Newsletter and Technical Publications
<Sourcebook of Alternative Technologies for Freshwater Augumentation
1.1.13 Sub-surface Dams, Small Dams, and Sand Dams
|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).
Figure 20. Subsurface dam (IRC, 1991).
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
Figure 21. Construction of the dam continues in stages
as silt or sand is deposited behind the wall (IRC, 1991).
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)
Figure 22. A gabion is a container filled with stones, the
typical dimensions of which are given
(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
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
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.