Engineering Geological Characterization of Oda Dam Site and its Catchement Area (Northern Ethiopia)
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Date
2003-06
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Addis Ababa University
Abstract
Dam 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.
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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 e
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Keywords
Engineering Geological