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[[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]]
'''Dams and intake'''<br>
Dams in streams are generally small; their purpose is to provide a small pond so that a controllable draw-off pipe can be built into the wall of the dam at a level higher than the bed of a stream. Unlike larger dams, which impound water to provide storage over a dry season, these small dams overflow for most of the time. The crest of the dam acts as an overflow [[Tyrolean_weir_-_Surface_water_collectionWater Portal / Rainwater Harvesting / Surface water / Tyrolean weir|weir]], except at the sides, where it is raised to prevent scouring of the banks.
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===The following project(s) utilize gravity-powered water flows.<br>{|style="border: 2px solid #e0e0e0; width: 7020%; text-align: justify; background-color: #f5f5f5e9f5fd;" cellpadding="2"|- style="vertical-align: top" |[[Image:akvorsr logo_lite.png|center|60px|link=http://akvo.org/products/rsr /]]|- style="vertical-align: bottom"|[[Image:project 786.jpg|thumb|nonecenter|200px140px|<font size="2"><center>Project 786</center></font>|link=[http://wwwrsr.akvo.org/rsr/project/786/]]|'''Akvo RSR Project786]<br>MWA-LAP:''' [<br>Honduras</center></font>|link=http://wwwrsr.akvo.org/rsr/project/786/ MWA-LAP: Honduras]In Honduras, MWA member Water For People will be implementing the program in the municipality of San Antonio de Cortes. Consisting of 36 villages and 11 urban neighborhoods, the region has an area of 227 square kilometers with a population of 30,000.]
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<br>==Manuals, videos=Links===* [https://sites.google.com/site/waterdistributionsoftware/neatwork NeatWork]: NeatWork is a free program specifically fashioned for the design of entirely gravity-driven water distribution networks for rural areas.* [http://aplv.org/technical_resources Air in Pipes], by Gilles Corcos (available in English and Spanish). This manual is intended as a complement to handbooks on the construction of rural gravity-flow water systems. It focuses on the problems and opportunities associated with the almost inevitable presence of air in the main pipeline from the spring to the distribution tank. The manual contains new material that the author has gathered in both the field and in the laboratory and links==which fits into a relatively simple theoretical framework.
===Acknowledgements===
* [http://www.wateraid.org/uk/what_we_do/sustainable_technologies/technology_notes/default.asp WaterAid Technology Notes.]
