Qanat

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Illustration of the qanat function
Excavation cones of several chains of qanat pits near Erfoud
Qanat Fir'aun
Exit of a qanat
Distribution grid of a qanat

A qanat or Kanat ( Arabic قناة, DMG Qanāh ) is a traditional form of fresh water pumping, mostly in desert areas , in order to obtain drinking and industrial water from higher regions. A qanat consists of a mother well , several vertical access shafts and the qanat channel. The Qanat Canal is a tunnel that leads with a slight gradient from the mother well over the access shafts to the Qanat outlet.

Designations

Qanate can be found in almost all countries on the Persian Gulf as well as in Afghanistan , Pakistan , Syria , Libya , on the edge of the Taklamakan and in the entire Maghreb as well as on the Canary Islands. Hence there are many different names for them. In Persian they are also called Kariz or Karez (كاريز, DMG Kārīz ). In Oman they are called Falaj , in North Africa, in the Maghreb, the name is Foggara , which means something like "underground tunnel". In Morocco the names Rhetara, Khettara, Hattaras or Käris are also used .

history

The origin of qanat water extraction is believed to be more than 2000 BC. BC in the area of ​​today's Iran , the first written references coming from a report on a campaign by Sargon II (722 to 705 BC). Water is still extracted this way today, especially on the edge of the Lut and Kavir deserts . One of the earliest qanats is that of Zavareh, which is over 5000 years old. Another example is the Qanat of Gonabad, with a mother well 350 m deep and over 2500 years old.

From Iran, the technique of underground irrigation channels spread mainly via the Silk Road in the ancient world and reached after the first Persian conquest in 525 BC. BC Egypt. Qanat systems have been found in the Charga oasis, for example . This technique later spread to the Roman Empire , where the technique of aqueducts was more decisive. An example of a Roman Qanat pipe was discovered in Brey on the Rhine, even further north, in the Düren district , at 1660 m, is the longest water tunnel north of the Alps that was built in this way: the Drover Berg Tunnel .

Technological dissemination of the qanats

In the opposite direction, the technology spread to northern India, like Megasthenes around 300 BC. Chr. Reported. But they can also be found in China, e.g. B. in Turfan's irrigation system . With the Arabs, the qanats spread to Algeria, Morocco, Sicily (e.g. the qanats of Palermo ), finally Spain and from here to South America.

In 2016, a selection of 11 qanats from Iran was included in the UNESCO World Heritage list under the title The Persian Qanats .

Planning and construction

The cause of the qanat and the origin of water

The reasons why the qanat technique developed and spread in the highland cultures in particular are primarily the lack of larger rivers and secondly the proximity of the settlements to a mountain or mountain range with a comparatively high rainfall. But the arid climate with its extremely high evaporation rates is partly responsible for this, as springs dry out quickly and superficial water conduction over long distances would lead to high evaporation losses. The downhill rains on the mountain slopes are primarily responsible for the abundance of groundwater , the water of which seeps away and collects in the depths above a groundwater non-conductor. These aquifers are often too deep for simple wells far away from the infiltration zones. If in the mountains but leachate on water impermeable layers meets creates layers of water above the actual groundwater; here there is the possibility of intercepting the water before it disappears into greater depths. However, these stratified water reservoirs have their own flow dynamics, so that this water is not available or accessible everywhere.

Preparations

For this reason, one first creates a test well ( gamaneh ) and determines the amount of water through scooping tests. It should be possible to detect an even and sufficient flow of water. If this is the case, the future route of the qanat will be determined above ground, and only then will the actual work begin with the creation of a mother well ( bir al-umm ) that extends to the groundwater .

These are usually carried out by work teams of three to four people - black slaves were used for this in the Maghreb . Because of the narrow shafts and tunnels, these are only equipped with the simplest of equipment. These include ropes, short spades or hoes, mostly simple light sources and the utensils required to determine the direction of advance (spirit level, plumb bob). Own, specially adapted tools were also developed regionally. The builders used hose levels as an additional tool .

The design and construction of a qanat require detailed knowledge of the subsoil and the resulting behavior of the water, especially to ensure the safety of workers. Still, working on a qanat is very dangerous. Only their small size protects shafts and tunnels against the forces acting underground, the risk of collapse or water ingress is omnipresent. In addition to these safety aspects, knowledge of surveying technology is essential, as you have to guarantee a constant gradient, the qanat should be as straight as possible and the work can only be successful if the base of the shafts is properly targeted. However, with comparatively primitive tools, poor lighting and generally adverse conditions, this is extremely difficult and requires a lot of experience.

construction

Starting from the mother well and the route determined by it, one begins to dig well-like shafts about every 20 to 35 meters from the destination of the water, i.e. the settlement area. However, there are also some cases where both sides dug at the same time. The shafts are in a row and point to the targeted mother well. Sometimes they are supplemented by branches in the area of ​​the aquifer to increase the amount of water, sub-qanates added to the qanat (underground tributaries, so to speak). The distance between the shafts requires the amount of work, which is why it is often expanded within the scope of what is responsible for the subsequent maintenance, especially when digging very deep. However, this makes it difficult to redefine the direction of excavation, which is always necessary. Even more important is the fact that the excavation during the later horizontal excavations can only be delivered to distant shafts with great difficulty, which therefore has a limiting effect on their maximum distance. The smaller the excavation depth, the cheaper it is to keep the distance between the shafts small (sometimes barely 20 meters). The total length of such manhole chains can normally be up to 16 km, but there are also records of up to 80 km. In these cases it can be assumed that construction began in the area of ​​the settlement towards the mountains without knowing when (and at what height) the aquifer was encountered. Due to the desertification (and thus the falling aquifer) it can also be assumed that these long qanats were used and extended over the course of centuries. The shafts are usually 20 to 200 meters deep, although in Iran, a qanat in the province of Khorasan is said to have reached 450 m. They are used for the manufacture and later for the maintenance of the tunnel and for air exchange.

The bottoms of the shafts are connected so that water flows out of the mountain to irrigate the fields. The condensation in the air also collects in the tunnel channel, dripping down from the rock walls. The water-bearing tunnel itself is 50 to 80 cm wide and 90 to 150 cm high. The tunnel must have a slight downward gradient to ensure that the water flows away. This gradient must not be too great, otherwise the water will reach too high a speed and the unpaved tunnel walls will be eroded . This would destabilize the tunnels and ultimately lead to their collapse, while at the same time severely reducing the water quality due to the water load. If the gradient is> 1 °, the aquifer may not be reached. In shorter qanats the gradient fluctuates between 1: 1000 and 1: 1500; in longer it is almost horizontal. If the difference in height is too great, steps can be built in, creating underground waterfalls. In some cases mills were connected to this in order to use the water power.

The advance speed is very different and depends mainly on the depth, the number of workers and the nature of the soil. At a depth of 20 meters, a work group of four people reaches around 4 m / day, at 40 m this value is halved. The qanat building therefore usually takes several decades.

The excavated material is usually transported out of the shafts using leather bags and piled up around the shaft exit. From the air, the shafts look like a long line of unusually large molehills.

maintenance

To obtain clean drinking water, it is necessary to constantly clean the drainage channels from mud and sand. This follows precise rules. In order to shield the manholes from outside material ingress, they were mainly covered in the direct settlement area.

Number and role

Historically, between 40,000 and 50,000 qanats were simultaneously active in Iran during the Persian Empire . Many ancient qanat systems were abandoned and fell into disrepair. They have recently been rediscovered during aerial flights. Today around 20,000 to 25,000 qanats are maintained in Iran. These have an average transport capacity of 2,000 to a maximum of 35,000 cubic meters per day. At the time of the Persian Empire there was a flow rate of around 1,000 m³ / s (32 billion m³ / a), which is roughly three times that of the Elbe in Dresden. If a permanent aquifer was not reached, the flow could fluctuate greatly depending on the season. The water was used as drinking water, but the majority was used as useful water in oasis agriculture, making this possible in the otherwise arid areas; because there were hardly any alternatives to the qanats in the corresponding areas.

literature

  • P. Beaumont, M. Bonine, K. McLachlan: Qanat, kariz and khattara: traditional water systems in the Middle East and North Africa. The Middle East Center, School of Oriental and African Studies, University of London in association with Middle East & North African Studies Press, 1989, ISBN 0-906559-35-9
  • Cornel Braun: Tehran, Marrakech and Madrid: Your water supply with the help of qanats. An urban geographic convergence on a cultural-historical basis. (Dissertation University of Bonn) Dümmler, Bonn 1974. ISBN 3-427-75521-5
  • Hamid Monadjem: Qanát, an engineering art from ancient Iran. In: Gahname. Trade journal of the Association of Iranian Natural Scientists and Engineers (VINI) in the Federal Republic of Germany V. No. 7, 2004.
  • Hamid Monadjem: Qanáte. In: Karl Gratzl, Robert Kostka (ed.): The mountains of Iran. Weishauptverlag, Gnas 2009, pp. 231-239, ISBN 978-3-7059-0297-8 .
  • Hamid Monadjem: Ghanat, an ancient irrigation technique . Dissertation at the Institute for Soil Mechanics and Foundation Engineering, Graz University of Technology , 1980.

Web links

Commons : Qanat  - collection of images, videos and audio files

Individual evidence

  1. ↑ Mohammed el Faiz: The garden strategy of the Almohad sultans and Their successors (1157 to 1900) . In: Michel Conan (Ed.): Middle East Garden Traditions: Unity and Diversity. Questions, Methods and Resources in a multicultural perspective. (Dumbarton Oaks Research Library and Collection) Harvard Press, Washington DC 2007, p. 97