Tag:usage=derivation
 usage = derivation
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| Description |
|---|
| Specific waterway usage allowing the derivation of water from a waterbody (reservoir, canal, river) to an other watershed or a temporary reservoir to prevent flooding |
| Group: man made |
| Used on these elements |
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| Requires |
| Useful combination |
| Status: undefined |
| Tools for this tag |
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Channels and canals are artificial structures are free-flow watercourses that use gravity to move water away from a water body and convey water for hydro-electric power, regulate water flows to assure flood protection, drainage, irrigation, water supply or other purposes.
Tag usage=derivation is used to indicate:
- Redirecting part of the watershed water to an other watershed for some hydro-electric projects. These derivation channels start from a waterway floodgate structure (plus spillway in major projects) that controls the quantity of water to deviate.
- Derivation to a temporary flooding basin
- Flooding channel structures in urban areas to deviate securly flow of water
Derivation channels may be in regular use or designed only to be used in emergencies. The flood control structures can be used with the tag [intermittent=yes].
Irrigation channels should be tagged with usage=irrigation.
How to map
See waterway=river for common guidelines on how to map waterways.
All derivation channels can be drawn as ways 
and tagged with waterway=canal + usage=derivation.
Larger derivation channels can be completed with an area 
and tagged with natural=water + water=canal + usage=derivation.
As any waterway, derivation channels should connect to the water body from which it drains and if it applies to the downstream outlet it's derived to.
Related tags
- waterway=dam - major derivation channels will usually be mapped in association with dams
- waterway=weir - where only water exceeding the weir height is deviated
- waterway=floodgate - control of water deviated through a floodgate
- waterway=sluice_gate - control of water deviated through a sluice_gate
- natural=water + water=reservoir - Water area.
- hazard=*
Possible tagging mistakes
Derivations from water body to agricultural irrigation areas should use usage=irrigation