Environmental and Social Impact Assessment - Gibe III

Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099
The construction of the dam will determine a reduction in the Omo River total solid transport downstream of
the reservoir.
The coarse fraction of the solid transport (i.e. gravel and sand), carried from the upstream catchment, will be
trapped into the reservoir.
However in the lower Omo river the sediment yield is substantially made (more than about 70 %) of
extremely fine particles (colloids, clay, fine silt), which will remain in suspension during the annual
operation of the reservoir.
On the base of the first analyses 21 and on the results of the previous studies in the river basin (Omo river
basin Master Plan, Gibe I, Gojeb and Halele Werabesa hydrological studies) it can be assumed that:
• The average total solid transport in the Omo river is composed of a relevant fine fraction (colloids,
clay, silt) totalling about 80 % and of a smaller coarser fraction (sand, gravel);
• The existing Gibe I reservoir, which has similar operating conditions to those planned for Gibe III
reservoir, indicates that :
� Average suspended solid transport (colloids, clay, silt) is not substantially reduced by the reservoir
trapping effect which regulate discharges during the year;
� The most relevant mean water quality parameters (phosphorus, nitrogen, COB, BOD, etc) will
probably not be greatly varied by the reservoir operation.
• Oligotrophic conditions (i.e. quite clear lake with low biological productivity) are likely to develop
in most of the reservoir;
• While the water quality and the mean sediment content will not greatly change because of the
reservoir operation, the trapping of the coarser sediments, coupled with the effect of restored
hydrological conditions of the Lake Turkana system on the longitudinal slope of the Lower Omo
River, may cause erosion at places previously characterised by sedimentation activity.
These local phenomena can be negative for the riparian environment and will need hence to be detected
through regular monitoring of the suspected critical locations, and where necessary prevented by means of
carefully designed river training works.
7.5.6 Agriculture
Under present ‘average’ flood condition, large tracts of floodplain are submerged annually along the lower
Omo River and around the river mouth. The annual flooding of the land bordering the Omo River soaks the
land for traditional recession cultivation and dry season grazing replenishes lakes and swamps on the
floodplain and favours fish breeding.
To satisfy the demand for traditional recession agriculture, dry season grazing and fishery resources,
seasonally more water will be released and flooding will be created on the land bordering the Omo River.
One of the immediate benefits of constructing the Gibe-3 dam will be the ability to avoid severe drought
periods and to control the large flooding downstream which cause the loss of lives, both human and animal,
as well as damage to property and infrastructure. However it is recognised that a complete cessation of the
average annual floods would be disastrous for the communities, which practice flood recession agriculture.
21 Downstream Impact Assessment Report
CESI SpA - Mid-Day International Consulting Engineers Page 230