Proposed features/Qanat

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Qanat
Status: Proposed (under way)
Proposed by: JoeG
Tagging: canal=qanat
Applies to: way
Definition: A man-made underground channel for transporting water from higher to lower areas, with shafts visible from the surface (excavation=qanat_shaft)
Rendered as: waterway=canal, typically with tunnel=flooded
Drafted on: 2018-09-01

canal=qanat

excavation=qanat_shaft

Proposal

Qanat is a traditional way of supplying water in hot and arid climates within limited distance of a mountain range. Developed some 3000 years ago, it still serves thousands of people worldwide. It originates from Iran or Oman and is most widespread across South-Western Asia, from Mediterranean to Himalaya with a presence in North Africa and south-eastern arabic peninsula.

It consists of an underground gallery that drains water from the aquifer at first (collection section) and then channels it (transport section) along a gentle 1/1000 slope, with a series of vertical shafts which are artifacts of the building process but also serve for ventilation and service access, with an opening of the channel section as it reaches the groundlevel and a a further open air canal.

You may read more about it in a great article at Wikipedia.

Qanat.jpg

For the purposes of tagging in Openstreetmap, the word qanat refers to all similar structures, e.g. those known as kariz, kahn, khettara, falaj, foggara. The word qanat is generally the best known in English.

The following features are considered essential:

  • The immediate source of water is groundwater, not a spring or river
  • Water flows by gravity in free flow (not pressurized or pipe flow)
  • A large part of the channel is underground, minimising evaporation
  • Construction and maintenance is through vertical shafts, which are then visible on the surface

New tags will likely be needed for water channels that do not satisfy these requirements, e.g. spring flow tunnels near Jerusalem (see Wikipedia)

Rationale

Despite the widespread use of pipelines and pumps, qanats still provide water for many people. They are gravity fed and physically limited in how much they impact groundwater levels. Compared to pumps, which require power and enforcement of pumping regulations, qanats can therefore be considered a sustainable and environmentally-friendly way of using water from natural resources.

Qanats are underground structures, vulnerable to unplanned ground works. Mapping them could hopefully save some of them from destruction.

Afghanistan is probably the country most dependent on qanat network, however after decades of wars large part of it lay in rubble. Having tools for mapping qanat could help rebuilding them in the future, when (hopefully!) the situation improves there,

Despite being mostly underground structures, qanats are quite easily traceable from satellite photos and surface surveys, due to the openings of vertical shafts (see below).

As they are used to access groundwater, mapping qanats can also help characterise groundwater resources and their use, and is therefore highly relevant to groundwater management in these regions.

A single village may have several qanats, and qanats can be found in a number of countries in Africa, the Middle East and central and south Asia. It is possible there may even be 100 000 qanats worldwide. Mapping them explicitly in Openstreetmap will help in determining their spatial distribution and significance.

Tagging

Qanats are firstly a means of conveying useful water in free vs pipe flow. It is therefore tagged with the generic waterway tag waterway=canal. Underground sections are tagged with tunnel=flooded. It is not a pipeline (man_made=pipeline), and is longer than a culvert used to convey water, e.g. under a road (tunnel=culvert). The term "aqueduct" is now also only used for bridge=aqueduct, rather than water channels generally.

However, qanats are a distinctive type of infrastructure, notably in that they have (more or less) regularly spaced shafts visible from the surface. It is therefore worthwhile also tagging them with the subtag canal=qanat, which does not affect rendering on the standard map, but makes them easier to identify.

Qanat shafts are to qanats what power towers are to a power line - the feature that is most obvious at ground level, and in aerial imagery. They are primarily excavation holes, not intended to access water (not man_made=water_well) or extracting another resource (not man_made=mineshaft). They are tagged with the generic man_made=excavation, and subtag excavation=qanat_shaft

Summary
Tag Purpose
waterway=canal free flow conveying useful water
canal=qanat identify qanats as distinctive infrastructure
tunnel=flooded located in a tunnel
layer=-1 tunnels should be rendered below other features
excavation=qanat_shaft identify the shaft itself
man_made=excavation the shaft is a type of excavation

Qanats that are local attractions might also be named, e.g. name:en=Qanat, name=قنات, and marked as tourism=attraction

Qanats of historic value can additionally be tagged with historic=aqueduct + aqueduct=qanat. This will cause the historic qanats to be rendered on the Historic Places map, e.g. aqueduct. If a qanat is only of historic value, i.e. does not convey water, it might be exclusively tagged as historic. If in doubt about historic value, do not tag it as historic. If in doubt whether it is able to convey water, assume it can.

This proposal builds on previous proposals for tagging qanats:

  1. Mark the way with man_made=qanat and shafts with qanat=shaft. This, however, will not be rendered by any existing engine.
  2. Mark the way with waterway=canal + canal=qanat + tunnel=yes. It leaves possibility to mark parts of qanat which are not underground (sometimes happens in the end sections) and gives chance to be rendered by existing engines. Still the question is how to tag shafts.
  3. Mark the way with waterway=aqueduct + aqueduct=qanat. This implies also introduction of aqueduct=surface and aqueduct=bridge for surface and Roman-style aqueducts.

As of 1 september 2018, there currently appears to be about 40 mapped qanats with various tags. In most cases, tags will be added rather than modified. Where this isn't possible, the original mapper will be contacted.

A previous abandoned version of this proposal was drafted by user emes

Previous discussion:

Examples

An example of mapped and unmapped qanats. East of town Mastung in Pakistan, south of Quetta.

Qanat mapped.jpg

The way is tagged with canal=qanat, while the nodes have excavation=qanat_shaft tag.

Rendering

Qanats are rendered as waterway=canal.

In specialised maps, it may also be useful to mark shafts as they are large and observable terrain features.

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