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1.1.4 Permeable Rock Dams
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
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.
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.
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
The control of gulley formation and the encouragement of silt deposition
can have positive effects on a river course and water quality.
Advantages to be obtained from employing this technology include:
- 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
- Enhanced groundwater recharge
- Reduced runoff velocities and erosive potentials.
The disadvantages of using this technology include:
- High transportation costs
- Need for large quantities of stone
- Site specificity.
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.
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.