Environmental and Social Impact Assessment - Gibe III

Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099
extension. None have been recorded in the area West and Northwest of the Omo river, although tremors were
felt at Mizan Teferi in 1971. This information suggests that presently the Chew Bahir rift system is
seismically more active than the Turkana rift system.
The project area (East-North East quaternary trending) does not seem to be active at this time, as illustrated
in detail in the above mentioned report.
Despite the evidences that a certain seismic activity affected the region in historical times, according to the
L1D Geological Report no evidences seem to exist of present seismic activity in the project area.
5.1.4 Reservoir Slope
As illustrated in the report [300 GEO R SP 002 A - Reservoir Seepage Risk Analysis], the possible
occurrence of major landslides or of potential slope instabilities has been inferred on the basis of positive
and/or negative landforms potentially related to accumulation and detachment areas along the main and
lateral valley flanks. However it shall be recalled that no evidence of sliding is directly and easily extractable
from the ASTER-derived photo-interpretation, as the 15 m ground resolution doesn’t allow for the
individuation of detailed discriminating features; to do this, an extensive aerial photographic survey will be
envisaged in the following design phases. Therefore, the areas already highlighted as potentially coinciding
with major landslides will be furtherly verificated on the basis of aerial photos and where necessary of field
surveys.
Along the river courses some areas characterized by high relief energy, i.e. by vertical or near-vertical walls
up to some tens of meters high have been detected. In most cases they probably correspond to massive
volcanite flows or sills. These areas are relatable to the risk of potential rockfalls, depending on the
fracturing state of the rock and on the geometric intersection relationships between the main joint sets and
the topographic surface; instability mechanisms such as toppling or collapse of wall portions might be
produced by saturation of the rock mass consequent to the reservoir filling. The presence of hard, fractured
rocks lying upon weak horizons such as ash levels, interbedded paleosoils or weathered volcanites, may
potentially increase slope instability, in consequence of the reservoir filling and saturation of the rock mass
on the valley flanks, provided that appropriate topographic and fracturing conditions occur.
As referred in the document 300 GEN R SP 003 B - Main Report, the preliminary analysis of the available
geological mapping and photos however has not currently identified major stability problems on the
reservoir slopes.
In the reservoir area, it is possible to meet two types of landslides in relationship with the principal
lithological types. Rock falls and toppling occur in Trachyte and rhyolite. Landslide occured in pyroclastic
rocks (tuff, ash, agglomerate and volcanic sand).
Below the reservoir operating levels, and therefore not relevant for the operation of the plant, some stability
issues might occur. The talus along the stream bed and also the presence of incoherent and semi coherent
pyroclastic materials might determine sliding.
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