Groundwater Dams  In most lowlands of Ethiopia where the annual rainfall is below 600 mm, the rains in the highlands drains towards the low elevation and cause floods for quite a short period of time. The sandy flood way valleys preserve the water until it is depleted by evaporation or/and by gravitational flow to lower elevation. The dwellers living along these valleys obtain water for domestic and other uses during the wet seasons and for couple of some months after the rains by digging pits in the valleys. When the depth of the water in the sand deposits goes deeper and deeper, the villagers find it impossible to dig any further and instead they travel long distances to fetch water and to water their livestock.

Types of Groundwater Dams: There are mainly two types of groundwater dams, they are:  Sub-surface Dams, which are dams constructed below ground level to store water of natural aquifers in reservoirs upstream of the dam.  Sand Dams, which are constructed across valleys to make the upstream reservoir filled with sand, gravel or stone to make artificial aquifer, which is replenished each year by the runoff in the valley.

Groundwater Dams compared with conventional dams  Simple to construct using locally available materials  Water treatment facilities are not required if to be used for drinking water  Less evaporation from the storage reservoir  Little loss of land  No damage due to dam failure  Pollutants such as mosquitoes and snails can not exist in the reservoirs  Siltation does not create any problems  Less vulnerable to pollution by animals or human beings.

Design Parameters of Groundwater Dams  As much as simple it looks, the design and construction of such dams require careful study of the different parameters which are likely to affect the proper functioning of groundwater dams. Reports indicate that quite a number of such dams have been built which are not functioning due to wrong selection of constructions sites, incorrect design or poor workmanship.  The following are the basic considerations that have to be thoroughly examined in the design of groundwater dams.

Site Identification  The first step to look for this resource is to study the valleys where villagers obtain their water supply by digging pits along sandy and dry riverbeds. The existence of green vegetation during all seasons along the banks of sandy valleys is another indication. Satellite photographs show also red colors along the valleys where green vegetation nourished by the subsurface water exists.  Observing water holes dug in river beds, digging trial pits on river beds and hand-auguring with auger rods or hand drilling are also alternative ways of identifying the appropriateness of a sandy valley for the construction of subsurface dams.  The choice of appropriate site can be best attained using contour maps of scale 1:20,000 or even with 1:50,000. Steeper sandy valley with narrow contours is mostly suitable for subsurface and sand dams while flatter areas with wide contour intervals are preferred for subsurface dams.

Topographic Consideration  The recommended slope to construct subsurface dam is less than 5%. This is to avoid depletion of the water through natural flow and to have acceptable size of sediments as well as to store good volume of water in the reservoir upstream of the dam. Lower gradient of less than 1% is not desirable as well. This is because steep gradients give run-off of higher velocity that flushes out silt and fine sand while course sand and gravel deposit. Low gradients slow down velocities and silt materials will be deposited. In general, the particle size of sediments accumulated along streams and in riverbeds is generally proportional to the topographical gradients where as on the other hand, the depth and lateral extent of such deposits is inversely proportional to the gradient.  Consideration should be taken to look for a narrow valley where the elevation of the bedrock outcrops to optimize construction costs. If the dam site is chosen in the upstream of the service area, gravitational flow could be attained.

Climatic Consideration:  In order to avoid unexpected flooding and minimize pumping from the construction sites, all construction works should commence and be completed in the dry months.

Hydrogeological Consideration  Precaution has to be taken to chose the correct foundation for the dam construction. The foundation material has to be a watertight solid rock or other impermeable materials such as consolidated clay.  Using hand digging or by conducting auger tests the type of the foundation material has to be determined. Besides the wing walls of the dam have to be well embedded in the side walls insuring no leak through this part.  The hydrogeology of the aquifer materials should also be studied. The type of parent rock in the catchment from where the sediments originate determined the amount of course particles in the total sediment load. The most favorable rocks are coarse granite, quartize and sandstone, but gneiss and mica- schists are also acceptable materials. As the amount of water to be extracted is a function of the void ratio and the grain size of the sand, it is always advantageous to look for course sand with grain size of over 5mm and to have higher porosity. Fine sand and silts are not appropriate reservoir materials.

Construction Materials  Groundwater dams can be constructed from locally available materials such as clay, brick, stone, concrete etc.  In Brazil, to store rainwater runoff entrenchments are dug below the ground surface in a shallow soil toward the impervious crystalline sub soil. Then, inside the entrenchment, earth or rock fill dams are built with a PVC sheet on the upstream face and around seeping zones avoiding seepage. On the watered upstream soil it is possible to plant crops or fruit trees. In addition there is almost always a shallow well to use the water for humans, animals or irrigation. Even in the first months of the dry seasons, it is possible to plant a second time and even in the driest years these impoundment are never without water.

Subsurface Dam from uPVC plastic

Water Abstraction Mechanism  The water in the saturated sand reservoir can be collected using one or any two of the following mechanisms:  from gravitational flow, by extending outlet pipes from the bottom of the dam to water abstraction spots.  from hand pump fitted wells  motorized pumping

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