Water Portal / Rainwater Harvesting / Groundwater recharge / Permeable rock dams
Permeable rock dams consist of long, low rock walls with level crests along the full length across valley floors. This causes runoff to spread laterally from the stream course. This is a floodwater harvesting technique that spreads and retains floodwater runoff for improved crop growth, as well as controls gulley erosion. Permeable rock dams can be considered a form of ‘terraced wadi’, though the latter term is normally used for structures within watercourses in more arid areas.
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.
Permeable rock dams for crop production can be used under the following conditions:
- Rainfall: 200–750 mm; from arid to semi-arid areas.
- Soils: All agricultural soils—poorer soils will be improved by treatment.
- Slopes: Best below 2% for most effective water spreading.
- Topography: Wide, shallow valley beds.
This system is generally used across relatively wide and shallow valleys. This technology is appropriate for regions with less than 700 mm annual rainfall, where gullies are being formed in productive land. This 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.
| - Increased crop production and erosion control as a result of the harvesting and spreading of floodwater.
- Improved land management as a result of the silting up of gullies with fertile deposits.
| - Reduced runoff velocities and erosive potentials.|
- High transportation costs.
Construction, operations and maintenance
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 down slope 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.
A typical rock dam providing erosion control and water supplies to plots of 2 to 2.5 ha costs about US$500 to 650 for transportation of material and about 300 to 600 person days of labour.
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.
- Sourcebook of Alternative Technologies for Freshwater Augmentation in Africa (UNEP-IETC, 1998, 182 p.) or (alternative link). UNEP.
- Rufino, L., Water Conservation Technical Briefs: TB 2 – Rainwater Harvesting and Artificial Recharge to Groundwater. Sustainable Agriculture Initiative (SAI). August 2009.
- Awulachew, Seleshi Bekele (IWMI); Lemperiere, Philippe (IWMI). Water harvesting and development for improving productivity. IWMI. Tulu, Taffa (Adama University). January, 2009.