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[[Image:gravity distr icon.png|right|80px]]
[[Image:gravity scheme.jpg|thumb|right|200px|Gravity flow schemes usually incur low running costs.<br>
'''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.
===Field experiences===
The following project(s) utilize gravity-powered water flows.
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|[[Image:project 786.jpg |thumb|center|140px|<font size="2"><center>[http://rsr.akvo.org/project/786/ RSR Project 786]<br>MWA-LAP: <br>Honduras</center></font>|link=http://rsr.akvo.org/project/786/ ]]
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===Acknowledgements===
* [http://www.wateraid.org/uk/what_we_do/sustainable_technologies/technology_notes/default.asp WaterAid Technology Notes.]
