Environmental and Social Impact Assessment - Gibe III

Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099
The absorption areas at dam axis are identified by means of a blue colour range scale; colour grey has been
used where no absorption has been recorded. Representative Lugeon units at the stage are added for each
step or sequence with same value. The absorption areas at the dam axis are classified into two main zones. A
first upper one located immediately below the Omo river rising through the left abutment. A lower second
one lies below 450m a.s.l. Both of them, detected with the same shape upstream or downstream, show a high
water absorption. It can be concluded that the permeability of the sound bedrock behaving as dam
foundation, below an upper layer with a low/moderate absorption, is practically nil.
The Confined Aquifer
During the geognostic campaign at dam site, a confined artesian aquifer has been reached between 90 and
110 m below the riverbed, whose elevation is 678 m a.s.l. Following the course of the Omo river from
upstream, the roof of the aquifer seems slightly rising, from the pre-cofferdam site (576m a.s.l.) to
downstream reaching the dam toe between 585 and 598m a.s.l. The aquifer still lowers before the dam axis
dropping down between 541 and 544m a.s.l. Downstream of the dam axis the level of the aquifer rises again
reaching 565m a.s.l. The measured water discharges in the boreholes are very modest giving no evidence of
relevant groundwater circulation, if compared to the Omo river flows.
5.1.5.3 Reservoir Seepage risk assessment
Further to document [300 GEO R SP 002 A - Reservoir Seepage Risk Analysis]:
• no receptive basins at an altitude lower than the reservoir bottom have been located within a distance
of at least 100 Km from the reservoir itself; elevations below 670 m a.s.l. are recorded only
downstream of the dam. The Main Ethiopian Rift east of the Gibe III reservoir lies at elevations
higher than the reservoir as well. Therefore, seepage from the Gibe III reservoir towards nearby
catchments can be excluded.
• the structural lineament study led to recognize two main lineament sets directed N20°E (parallel to
the MER) and N120°E, and subordinate ones with directions N50°E, N70°E, N135°E, N160°E, N-S
and E-W. Single lineaments have a length in the order of 1.000 to 4.000 m, whereas major composite
structures have an observable persistence in the order of 30-40 Km. The structural pattern observed
in the study area (no large scale interconnection) allows to exclude the risk that significant
infiltration of water from the reservoir may occur along fault zones. A preferential filtration direction
can be inferred parallel to the N20°E set, i.e. to the main river course, further limiting the possibility
of consistent water loss away from the reservoir area.
• the supposed seepage from the reservoir towards the banks (saturation of the banks) doesn’t
influence negatively the filling time being the filtered volume as much as 11% of the total reservoir
volume. The filling time of the reservoir ranges between 12 and 16 months, i.e. 1 to 2 months longer
than the result obtained without considering seepage into the banks.
5.1.6 Soils
The alteration of the geologic basement in the area has led to the appearance of different types of soils, that
can be classified in the following groups.
To a first general soil group belong Eutric Nitosols, Ortic and Chromic Luvisols and Orthic Acrisols
(according to the FAO classification), all these soils have a significant clay Bt level of dark reddish brown
CESI SpA - Mid-Day International Consulting Engineers Page 91