Rainwater Harvesting with Subsurface and Sand Dams
Mesfin Shenkut, Ethiopia, email: email@example.com
Ethiopia has a yearly surface water potential of 110 billion cubic meter coupled
with an estimated groundwater resource of over 2.6 billion cubic meter. One
way or another these water resources are derived from a yearly rainfall. The
great diversity in geography associated with high rugged mountains, flat topped
plateaus, deep gorges and the extreme variation in altitude ranging from an area
of below sea level to peaks reaching over 4,500 meters result with high variation
in annual rainfall ranging between 200 to 2000 mm per year.
The present coverage of water supply of the country is about 17%. Given the
current population of about 60 million accompanied by annual growth rate of
over 3% with incompatible economic development, it is unlikely that substantial
improvement in the water supply coverage will be attained within a foreseeable
Even though rainwater harvesting ranks high on the priority list as a cheap
possibility to alleviate the severe drinking water shortage, roof water catchment
is not popular in the country, particularly in the rural areas, since the vast
majority of the residential houses have thatched roofs, not suitable for rainwater
collection, and the high costs of the water storage facilities.
Ponds are the most prevalent means of rainwater harvesting in rural areas of
Ethiopia. In the Gambella region for instance, over 30% of the water supply for
drinking purpose comes from ponds. They are also very common in the arid and
semiarid parts of the country where alternative water sources are not available.
These ponds are planned, constructed and managed by the communities.
Besides its high turbidity and bacteriological load, the water stored in the ponds
is not available all year round due to water loss through seepage, sediment
deposit and evaporation. Apart from natural self-purification, the ponds do not
have any sort of water treatment facilities and improved water abstraction
mechanisms. Due to the unsanitary situation around the ponds, they create
conducive breeding areas for mosquitoes as well as for spreading of water borne
In most lowlands of Ethiopia the annual rainfall is below 600 mm, however,
there are periodic short-term floods due the drainage of highland rains. The
sandy flood way valleys preserve the water until it is depleted by evaporation
or/and by gravitational flow to lower elevation. The dwellers living along these
valleys obtain water for domestic and other uses during the wet seasons and for
some months after the rains by digging pits. When the depth of the water in the
sand deposits gets deeper, the villagers find it impossible to dig any further and
they travel long distances to fetch water for themselves and their livestock.
Damming the water during the rainy season and using it in the dry period is an
obvious solution. However, the construction of conventional surface dams is
beyond the technical and financial capacity of the users.
Technically two ways of dam construction are possible. The first is the common
surface water dam, which can be built across the river valleys to hold back
water and control its flow from the reservoir. The second alternative is a
groundwater dam. It is a barrier constructed across a permeable sandy valley to
store water below ground level in the upstream reservoir.
Though the water holding capacity of surface dams is very high, its construction
cost is extremely discouraging and is beyond the financial capacity of small
Apart from the enormous cost differences as well as the severe problems of
siltation, causing the most dam failures in the country, groundwater dams have
the following advantages over surface dams:
•Simple to construct, using locally available materials
•Water treatment facilities are not required
•Less evaporation from the storage reservoir
•Little loss of land
•No damage due to dam failure
•Pollutants such as mosquitoes and snails can not exist in the
•Siltation does not create any problem
•Less vulnerable to pollution by animals or human beings
Types of Groundwater Dams
The two techniques of damming ground water are:
1 Sub-surface Dams, which are dams constructed below ground
level to store water of natural aquifers in reservoirs upstream of
2 Sand Dams, which are constructed across valleys to make the
upstream reservoir filled with sand, gravel or stone to make
artificial aquifer, which is replenished each year by the runoff in
Design Parameters of Groundwater Dams
As simple as they might look, the design and construction of such dams require
careful study of the different parameters which are likely to affect the ideal flow
condition of groundwater dams. Reports indicate that quite a number of such
dams have been built which are not functioning due to wrong selection of
constructions sites, incorrect design or poor workmanship.
The following are the basic physical considerations that have to be thoroughly
examined in the design of groundwater dams.
The first step is to study the valleys where villagers obtain their water supply by
digging pits along sandy and dry riverbeds. The existence of green vegetation
during all seasons along the banks of sandy valleys is another indication.
Satellite photographs show also red colors along the valleys with green
vegetation nourished by the subsurface water.
The recommended slope to construct subsurface dam is less than 5%. This is to
avoid depletion of the water through natural flow and to have acceptable size of
sediments as well as to store good volume of water in the reservoir upstream of
the dam. Besides, consideration should be taken to look for a narrow valley
where the elevation of the bedrock outcrops to optimize construction costs. If
the dam site is chosen in the upstream of the service area, gravitational flow
could be attained.
In order to avoid unexpected flooding and minimize pumping from the
construction sites, all construction work should commence and be completed in
the dry months.
Precaution has to be taken to chose the correct foundation for dam construction.
The foundation material has to be a watertight solid rock or other impermeable
materials such as consolidated clay.
Using hand digging or conducting auger tests the type of the foundation material
has to be determined. Besides the wing walls of the dam have to be well
embedded in the sidewalls insuring no leak through this part.
The hydrogeology of the aquifer materials should also be analyzed. As the
amount of water to be extracted is a function of the void ratio and the grain size
of the sand, it is always advantageous to look for course sand and gravel with
grain size of over 5mm. Fine sand and silts are not appropriate reservoir
Groundwater dams can be constructed from locally available materials such as
clay, brick, stone, concrete ,etc.
Water Abstraction Mechanism
The water in the saturated sand reservoir can be collected using one or any two
of the following mechanisms:
•from gravitational flow, by extending outlet pipes from the bottom of
the dam to water abstraction spots
•from hand pump fitted wells
The potential of sand dams and subsurface dams to supply good quality of water
at a reasonably low price is undoubtedly very high in Ethiopia as well as in
many parts of Africa. However low awareness on their resourcefulness and
limited knowledge of planners on how to design and construct these units have
hampered their development. It is therefore necessary to give the issue the
importance it deserves and to promote this technology.
Erik Nissen-Petersen (2000), Water From Sand Rivers, Technical Handbook
No.23, RELMA, Nairobi, Kenya
Nilsson Ake (1984) Groundwater Dams for Rural Water Supply in Developing
Countries. Pub. royal Institute of Technical Dept. of Land Improvement and
Drainage Stockholm, Sweden
Shenkut Mesfin (1999), Design of Subsurface Dam for Bori Village, Addis
Shenkut Mesfin et al. (2000), Rainwater Harvesting in Ethiopia, Status,
Potential and Problems, Nairobi, Kenya
Waterlines (1986), Water from sand, Vol.4, No. 3, January 1986, London, UK