was in Aspendos (Turkey).

Vitruvius - a Roman architect from the 1 c BC who left a treatise called 'de architectura' - made reference to a venter (the lowest part of the siphon) and geniculus (vertical bends), but he also introduced the term 'colliviaria' a term which has caused much discussion. One of the functions of this device could have been to let air-bubbles escape from the aqueduct pipes. Some aqueducts were indeed equipped with this type of device. Later instances were found in Turkish siphons, called suterazi (water balance).
Name aqueduct Length (m) Literature Remarks
Smyrna (Turkey) 4.400 Kessener 2001, 2004  
Lyon, Yzeron (France) 3.600 Burdy 1991, Kessener 2001,2004 Craponne II
Lyon, Brevenne (France) 3.500 Burdy 1993, Kessener 2001,2004 Ecully (Grange-Blanche)
Lyon, Mont d'Or (France) 3.500 Burdy 1987, Kessener 2001,2004 des Planches (Ecully)
Pergamon, Madradag (Turkey) 3.250 Kessener 2001,2004  
Alatri (Italy) 3.000 Kessener 2001,2004  
Lyon, Gier (France) 2.600 Burdy 1996, Kessener 2001,2004 Yzeron (Beaunant)
Lyon, Yzeron (France) 2.200 Burdy 1996, Kessener 2001,2004 Craponne I (Grezieux)
Aspendos (Turkey) 1.670 Kessener 2001,2004 total length
Termini Imerese, Barratina (Italy) 1.300 Kessener 2001,2004  
Lyon, Gier (France) 1.210 Burdy 1996, Kessener 2001,2004 Garon (Soucieu)
Laodikeia ad Lykum (Turkey) 800 Kessener 2001,2004  
Lyon, Gier (France) 700 Burdy 1996, Kessener 2001,2004 Dureze (St. Genis)
Oinoanda (Turkey) 600 Stenton 1996, Kessener 2001,2004  
Lyon, Gier (France) 575 Kessener 2001,2004 Trion (St. Irenee)
Lyon, Brevenne (France) 500 Burdy 1993 hypothetically
Lyon, Mont d'Or (France) 420 Burdy 1987, Kessener 2001,2004 Limonest (Cotte-Chally)
Patara (Turkey) 260 Kessener 2001,2004 Delik Kemer

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