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

future.

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

diseases.

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

communities.

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:

reservoirs

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

the dam.

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

the valley.

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

materials.

Groundwater dams can be constructed from locally available materials such as

clay, brick, stone, concrete ,etc.

The water in the saturated sand reservoir can be collected using one or any two

of the following mechanisms:

the dam to water abstraction spots

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

Ababa, Ethiopia

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