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[[Image:Sand dam.JPG|thumb|right|150px|Sand dam construction in Kenya]]
Regions with highly erratic rainfall often have rivers with temporary seasonal sandy bed streams. They only experience heavy water run-off for short periods of time after rain. During such periods of high flows, large quantities of sand are transported downstream. Some sand can get trapped on the upstream sides of rocks outcroppings along the stream. Such sand traps form natural water storing aquifers. The sand dam technology provides a mean means to reproduce this water collecting mechanismand to increase the size of existing aquifers. Water in natural collection sites is usually clean for drinking but quite finite and quickly gets depleted. Sand dams are an artificial improvement of sand beds where more water can be recharged and stored for water use. A dam of concrete, clay soil or stone masonry is constructed across the channel at specific sites to trap and hold back the sand during flooding; this creates an additional sub surface water bank to be harvested during the dry season.
==History and social context==
In Kenya, it has been used with good outcome in the Kitui, Machakos and Samburu districts. Other countries with similar dry environments such as the U.S.A, Thailand, Ethiopia and Namibia also have used it in numerous and divers diverse forms. Sand dam projects here have not only improved water availability, but benefited the community socially and economically. Locals are trained to participate in construction of the dam, maintenance, financial management, and also in natural resource management.
==Suitable conditions ==
Expert input is required to determine the best sitefor the dam and to make sure the project is adapted to the physical conditions as well as social circumstances of the local community. Problems primarily relate to aspects Physically, the site must be suitable for construction of a dam locations , provide safe drinking water, and constructionbe able to use sand sedimentation to increase water availability. Accurate estimation Firstly it must be decided if the river is appropriate, secondly the best section of riverbed must be chosen, and then the optimum location for extraction. Generally, the dam requires a sandy riverbed in a hilly area. Preferably the groundwater reserves riverbed consists of coarse sands layed on impermeable bedrock. The river should be seasonal but must have a base flow. Whether this is difficultthe case can be determined by the type of vegetation growing around the river. The river shouldn’t be too wide (max. Risk of erosion 25m) and contamination during the rainy seasonshould have two high riverbanks. With an accurate These conditions and appropriate location choice also others related to, for example; climate, presence of boulders, or gradient of the total amount riverbed must be analyzed in detail to determine suitability of water available in the sand dams site. Also, though it can be over 6000m³artificially improved, the water quality must be good enough for drinking (not too saline, contaminated, etc.)
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* regeneration of vegetation along riverbanks <br>
* The effects of sand dams on downstream river discharge are generally small (< 10% of runoff). <br>
* Improved water quality compared to surface water dams since sand dams protect the water against evaporation, contamination (by livestock, etc.) and insect & bacteria breeding. <br>
* Additionally the sand deposits can be collected and sold for extra income. <br>
* Long lifespan if maintained correctly.
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* external expert knowledge needed <br>
* labour intensive construction <br>
* capital intensive <br>
* The site must fit very specific conditions for the sand dam to be successful <br>
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====Operation====
First an appropriate site needs to be selected, depending on whether it is suitable for a sand dam or not. Then the best section of the riverbed is chosen. Then the dam is designed, depending on the conditions and the water needs of the community, and built. It takes a few rainy seasons to let the aquifer fill up. After that an extraction point is constructed such as a scoop hole or well. Then water can be extracted and used for drinking, agriculture, etc. Meanwhile the local community needs to be trained to manage and maintain the dam.
====Maintenance====
====Manufacturing====
First a trench needs to be dug. Its position is marked out using pegs. Then it is excavated and the soil placed downstream. It can also be dug out into bedrock. It should then be checked for weathering zones and cracks. Then the dam is constructed. The method depends on the type of dam and the type of ground. The construction of sand dams in cascades improves total storage and efficiency and minimises minimizes seepage losses. Dams made of concrete, stone-masonry and brickwork require skilled labour for construction, but are stronger and have a longer lifespan.
Round bars are placed vertically along the trench to reinforce it. Then a foundation is layed consisting of 2 layers of cement with barbed wire in between. Once this has set the trench is filled with masonry of mortar and hardcore. The wing walls and final dam wall can then be constructed. Finally any exposed parts of the construction are plastered.
Then an outlet for water extraction must be constructed. Hand dug scoop holes are the simplest possibility, however the water can easily be contaminated. Covered wells, with hand or rope pumps protect the water much better. It is also possible to construct an outlet pipe with a tap. This however are difficult to maintain, and can weaken the dam.
====Estimated Lifespan====
The lifespan depends on the materials used and expertise with which it is constructed and maintained. The dams in Kitui Kenya were built at a cost of 7500 USD and have a minimum lifespan of 50 years.
==Cost==
==Country experiences==
Kitui district, Kenya:
SASOL Foundation has since 1995 built over 500 dams. They are built using locally available materials and are partially (40%) financed by the local community. The community is also involved in the construction and maintenance. Sand management groups were formed to aid in the construction of dams and to maintain them after.
The dams not only provide a steady source of drinking water, but also have further social and economic benefits. They provide irrigation for cash crops and other commercial rural activities, piped water for industrial activities in nearby villages. Further due to the large number of dams, the water table rises over a large area and therefore stimulates ecological regeneration.
Borana Zone, Ethiopia:
Communities in this area depend on agriculture and livestock farming, which is very limited due to the unstable water supply. In 2007 several NGOs implemented 7 sand dams and 10 surface runoff tanks. This combination should provide a reliable source of water for 10 communities in the area. The project will in the future expand to other parts of the country also.
==Manuals==
http://www.rainfoundation.org/fileadmin/PublicSite/Manuals/Sand_dam_manual_FINAL.pdf
==Movies==
* [http://www.thewaterchannel.tv/index.php?option=com_hwdvideoshare&task=viewvideo&Itemid=53&video_id=63 Kitui Sand Dams - CPWC]
* [http://www.youtube.com/watch?v=aOH7ar274S4 RAIN sand dam workshop & field visit in Ethiopia, 2009], by RAIN foundation and EHRA.
* [http://tv.oneworld.net/mediamanage/play/3820//148271/ Sand dam movie - Excellent Development.com]