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<Sourcebook of Alternative Technologies for Freshwater Augumentation in Africa> 1.1.4 Permeable Rock Dams Technical Description. Permeable rock dams are long, low structures across valley floors which have the simultaneous effect of controlling gulley erosion while causing deposition of silt, and spreading and retaining runoff for improved plant growth (Figure 5). This is a floodwater harvesting technique.
Figure 5. Permeable rock dams (Critchley et al., 1992). Permeable rock dams are usually constructed across relatively wide and shallow valleys. Some dams may require central spillways, especially where the water course is incised, but the majority of permeable rock dams will consist of long, low rock walls with level crests along the full length. This causes runoff to spread laterally from the stream course, and, if the dam is overtopped, results in water being distributed evenly along the length of the crest. Wing walls or spreading bunds on the dam should follow the contours away from the centerline of the valley or gully. In addition, contour stone bunds are sometimes used in association with rock dams, especially when the dams are widely spaced. Stone bunds are placed to prevent the overflow from the dam creating a gully downstream of the structure, which could erode back to, and undercut, the dam wall. In general, poor siting of dams, such as at the head of gullies, leads to their failure. Each dam is usually between 50 and 300 m in length. The dam wall is usually 1 m in height within a gully, and between 80 and 150 cm in height elsewhere. The dam wall is also flatter (2:1) on the downslope side than on the upslope side (1:2), to give better stability to the structure when it is full. A shallow trench for the foundation improves stability and reduces the risk of undermining. Large stones are used on the outer wall and smaller stones internally. Extent of Use Several hundred permeable rock dams have been constructed on the central plateau of Burkina Faso. Operation and Maintenance No data are available on the operational and maintenance costs associated with this technology. Level of Involvement Several organisations have been involved in the promotion of this technology in Burkina Faso. The structures are labour-intensive and require provision of mechanised transport for moving the quantities of stone required. Villages that request application of this technology usually pay half of the transportation costs, contribute all of the labour, and manage the dam . Where rock dams have been installed, a spill over into the construction of smaller rock dams by individual land owners has been seen. Governmental or agency involvement is comprised primarily of provision of technical advice needed during the construction stage. More recently, villages have been asked to form a land resource management committee to serve as a focal point for coordination of land and water resource management activities with extension agents. This committee draws up a land use management plan that provides the planning context for dam construction and other environmental protection projects. Costs A typical rock dam providing erosion control and water supplies to plots of 2 to 2.5 ha costs about $500 to 650 for transportation of materiel and about 300 to 600 person days of labour. Effectiveness of the Technology Permeable rock dams provide a more effective and popular technique for controlling gully erosion than gabions. Permeable rock dams, in addition to the effective control of gullies, have resulted in considerable crop yield increases behind the dams. Gullies are rehabilitated by the deposition of silt behind the dams, increasing the depth and quality of the soil immediately behind the dam as a result of the deposition of fertile silt.. They have also improved the amount of moisture available for crops. Yields of sorghum from land restored with permeable rock dams range up to 1.9 t/ha compared with a yield of 1 t/ha from equivalent, untreated land. Other crops planted behind permeable rock dams include rice (on heavy soils), pearl millet and peanuts. Suitability This technology is appropriate for regions with less than 700 mm annual rainfall, where gullies are being formed in productive land. It is particularly suited to valley bottoms with slopes of less than 2%, and where a local supply of stones and the means to transport them is available. Environmental Benefits The control of gulley formation and the encouragement of silt deposition can have positive effects on a river course and water quality. Advantages Advantages to be obtained from employing this technology include:
Disadvantages The disadvantages of using this technology include:
Cultural Acceptability This technology is an acceptable technology within local communities of Burkina Faso. Different approaches by various agencies have resulted in confusion in cases where some communities have been asked to pay a proportion of the costs and others not. Further Development of the Technology Implementation of a programme of permeable rock dam construction requires technical knowledge. In addition, development of a community-centred, sustainable resource management system is required which builds upon the existing demand for implementation of resource conservation practices in villages. There is need to develop cheaper construction techniques if this technology is to be used more widely and if construction is to keep pace with demand. Information Sources Critchley, W., C. Reij, and A. Seznec 1992. Water Harvesting for Plant Production. Volume II: Case Studies and Conclusions for Sub-Saharan Africa. World Bank Technical Paper No. 157, 133 p. Critchley, W. 1991. Looking After Our Land, Soil and Water Conservation in Dryland Africa, Oxfam, London, 84 p.
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