__NOTOC__<small-title />[[Image:gravity distr icon.png|right|80px]]
[[Image:gravity scheme.jpg|thumb|right|200px|Gravity flow schemes usually incur low running costs.<br>
Photo: WaterAid / Caroline Irby]]
The usual components of a gravity scheme are the source (stream, spring catchment, dam, or intake), main pipeline, storage and break-pressure tanks, distribution pipelines and tapstands. The communities who benefit from the scheme are usually involved in large commitments of time and effort in the construction work associated with these components.
===Suitable conditions===
[[Image:gravity scheme3.jpg|thumb|right|200px|Break pressure tanks can be used to store water to meet peak demand.<br>
Photo: WaterAid / Caroline Penn]]
A dam is usually constructed of concrete, blockwork or masonry, preferably founded on rock. Rock, or some other impermeable material, should also form the basin of the impoundment. Twin intake pipes (one in use, one in reserve) are built into the wall of the dam; on the upstream side of the dam, they have strainers or screens; on the downstream side they are fitted with control valves. A scour pipe is also built into the dam, at low level, with a stop valve on the downstream side, and is used periodically to drain the pond and to clear accumulated silt, etc.
===Construction, operations and maintenance===
[[Image:gravity scheme2.jpg|thumb|right|200px|Group of men carrying a pipe to be laid for a gravity flow scheme in Ethiopia.<br>
Photo: WaterAid / Caroline Penn]]
Tapstands should have a fence around them to keep animals away and each one should have a nominated person, or caretaker, to keep the area clean and tidy.
===Costs===
===Field experiences===
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===Manuals, videos, and links===
===Acknowledgements===
* [http://www.wateraid.org/uk/what_we_do/sustainable_technologies/technology_notes/default.asp WaterAid Technology Notes.]
