Water resource development projects in Ethiopia: Evaluation of Damte dam operation

Ethiopia is one of the largest and most populated countries in Αfrica and is situated on the most eastern side of it, the Horn of Africa. It is a country with a very diverse topography, which affects the climate, the ground conditions, the flora and the distribution of population. Although it is considered one of the poorest countries in the world, in recent years it has had a very high growth rate, with increasing electricity production and improving infrastructure being the key pillars of this change. One of the main problems faced by the country is that a large percentage of the population doesn't have access to good quality drinking water. Due to the fact that Ethiopia bases its economy primarily on agricultural production, the lack of water introduces restrictions on irrigation of crops with implications for food security of the country. The lack of water is not due to the climate of Ethiopia, since most of it is characterized by abundant rainfall and average temperatures. The problem lies in the seasonal character of the rainfall and the lack of infrastructure for water management in order to meet the needs of the population through the whole year. In recent years there has been a tendency to build large dams which, although they have the overriding goal of generating electricity, can be exploited for annual water regulation. Under these circumstances was constructed in 2005 the small-scale irrigation dam Damte, which was built to relieve a local community in Damot Gale Worenda in North Omo Zone in south-west Ethiopia. For the funding of the project there was a major contribution of the Greek state, the Consulate of Ethiopia in Greece and the Ethiopian community of Athens. Also the study and construction of the dam were carried out from engineers of the Department of Water Resourses and Environmental Engineering of the School of Civil Engineering of the National Technical University of Athens and the Non Governmental Organization "Engineers without Borders ". Its purpose was to collect water to upgrade the quality of life of 400 families. This embankment dam has a height of 15 m. Water is transported downstream through a water intake - outlet system, while safe drainage of excess water is achieved with the frontal spillway structure on the body of the dam. The total storage capacity of the dam is 560,000 m3 with the dead storage is estimated at 150,000 m3 for the 25 year life span of the dam. The average annual rainfall in the project area according to the study is 1627 mm and the average temperature is 19.8 °C. The soil in this area is tropical and residual, characterized by severe erosion and change in shear strength with repeated wetting and drying of the material. The main objective of the thesis is to assess the operation of the Damte dam and for this purpose an on-site visit to the project area. The overall condition of the dam is good with slight wear on its body and on gabions of the spillway, but the rapid accumulation of sediment in the reservoir risks undermining its continued operation. The benefits of the project in the everyday life of the local community are impressive and the standard of living for the inhabitants has improved dramatically. Finally, due to the educational nature of the visit, a guided tour was made of the largest at the present time dam in Ethiopia, Gilgel Gibe III. The solutions proposed are aimed at repairing the damages that have been identified, mainly dealing with the reduction of reservoir capacity due to siltation. The causes of accumulation of sediments are due to erosion of the basin as well as the inflow of soil material from surface slips of the lake slopes. Due to the fact that during the visit to the dam the high water level did not allow an estimation of the dead storage, an effort was made to estimate this volume with help of aerial photos from Google Earth. Approximately the total volume of sediments for 13 years of operation of the dam was estimated at 123 000 m3. Removal of the sediments can only be done by mechanical extraction, therefore the most suitable solution is dredging. After a survey, the Finnish dredger Watermaster was found, whose technical characteristics are ideal for this project. Finally, a solution was sought to protect slopes from erosion and two alternatives are presented, one using gabion mattress and the second one by using rip – rap in combination with geotextile.