|Sourcebook of Alternative Technologies for Freshwater Augmentation in Africa (International Environmental Technology Centre - United Nations Environment Programme, 1998, 182 p.)|
|Part B - Technology profiles|
|1. Agricultural technologies|
|1.1 Fresh water augmentation|
A sub-surface dam consists of a vertical, impermeable barrier through a cross section of a sand - filled, seasonal river bed (Figure 20). A ditch is dug at right angles across the river and into each bank, preferably where a rock dyke protrudes. This provides a solid, impermeable base onto which a simple masonry wall can be built within the trench. In some situations, the wall is raised gradually as sand from upstream accumulates behind the structure, forming a sand dam (Figure 21). The same approach may be used to control erosion in stream beds and to encourage deposition of alluvial deposits for agricultural purposes.
It is important to ensure that there is a seal between the vertical barrier and the impermeable layer beneath the sand to avoid seepage of water. Similarly, the barrier must also be extended into the banks to prevent lateral seepage and side erosion.
Water is taken out through a shallow well in the sand bed, or through a filter box, into a gravity pipe which runs through the dam to the point of use downstream.
In other situations, a small earth dam is built to hold back water and soil. The soil deposited is cultivable, and the water held back penetrates to the water table, providing a degree of groundwater recharge as well as increasing soil moisture. Cultivation usually starts late in the wet season and relies on the residual soil moisture in the alluvial bed.
For soil conservation purposes, the dam should be built as close as possible to the head of the stream as this is where the water begins to erode the soil. For water supply augmentation and soil conservation purposes, it is better to build a series of small dams along the same stream, rather than building one large dam. A sequence of small dams increases alluvial deposition and improves infiltration more than a single large dam.
Gabions (Figure 22) are often used as permeable rock dams, slowing down water flow and increasing infiltration, and reducing erosion and increasing silt deposition (Chleq and Dupriez, 1988)
Construction of the wall requires specialist advice to ensure it will withstand the pressure of the water behind it.
Extent of Use
Small dams of various types are common in southern Kenya, and occasionally found in Zimbabwe.
Operation and Maintenance
Once constructed, recurring costs are negligible. The structures may be assumed to last for 30 years.
Level of Involvement
The level of involvement depends on the extent of the project. Generally, small dam design and construction is within the capacity of local agencies. Often, governmental agencies and extension services are involved in the initial production of standardised designs for dissemination to communities.
A 3 500 m3 dam is estimated to cost approximately $8 250, resulting in an annual equivalent cost of about $0.11/m3
Effectiveness of the Technology
This technology is an effective means of augmenting drinking water supplies, providing additional arable lands, and protecting watercourses from sedimentation.
It is most suitable for use in sandy, seasonal rivers prone to siltation.
Reduction of erosion, management of silt deposition within river basins, and increased moisture infiltration within the soil profile and into the groundwater are environmental benefits associated with sub-surface dams.
Small dams store water from seasonal flows and are less vulnerable to siltation. Water is of good quality for consumption due to the filtering effect of the sand. When used for agriculture, the dams are effective in slowing down flows, and encouraging silt deposition and water infiltration, providing both soil and water conservation benefits. They allow crop production where otherwise it may not be possible, and reduce siltation in other, conventional water storage systems downstream.
The use of these structures is limited to drinking water augmentation in most cases.
There are no significant cultural problems.
Further Development of the Technology
This technology has not been widely adopted, probably due to the lack of indigenous knowledge on the principles of small dam construction.
Lee, M.D. and J.T. Visscher 1990. Water Harvesting in Five African Countries. IRC Occasional Paper No. 14, 108 p.
Chleq, J.L. and H. Dupriez 1988. Vanishing Land and Water. Soil and Water Conservation in Drylands. Macmillan, 117 p.