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[[Image:Sand dam.JPG|thumb|right|200px|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 rock outcroppings along the stream. The sand traps form natural water storing aquifers. Sand dam technology provides a means to reproduce this water collecting mechanism and to increase the size of existing aquifers.
With an accurate and appropriate location choice the total amount of water available in the sand dams can be over 6000m³.
[[Image:Sand dam.JPG|thumb|right|150px|Sand dam construction in Kenya]]
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 ==
[[Image:Sand dam flood.JPG|thumb|right|150px200px|Sand dam during flooding]]
Expert input is required to determine the best site for the dam and to make sure the project is adapted to the physical conditions as well as social circumstances of the local community. Physically, the site must be suitable for construction of a dam, provide safe drinking water, and be able to use sand sedimentation to increase water availability. 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.
==Construction, operations and maintenance==
[[Image:Sand dam diagram.JPG|thumb|right|150px200px|Cross section of a sand dam. (Click to see details)]][[Image:Sand dam trench.JPG|thumb|right|150px200px|Excavation of a trench for sand dam construction]]
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. Round bars are placed vertically along the trench to reinforce it. Then a foundation is laid 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.
====Building the sand dam (and wing wall)====
[[Image:SandDamConstruction.JPG|thumb|right|150px200px|Sand dam under construction. Somaliland. Eric Fewster, BushProof / Caritas]]
* 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 minimizes seepage losses. Dams made of concrete, stone-masonry and brickwork require skilled labour for construction, but are stronger and have a longer lifespan.
* Key into banks (sub-surface dams) or construct wing walls to avoid erosion around edges of the sand dam. Where wing walls are built, a good technique is to start from the wing walls and work inwards to the centre, since community enthusiasm lags by the time wing walls are constructed (if not built first), yet they are essential to proper functioning. Length of the wing wall varies according to bank characteristic: loose riverbanks, 7 metres; hard soils, 5 metres; hard & impermeable soils or rocks, wing wall is not needed. Planting napier grass along upstream riverbanks controls erosion and fixes the course of the river in a flood.
After the dam is built, 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 is difficult to maintain, and can weaken the dam. Meanwhile the local community needs to be trained to manage and maintain the dam.
[[Image:Sand dam scoop hole.JPG|thumb|right|150px200px|Women fetching water from a local aquifer through scoop hole]] [[Image:Sand dam well.JPG|thumb|right|150px200px|Closed well near a sand dam]]
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. 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.