Ayenew, Tenalem (PhD)Abay, Asmelash2018-06-112023-11-302018-06-112023-11-302003-06http://etd.aau.edu.et/handle/123456789/313Dam is one of the hydraulic structures that demand the integration of many disciplines during its investigation. Geomorphological, geological, hydrological and engineering geological investigations were carried out at the proposed Oda dam so as to understand and characterize the soils and rocks of the research area from engineering geological perspective. Oda dam is proposed to be either a masonry type its combination with a rockfill type of dam located in Tigray (Northern Ethiopia) across a seasonal river called Oda River for irrigation purpose. The catchment area of Oda is 65km2 and according to the preliminary engineering design done so far by REST, the crest length and maximum dam height are 265m and 46.5m respectively although this can vary a little bit depending on the final choices of the dam axis. Slope, land form and relief, soil and land use maps of the catchment area are produced at a scale of 1:50,000 to see their impact on the dam project and each unit are described. High attitude difference and steep slopes characterize the catchment of Oda, which in turn has result in the high rate of erosion. About 74% of the total area of the watershed is covered by the sloping to moderately steep (hilly) landforms. Textural classification of the soil show that the Oda soils fall into the Sandy loam (residual origin mostly) which represent the thin soil cover (25 - 50cm) for majority of the catchment, and the silty loam soils which are restricted to the stream banks and foot slopes. Four land uses namely grazing land, shrub land homestead and others (rock outcrops, gullies, etc.) are identified and, out of these grazing land has the largest aerial coverage (84%). The main geomorphological processes that prevail in the study area are weathering, erosion, mass wasting, and geologic structures (faulting and dyke) which are responsible for the formation the various geomorphic units in the area. ii Geological maps of the whole catchment are, and reservoir and foundation at scales of 1:50,000 and 1:1000 are produced respectively. The main lithological units of the area include the alkali olivine basalt, aphaniticbasalt, welded tuff and very small outcrops dioritic rocks. The area is covered dominantly by the oldest alkali olivine basalts, which are medium to highly weathered. Slightly weathered aphanitic basalt, which occurs as dykes, sills, cape rocks and they have variable trends. The welded tuff is found at the upper part of the Oda catchment. Overlying the basaltic units and are characterized by sparsely and scattered small sized lapilli (2mm to 4mm in diameter) through out the matrix of ash. Small and negligible outcrops of dioritic outcrops are also observed in a small part of the central part of the Oda River (not mappable) and to the north of the reservoir area just on the water divide of Oda-Tengago catchments. The quaternary deposits are recent deposits found below the dam axis forming the plain areas. Dominantly two joints sets trending WNW-ESE and NNE-SSE affect the basaltic units although some third joint sets trending nearly E-W are also observed at places. 23 years of rainfall records of both the Alamata and Korem station is used to calculate arithmetic mean annual rainfall of the study area and is found to be 869mm. The calculated actual evapotranspiration value (based on long-term water balance approach) is 478.6mm. Comparison of rainfall and PET shows that rainfall eenEngineering GeologicalThesis